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Marcel
27-07-2017, 03:58 PM
As some of you know in the recent past I built a 2204 style head out of an old defunct 1970's Playmaster chassis. I didn't do a build diary for it and only have the result to show everyone. So to make a difference I'm doing a build diary on my latest amp build that is loosely based on a mid '90's AC15 circuit.

The objective is to build a clean 15W EL84 based amp with tremolo..

After scouring dozens of circuits from VOX, WEM, Gretch, Hohner, Hynes, Watkins, Marshall and the AX84 web site, and researching parts suitability and availability, the VOX AC15 circuit suited most of my needs. As I couldn't find original VOX transformers but could find 18W Marshall types I'll be using those instead and then making adjustments to the circuit as required. Marshall do have a reasonable 18W combo amp (from where the transformers came) but it does not have a tremolo feature, however some aspects of that circuit may be needed to make this project work.

As it stands, I have most of the important bits like HV caps, tag strips, pots, knobs, tubes, sockets, transformers and a chassis from Jaycar. Still waiting on delivery of numerous resistors and various caps from RS-online.

Marcel
27-07-2017, 04:02 PM
Got stuck into doing some of the much needed metal work... Cabac make and awesome multi sized hole reamer...

The power transformer is just sitting there. needs the square hole for the windings to be cut out. The other parts are pretty well in their correct positions.

WeirdBits
27-07-2017, 04:50 PM
I was going to suggest ClassicTone (http://www.classictone.net/), but clearly there's no need.

Marcel
27-07-2017, 05:22 PM
I'd like to hear your reasoning behind the suggestion WeirdBits.

Main reason I got them was availability... would be good to have more info...

Guvna19
27-07-2017, 06:07 PM
Great thread Marcel, much amp building jealously !!

fortunately for me my tube amp desires are currently satisfied..... but that wont last ;). trying to save for a custom pitbull kit atm. fingerscrossed, scratchy lappy has left me wanting more and to really feel part of this awesome commune

look forward to watching your progress

regards
Guv

dave.king1
27-07-2017, 06:19 PM
I know it's somewhat after the horse is through the gate but have you also had a look at 18Watt.com and all it's sub branches

https://www.18watt.com/

Guvna19
27-07-2017, 06:36 PM
I know it's somewhat after the horse is through the gate but have you also had a look at 18Watt.com and all it's sub branches

https://www.18watt.com/

Is this normal ? ...for some reason, every time i read the various articles on this site , i.e guitars / amps / pedals / upgrades etc. my brain goes fuzzy, my credit card gets twitchy and i sense a whining noise coming from the mrs.....?

andrewdosborne
27-07-2017, 06:55 PM
Is this normal ? ...for some reason, every time i read the various articles on this site , i.e guitars / amps / pedals / upgrades etc. my brain goes fuzzy, my credit card gets twitchy and i sense a whining noise coming from the mrs.....?

Perfectly normal....

WeirdBits
27-07-2017, 07:11 PM
... hear your reasoning ...

Just availablility, price, range and a few from here have purchased from them before. They seem to have options not available elsewhere or at least at better prices.

Marcel
27-07-2017, 08:18 PM
I know it's somewhat after the horse is through the gate but have you also had a look at 18Watt.com and all it's sub branches

https://www.18watt.com/

It's never too late... One can always jump onto another horse and rope in the buggar that bolted...

I went and had a quick look at the 18watt site. Lots of good info there but as yet haven't found anything to change my current plans on this build. Truth be known I've been actively planning this amp build for about 4 months now so barring unexpected events there are only a few things left open for modification. That also doesn't mean that once power does finally flow through this little beastie that there will be nil tweaking and sweetening.

As for the 'whining noises' from the better half I have that under control Guvna19... In the next few months She's getting a new sowing room, an en-suite and a kitchen makeover... Maybe it's all getting out of control...lol

Guvna19
27-07-2017, 08:55 PM
As for the 'whining noises' from the better half I have that under control Guvna19... In the next few months She's getting a new sowing room, an en-suite and a kitchen makeover... Maybe it's all getting out of control...lol[/QUOTE]

no sowing room here but done the kitchen / bathroom thing last year and new car prior to that, and guess what, cant shake the noise....lol
must be some sciency hearing device we could all buy to fix it......just kidding....all good, cant really complain...out loud anyway:cool:

Simon Barden
27-07-2017, 09:00 PM
In the next few months She's getting a new sowing room, an en-suite and a kitchen makeover...

Wow! You must have done something really bad!

Marcel
27-07-2017, 09:24 PM
You don't know the half of it Simon...lol

Marcel
28-07-2017, 04:19 PM
More metalwork... put extra holes for possible later modifications and experiments, and with some drilling and filing got the large hole for.the power transformer done. Then bolted on the sockets and the transformers.

Not your standard look for an amp head which is okay as it is unlikely to ever find itself in a performance on any real stage with a real audience...

Next up for work is the front and rear panels...

Marcel
29-07-2017, 12:41 PM
Front and back panels drilled and assembled.

To stay with the VOX theme I found some chicken head knobs for all controls. For the time being and convenience using all Alpha pots on all controls.

Front will read - input A & B, gain, treble with bass underneath, master vol, vibrato/tremolo intensity with speed underneath, then a space for either a reverb level or top cut, and the power switch.
Rear is - IEC power in, main fuse, HT fuse, footswitch, speaker out A & B, and the speaker impedance selector.

Access inside is from underneath and is not as roomy as I'd like but I suppose I'll just have to cope...

wokkaboy
29-07-2017, 12:48 PM
nice work Marcel looks awesome

Marcel
30-07-2017, 03:35 PM
Thanks Wokka. The real 'rats nest' photos are yet to come...

I wanted to tidy up the transformer wires before I set this project aside for a few days while I work on the RCM-4F. Ended up wiring up all the 6.3VAC heater connections plus all the Mains 240VAC wiring. As this will have a solid state HT rectifier there was a spare 5.0/6.3VAC 2A output from the main transformer which has been pressed into service as the heater voltage for the output tubes instead of a rectifier tube. As it stands the output tube heaters will be floating but if hum in the finals manifests itself I can always retrofit a pair of 100 ohm resisters to ground so as balance them up.

As a side line I've decided to loom all cables with older school harnessing. The aim is to avoid using zippy ties on the entire project.

Dedman
30-07-2017, 04:19 PM
I look at that and think "I could do that" then I fall down laughing at my own joke. :)

Thats super neat and tidy.

stan
30-07-2017, 04:24 PM
This is great work Marcel, very cool build

WeirdBits
30-07-2017, 05:12 PM
Looks choice.

DrNomis_44
30-07-2017, 06:20 PM
Looking cool Marcel, once she's up and running you'll have to do a demo of it so we can all hear what the amp sounds like mate.


I'm definitely going to have to start ordering the new bits and pieces I need for rebuilding the amp head I'm making, it's been languishing in a partially worked-on state for far too long for my liking.

Marcel
30-07-2017, 07:59 PM
Thanks Dedman...

Don't rush Doc... though it does hang heavy on the heart when a project like that sits unusable and dormant...

Last night I did some receipt number crunching and Ebay auction surfing and the better value odds are on the 2nd hand market. It turns out that I could have bought a real complete near new 2nd hand VOX AC30 plus a trip to Melbourne to pick it up for near the same dollars this clone AC15 head has cost me. Well almost, I will have a small stack of spare bits and tubes left over as sometimes (most times) I had to buy 10 or 25 when I only needed 1 or 3. Much food for the digestion of others who seek to follow my footsteps...

However, The money is spent now, and unlike a real VOX AC30 this build is at the mercy of my customising whims... so I'll just march (build) on forward and we all will see/hear wherever I end up...

(Note to self - learn how to upload to soundcloud...)

DrNomis_44
30-07-2017, 11:00 PM
Thanks Dedman...

Don't rush Doc... though it does hang heavy on the heart when a project like that sits unusable and dormant...

Last night I did some receipt number crunching and Ebay auction surfing and the better value odds are on the 2nd hand market. It turns out that I could have bought a real complete near new 2nd hand VOX AC30 plus a trip to Melbourne to pick it up for near the same dollars this clone AC15 head has cost me. Well almost, I will have a small stack of spare bits and tubes left over as sometimes (most times) I had to buy 10 or 25 when I only needed 1 or 3. Much food for the digestion of others who seek to follow my footsteps...

However, The money is spent now, and unlike a real VOX AC30 this build is at the mercy of my customising whims... so I'll just march (build) on forward and we all will see/hear wherever I end up...

(Note to self - learn how to upload to soundcloud...)


Uploading to soundcloud is relatively easy, although you need to go through the initial registering process, which reminds me, I need to pay for more time if I want to upload more stuff to soundcloud.

Marcel
02-08-2017, 07:11 PM
I lashed out and ordered a Behringer U-PHORIA UMC204HD USB 2.0 Audio Recording Interface as my current AtoD interfaces just are not up to the job. Hopefully the new input device will yield better and more consistent results.

Did some hunting in the shed and found some items of golden info. The Radiotron book is circa 1940 and is chock full of tube theory, as is the 1947 RCA receiving tube manual. The 1974 Philips pocket book and the RCA book both have lots of tube data text and info but neither is the book that I was actually looking for.

The guitar book is from 1982 and has plenty of interesting tid bits of current and forgotten info....

Fretworn
02-08-2017, 07:15 PM
I have a very well worn copy of "The Guitar Handbook". I think it is in my daughter's room because she uses the chord book in the back.

WeirdBits
02-08-2017, 07:22 PM
Yep Frets, excellent book, was glancing through my old copy just the other day.

DrNomis_44
02-08-2017, 08:24 PM
I've got a more up to date copy of that guitar handbook, definitely worth the money, and I'd be the first one to recommend that all guitarists buy themselves a copy, whether they play bass or guitar, the music theory section alone is worth the money, plus there's a useful preamp project that you can build on a small piece of Veroboard and install in your guitar, I've built it and it does work well.

Marcel
04-08-2017, 04:27 PM
Output stage and HT power supply wired.

Not exactly happy with the current amplifier main common ground point which may get changed if it all bugs me too much. Ground commons point for the amplifier will not be chassis earth but a point separated by a 10 ohm resistor, two diodes and a 220nF cap in an effort to minimise potential ground loops with external connected equipments (like pedals).

FYI - the tremolo oscillator output feeds into the amp via the two 220k output tube grid resistors that in other amps are normally held solid at ground potential.

Guvna19
04-08-2017, 08:09 PM
impressive skills - much jealously watching this thread - I want a 'Marcel' built amp

Marcel
05-08-2017, 06:35 AM
Thanks for the compliment Guv... but lets wait til we hear it before making rash decisions....lol

dave.king1
05-08-2017, 06:53 AM
It doesn't appear to be an issue here but thought I'd drop this for future reference.

Back in the day I had a Lennard GB150 and later on a Dan Armstrong 100W, both had discrete components mounted on poorly mounted rather long tag strips, the flexing in the strips caused by the vibration of transport, bumping in and out and very loud playing caused the tails on the components to break regularly ( mainly the larger caps )

Wonderful sounding amps but once the problems started they became too unreliable.

Just something to keep in mind when mounting long boards

Marcel
05-08-2017, 07:19 AM
I know what you are suggesting Dave however I have my doubts it will ever be a problem here.

My long boards are at best about 12cm and held at each end so the there is the potential for flex in the centre. But at 15W and with the tubes so exposed on the top and unless it is a dire emergency I doubt this amp will ever hit the road or see a gig. It will most probably see out its days in someone's bedroom or if it's lucky get to live the life of Riley hooked up to a 1x12 in the air conditioned comfort of a small recording studio.

But one never knows. I'll check it out and might put a third stay under each of the long boards.

dave.king1
05-08-2017, 08:41 AM
I couldn't see a problem with it either Marcel, just putting it out there for when/if folks go bigger

Marcel
05-08-2017, 04:11 PM
Layout - layout - layout....

A huge headache has developed. My brain is hurting...

I wired the input jacks and started installing components around V1...all good.. tight...but all good.

Hmm... The HT on V1 has gotta come from somewhere!... Hmmm... Vox in their wisdom use separate HT drop down resistors and HT filter caps for each stage whereas Marshall cascade the HT on each stage with the highest on V3 and dropping down in voltage to V2 and with the lowest HT voltage on V1. So in a Marshal (and most other brands) it is a chain but in a Vox (and Hohner) it is a star arrangement.

I've got all the needed caps... no problem....
Where to put the caps and various dropping resistors so the are reasonably close to their respective tubes and I don't have dozens of wires carrying top end HT all over the place.....In the tiny little space that is my amp ...Big headachey problem.!!!...

Simon Barden
05-08-2017, 06:16 PM
Can't you modify the circuit so that it does cascade?

DrNomis_44
05-08-2017, 06:48 PM
It should be relatively easy to mod the HT power supply so that it cascades like in a Marshall amp, doing that may also come with some benefits too, for example getting rid of hum noise from the HT power supply in the first gain-stage (which is probably why Marshall chose to do it that way), incidentally, Fender have the HT power supply cascading too.

Marcel
05-08-2017, 09:34 PM
It would be all too easy to change to a cascade HT supply. And in some aspects a cascading HT is a better way to do things as any failure (burn out) of any of the supply components will effectively kill the V1 tube thus stopping the amp from trying to do anything....

BUT ...

Part of the charm of a VOX amp is how it breaks up with increasing volume/gain.... and I think the HT arrangement is integral to that break up behaviour... So therefore in my build the only significant changes to the HT config will be the use of a Marshall 18W power transformer (which isn't very significant as the HT voltage and currents are very close), and solid state rectifiers instead of a tube rectifier.

Marcel
06-08-2017, 09:27 PM
Ever had one of those days? The sorta days where one of the kids calls for help at 3:30am? ... "..and Dad, can you bring the big trailer ?"...

So, this evening it is a trip down memory lane YouTube entertainment style from a time before computers, VCR's and iPods.... from when and how I was first introduced to the magic we work with today...

https://www.youtube.com/watch?v=ryCr4Y7ypLA

dave.king1
07-08-2017, 05:23 AM
Reminds me of Electronics 3 in first year at college, damn that was a long time ago :D
( a certain long haired band from Liverpool toured Australia that year )

Marcel
07-08-2017, 10:21 AM
One of the smaller issues I have is as V4a will be the tremolo oscillator and as this build will not have a reverb circuit thus at the moment V4b has nil purpose or job to do. Normally in a VOX AC series amp V4b would be the reverb recovery amp and a V5 tube the reverb drive amp. So what to do with the spare V4b?

Anyone have any useful suggestions?

As a side line, in my searching I have uncovered a number of useful and informative web pages.
https://robrobinette.com/How_the_Marshall_JCM800_Works.htm is a bit technical however gives excellent explanations of why thing are as they are in Marshall designs and helps with understanding other tube amp circuit designs, and to some substantial extent about tube amps in general.

Marcel
08-08-2017, 04:13 PM
Only the tone stack and the tremolo oscillator to go before the real testing starts...

DrNomis_44
08-08-2017, 05:45 PM
I checked out that link you posted, it's a good read so I've bookmarked it, cheers Marcel.

dave.king1
08-08-2017, 07:50 PM
That loom lacing looks very much like what I learned as a TiT more than half a lifetime ago, nice work

Marcel
08-08-2017, 09:20 PM
It's wonderful how memory works Dave.

It took me a number minutes of tying a few weird random knots before it all suddenly fell into place and I was getting it done again like a pro. I first tried loom lacing when I first started high school and later in the RAAF spent quite a few hours making up huge test harnesses with hundreds of wires for gear that was in service back then, and there only a few rules to remember, and it is so easy.

It will be interesting to see in a few years if anyone can accurately date this amp build. Add a bit of dust build up here and there, a spot of rust... I think most will guess wrong...

Might re-visit my 2204 build and give it the loom lacing treatment too ...

Marcel
09-08-2017, 04:24 PM
It's all done... well the basic build is done. All the components I want in on this first build run are fitted.

Connected mains power to it and the HT volts and filament volts appear okay as what I measured is what I expected considering there are nil tubes fitted. HT came in at 429VDC which is a it high but should sag once tube idle current load comes on. Self discharge to a moderately safe 50VDC of HT without the tubes fitted took nearly 10 minutes. This time should substantially decrease once the tubes are fitted.

Still need to double/triple check it all that everything is in the right places, and then fit a set of tubes, and then check that it actually works....

dave.king1
09-08-2017, 05:36 PM
And then the mandatory sound clips ( please )

Marcel
09-08-2017, 06:57 PM
That's something which is happening in the background Dave.... Got a new Behringer UMC204HD box to get those quality real life sounds of sweetness into this darn tarnation Internetty thingamy-bob... just gotta work out how to make the darn com-putter play back what I think I put in .....lol

Might start by plugging in my SM57 into it and see what I can get out of this new USB dongle. It certainly seems a lot better and easier to use than the modified iMic that I was using before.

Marcel
10-08-2017, 01:32 PM
Yeah, I know...Where is the sound clip?.... It's coming, okay...

It seems I have a working AC15 clone ...... Yay!!

Plugged in a set of JJ tubes. Fired it up... All the right glowy bits started glowing, and no smoke from unexpected places was a relief. Hooked up some multi-meters and started measuring... HT now at 345VDC, Cathode of V5 & V6 at 11.66V, Anodes at 331V....Hmmm, That makes out to be 14.9W of plate dissipation. They don't look Red at all.... I'll finish all my readings of the other tubes and come back once it has truly warmed up.

Long story short, after a good 15 minutes all the reading had remained pretty static and the EL84's were showing no signs of Red plating... Interestingly the 4 ohm 160W speaker that I was using as a test load made all the correct noises as I probed around inside the amp with my volt meter which gave me an abundance of confidence to try a real signal from a guitar.

My confidence was rewarded.... 15W of audio out isn't a lot, but it is respectable. A hour later I had tried my LP-1SSP with P90's, a LP Studio, a SD rails loaded Strat and even a JB and all sounded the way I thought they should. Winding up the gain control brought on a decent amount of crunch, tone controls seem a little weak but do as they should, and opening the master volume to full was loud but not ear damaging.

Only thing that didn't work was the tremolo. Checked the circuit again. The foot switch that controls the tremolo simply shorts out a 47K resistor, and in the default state that I had the amp the resistor was not shorted out.... so I fitted a alligator lead to see what would happen. Bingo, I had a working tremolo. Switched off the amp and changed the rear footswitch socket so the default is to have the tremolo switched on and then when a footswitch is plugged in the tremolo can be switched off an on.

So it is complete and it works.... I will need to re-visit the EL84 bias issue and cool the tube down to less than 12W plate dissipation at idle by fitting a larger cathode resistor once replacements arrive from RS-online.... but there is no hurry.... :D

And yep, I did my usual..... forgot the knob/socket labels on the front and rear panels.... You'd think one day I'd learn to at least remember that....

dave.king1
10-08-2017, 02:01 PM
Great work Marcel, I guess building an amp you don't have to wait for the finish to dry & cure

Dedman
10-08-2017, 02:23 PM
Bravo! it's been interesting and educational watching this come together.

Chuck
10-08-2017, 05:21 PM
Great work Marcel - it's been fascinating following along.

Marcel
10-08-2017, 08:18 PM
Thanks guys... building this amp was quite a lot of fun, and very rewarding...

I left a lot of the technical details out as I fear most of what I could have written would have gone straight over everyone's heads like a supersonic F1-11 on 50' TFR.

Suffice to say I managed to limit the number of metal film resistors to only 4 with the rest being the more era correct Carbon based or wire-wound types. Poly and Mica caps and nil Ceramic caps. Not that anyone will be able to pick or hear the difference.

Personally I still think it looks a bit of a rats nest mess, but at the same time it looks 'just right'. There are things that had to be done (like bending leads so they can expand when warm and not fatigue and break) and there are things I wish I had done (Like arrange the components better in neat little rows) which I suppose all add to the charm of the small tube amp that it is....

DrNomis_44
10-08-2017, 09:26 PM
Looks like a really cool amp, very much looking forward to checking out the audio demo of it so I can hear what it sounds like.

Marcel
11-08-2017, 03:45 PM
It's all a bit of a numbers game when it comes to biasing amps.... and even then the tubes dictate which numbers are correct for them...

Let me start by saying that "plate dissipation" and "power out" are both expressed in Watts (W) and are NOT the same thing. One is how much heat is generated by the tube by just being there and switched on, the other is the energy being sent to the loud speaker....

My common cathode resistor on the EL84's was originally a 120 ohm 3W resistor... which turned out to be too low in value for the HT that the power supply was capable of delivering. With 330VDC on the plates my plate dissipation calculated out to be 14.9W per tube which is 2.9W more than the maximum allowable 12W for a EL84. So I had to increase the value of the cathode resistor.
I tried a 135 ohm 6W resistor. Everything got worse. My anode voltage had also increased (due to less current draw) to 355VDC and my dissipation had bumped up to 15.1W. Not a good place for the EL84's to operate, but the logic of increasing the cathode resistor to reduce anode dissipation is a long standing proven correct one, so I summarized that the operating curves of the power supply loading and of the tubes current draw were at odds and so I needed to go with an even higher cathode resistor.
In the end after various other failed attempts I decided to try a 270 ohm 3W resistor. This time things were far better. My anode voltage had sky rocketed to 373VDC and my cathode voltage was a staggering 15.8V but that was okay as my plate dissipation had now dropped to a mere 10.1W which is well within the 12W maximum....I noted down other voltages throughout the amp and was not surprised to all had significantly risen higher.... time to plug in a guitar...

Yeah... there is a tiny bit of hum.... and a tiny bit of noise at full gain...tremolo works well, can easily hear the preamp noise being modulated by the tremolo...

turn up the guitar....

:D ... :D ... :D .... :D .... :cool:

Why did I ever bother building a 2204 clone..... Damn this thing kicks !!! ... and the higher voltages on the preamp tubes has really made them come alive !!

Guvna19
11-08-2017, 07:47 PM
Looks awsome Marcel !!!

Dedman
11-08-2017, 07:55 PM
https://www.youtube.com/watch?v=FPQlXNH36mI

Marcel
11-08-2017, 08:01 PM
Amen to that Dedman.... :)

andrewdosborne
11-08-2017, 08:40 PM
Awesome thread Marcel I’ve just finished reading it great amp! This level of electronics is way over my head it’s quite an education for me lol


Sent from my iPhone using Tapatalk

Marcel
11-08-2017, 11:08 PM
I'm quite the opposite Andrew.

For me the theory and the build are the easy parts, the worst part is all the math needed to calculate plate dissipation which really boils down to only a bit of multiply and divide and occasional addition and subtraction...

Marcel
17-08-2017, 02:46 PM
Latest update, and probably the last few things to say about this amp build....

My stocks of parts is rather limited, and for this project I only ordered the essentials I needed at the time. As Murphy is always lurking, I had to put in another order for parts (mostly resistors) so that I could make proper bias or supply voltage adjustments on pretty well every tube.

Another long story short, I ended up making six major changes to the original AC15 circuit that I used to build this amp...being...
1/- Drop the reverb circuits... not required at this time.
2/- Used 'Colortone' power and output transformers which were readily available and are typically found in Marshall 18W combo amps.
3/- Increase the EL84's cathode resistor from 120 ohms (125 in a Marshall) up to 180 ohms. Gives a 11.3W anode dissipation which is perfect 95% biasing.
4/- Increase the power supply dropping resistor from 2W 270 ohms to 5W 1k ohms. Returns better voltages for all the 12AX7's
5/- Increase the EL84's screen resistors from 100 ohms to 1k2 ohms. Screen voltage now 10V lower than the anode and dissipation now just under 1W.
6/- Fit a 'Hi Cut' pot. Identical to as implemented in an AC30. Not normally installed in an AC15.

Incredibly the noise floor of the amp is lower now, but the loudness of the noise it can make is truly astounding...

I'm very happy with this build, and even learnt some things in the process... A re-visit to my 2204 build seems in order.

Simon Barden
17-08-2017, 05:30 PM
Sweet. I bet you wished you'd built it in a prettier casing now!

Marcel
17-08-2017, 05:33 PM
Nah Simon....lol

Have always thought about putting a cage over the top, but I like the semi Steam punk look it has now...

Marcel
17-08-2017, 08:38 PM
A tid bit of info for tube enthusiasts....

A web site that has data sheets for virtually every kind of tube that has ever existed....

https://frank.pocnet.net/sheets11.html
https://frank.pocnet.net/sheetsE1.html

No excuses now for getting the biasing wrong ....

Marcel
28-08-2017, 07:49 AM
Was surfing the net and found this...

https://www.farnamstreetblog.com/2017/08/amateurs-professionals/

On some things I see myself as one, and on other topics I feel I'm more the other... I suppose it's all about your attitude to the subject at hand ....

Marcel
06-09-2017, 10:38 PM
So I'm super happy with how this amp build has turned out. Get some nice overdrive if I open it up, and for the low power it sure pushes out some volume...

However,
After reading Doc's thread, and doing some web surfing. and in the never ending search for new and better tones, and because I can, I've decided to mess with the first 12AX7 stage and add an old school EF86 stage as was used in the very earliest AC15 and AC30 amps. The intention is to get away from the 12AX7 first stage as I have in my 2204 build and in my JVM210c and see if I can bring in a different front end overdrive that is more akin to what early VOX amps are identified with....

Not sure yet if I'll keep the existing 1st stage as is and add the EF86 beside it, or pull the 1st 12AX7 out and build a new EF86 stage in its place. My current preference is to try and have both but I suspect I may end up having to rebuild the entire 12AX7 chain to achieve that. Most likely it will be out with the old stage and in with the new stage...

Marcel
11-09-2017, 08:35 PM
A bit late, but late is better than never, so a chunk of background on the basis for this amp...

Obviously there is history and charm when it comes to the VOX AC15 and AC30 series amps, and along with the Marshall brand it has left its mark on what I like an electric guitar to sound like. So after re-building an old amp head that I had kept for over 30 years in my shed into a Marshal 2204 clone, and exploring the joys of my JVM210c and a tiny solid state 10W VOX practice amp, and my VOX VX1, I made the decision to build something in between and an AC15 clone build fulfilled the desire perfectly.

After looking at the dozen or so different circuits I have for various releases of the AC series amps I finally settled on a 1996 version of the AC15. The main reasons being due to the availability of the much needed Marshall type transformers and the use of only 12AX7's and EL84's similar to the Marshall 18W series of amps, and due to having a much simpler tremolo circuit than the designs from the 1960's.

In reality I would have preferred to build a 1960's style tremolo however it requires the use of at least two 12AX7's to achieve the tremolo effect and the '96 version uses only half of one 12AX7. The audible difference between the two circuits is dramatic. The designs from 1978 onwards with half a 12AX7 are pure volume control in function, offering only a variation in volume at the tremolo speed, thus the effect. The 1960 and earlier design operates in three stages where one tube oscillates at the tremolo speed on one half and a phase splitter on the other half, and on the 2nd tube runs two low gain stages in parallel driven alternately and with different spectrally filtered inputs that gives a varying volume alternately varying on different parts of the sounds passing through the circuit.... a much more interesting and complex arrangement...

For the time being I've stuck with the simpler tremolo circuit that uses only half of one 12AX7.

On to the 'Clean channel" EF86. While the tremolo channel in a AC15 or AC30 with its Bass and Treble controls is great and offers a lot, it is still a triode design based around the ubiquitous 12AX7 tube. The EF86 as used in the 1960's AC series amps is a hi-gain Pentode, which has a completely different way of dealing with signals when compared to our old friend the 12AX7. Originally the EF86 was designed back in the early 1940's to be the tube to drive a loudspeaker with anywhere up to 5W of audio (1W being more typical) directly from the rectifier in old AM radios, however that is not where this tube found fame and a second life. With other tubes being more suited for the job of driving a loudspeaker, some ingenious designer rather than use a 12AX7 with a gain of 100 went and incorporated the clean hi gain of 150 to 210 of the vast unused stocks of the EF86's into an audio amp, which was later adapted by the VOX engineers into the early AC series amps of the 1960's. Production of the original EF86 stopped shortly after 1955, but was re-started by Russian entrepreneurs at Electro-Harmonics due to demand from guitarists some time in the late '90's.

So, it will be the Electro-Harmonics version of the EF86 that will find its way into my AC15 build. However there will be other major differences in my build when compared to an actual AC15. I can't do without Bass and Treble controls, a personal preference rather than an actual need. To that effect instead of the EF86 feeding via only a volume control straight into the phase splitter to the EL84's as is in a AC15, the tube in my amp will feed via a pad into the 2nd 12AX7 cathode follower which is the tone stack driver. Actually the plan is to have a switch-able choice. One set of inputs with a toggle switch to choose between either EF86 or 12AX7 first stage both going via a volume control into the tone stack second stage, and then on to the master volume and phase splitter and then simple tremolo/Hi-cut//PA final stage.

I suppose it isn't an AC15 at all any more... probably why I gave it the AC18W designation....

Andy40
14-09-2017, 06:26 PM
I am so going to build one of these. one day this thread will make sense to me. Thanks for posting it Marcel:D

Marcel
15-09-2017, 07:42 AM
I'm happy to elaborate on anything that doesn't make sense Andy. Just ask the question...

Marcel
15-09-2017, 08:41 AM
Just putting this info out there...There is not a lot of places that sell guitar amplifier kits, however there are two American sites that do.

As so far all my amp builds have been totally from scratch through individual parts dealers that I have used in the past it should make sense that I have never bought from either of these kit suppliers, however if I were to buy a full kit then these kit suppliers would be high on my list to look at.

From my perspective as the sites are in US$ they both are more expensive options but you do get all the correct parts and a real set of instructions to build a complete amp head or combo that most of the general public would consider was professionally made. Not as expensive as buying a new name brand amp, on a par price wise with many of the 2nd hand equivalents, certainly a cheaper and a more time intensive option for the more boutique type amps. For anyone with reasonable hand tool and soldering skills and moderate electronics knowledge there are plenty of variations that are certainly worth looking at.

One warning or important point to note.... Most of these kit offerings come with a build estimate time which I would ignore. For someone like myself with all the right tools and the right experience then yeah maybe on a good day the estimate might be somewhere near accurate, for most people I'd double if not triple the time estimate to get a working result and double that again for a semi-pro looking completed finished product. And if you wire up something and it's wrong then all time estimates and cost savings can be thrown out the window and hopefully nothing but your pride (and wallet) is damaged in the process...

http://www.mojotone.com/kits

https://www.tubedepot.com/t/diy-central/kits-tube-amp

Simon Barden
15-09-2017, 08:45 PM
Also in the US there's Ceriatone, one of the better known US suppliers in the UK. http://www.ceriatone.com/

And my ex-Blackstar mate suggested this lot. https://www.trinityamps.com/kits/

Marcel
17-09-2017, 08:35 PM
That Trinity site seems a good site for kits there Simon.

I like in particular the Trinity 18 and the Trinity TC15, and both seem to be at a good competitive price too...

Marcel
09-10-2017, 06:38 PM
So everyone is on the same page I'll put it here that I've been using this amp build quite a lot. On guitar builds, on Bass builds, fiddling with effects, and everything else into a 2x12 vented cab loaded with a 16 ohm V30 and a 16 ohm G12H75....and it sounds great !!! It has more overall bottom end than maybe it should which I put down to the home made vented cab and I can adjust out using the bass and treble controls, but otherwise it is so easy to dial in sounds, the nice sounds, the sounds I want to hear.... it's great !!... Just sometimes I wish I had a real AC15 or AC30 to be able to do a worthwhile A/B comparison...

I hadn't noticed until today, but the amp has developed a low level 50Hz hum... not enough hum to stop using it for practice or testing or even gigging, but it would be or is too much for recording at lower volumes. High volumes it's no issue, low volumes it's a real pain... and the hum varies up and down with the master volume control but is not affected by the gain control, so it is getting into the amp via the 2nd pre-amp tube or adjacent circuitry. Yet another item to add to the list for when I decide to take the covers off and wire in the EF86...

DrNomis_44
10-10-2017, 03:01 PM
So everyone is on the same page I'll put it here that I've been using this amp build quite a lot. On guitar builds, on Bass builds, fiddling with effects, and everything else into a 2x12 vented cab loaded with a 16 ohm V30 and a 16 ohm G12H75....and it sounds great !!! It has more overall bottom end than maybe it should which I put down to the home made vented cab and I can adjust out using the bass and treble controls, but otherwise it is so easy to dial in sounds, the nice sounds, the sounds I want to hear.... it's great !!... Just sometimes I wish I had a real AC15 or AC30 to be able to do a worthwhile A/B comparison...

I hadn't noticed until today, but the amp has developed a low level 50Hz hum... not enough hum to stop using it for practice or testing or even gigging, but it would be or is too much for recording at lower volumes. High volumes it's no issue, low volumes it's a real pain... and the hum varies up and down with the master volume control but is not affected by the gain control, so it is getting into the amp via the 2nd pre-amp tube or adjacent circuitry. Yet another item to add to the list for when I decide to take the covers off and wire in the EF86...


Could very well be an earth-loop that's causing the hum, I was watching a video on youtube about a guy who was in the process of diagnosing a hum-noise issue in one of his Fender Tweed Bassmans, and he eventually traced the cause to the 6.3V AC heater supply going to the two 6L6 power valves and three 12AX7 preamp valves, his conclusion was that the heater-wiring between the two 6L6 power valves hadn't been done correctly at the Fender Factory as the amp was being manufactured, the Fender Bassman in question was an old early 50's vintage amp.


Incidentally, I wouldn't mind having a go at building a 50's Fender Tweed Bassman from a kit of parts, would be a fun project.

Marcel
10-10-2017, 07:35 PM
It could be as you said Doc.

In this amp build the heaters for the 12AX7's are from the grounded centre tap 6.3VAC winding, so if the centre tap isn't exactly dead centre then there will be a imbalance that could cause hum. As this has a solid state rectifier diodes the EL84 heaters are on the otherwise unused winding that would have powered the rectifier tube with two 1W 100 ohm resistors to ground to balance up that sub-circuit. However from initial operational diagnosis and knowing the circuit as well as I do I'm more inclined to think it is a 22uF filter cap beginning to fail and starting its climb up the ESR ladder. When I open it up it will one of the first things I check.

I also have a 2.9H 120mA Fender champ power supply filter coil that I want to incorporate, so that may also go some way to eliminating all hum.

As for the Tweed amp Doc... I've lost count on how many times I've had to stop myself from getting a Tweed kit.. or even a small kit from one of those places Simon recomended... I gotta get my spending under better control before my wife gets fed up and takes over control by hiding my credit card...lol

Marcel
10-10-2017, 07:57 PM
FB has some gems occasionally..

From the 'Tube Amp Builders (DIY)" group, a link to a very informative Mullard manual... theory and some practical circuits...

http://basaudio.net/blog/wp-content/uploads/2013/01/19400164-Mullard-Circuits-for-Audio-Amplifiers.pdf

Marcel
10-10-2017, 08:08 PM
And another one ...

http://www.tubebooks.org/tubedata/RC30.pdf

DrNomis_44
11-10-2017, 09:06 AM
It could be as you said Doc.

In this amp build the heaters for the 12AX7's are from the grounded centre tap 6.3VAC winding, so if the centre tap isn't exactly dead centre then there will be a imbalance that could cause hum. As this has a solid state rectifier diodes the EL84 heaters are on the otherwise unused winding that would have powered the rectifier tube with two 1W 100 ohm resistors to ground to balance up that sub-circuit. However from initial operational diagnosis and knowing the circuit as well as I do I'm more inclined to think it is a 22uF filter cap beginning to fail and starting its climb up the ESR ladder. When I open it up it will one of the first things I check.

I also have a 2.9H 120mA Fender champ power supply filter coil that I want to incorporate, so that may also go some way to eliminating all hum.

As for the Tweed amp Doc... I've lost count on how many times I've had to stop myself from getting a Tweed kit.. or even a small kit from one of those places Simon recomended... I gotta get my spending under better control before my wife gets fed up and takes over control by hiding my credit card...lol


Have you thought about converting the 6.3V AC heater supply from AC to DC?, that may reduce the hum level a bit, I think some well-known guitar amp manufacturers use a 6.3V DC heater supply in some of their high gain amps that they manufacture.


Cheers for the links to the two pdfs, I downloaded the Mullard pdf and I'm in the process of downloading the other one, the Mullard pdf has some really interesting projects in it, most of the theory in the Mullard pdf is geared towards HiFi applications, but it still applies to guitar amplification applications as well, definitely worth downloading a copy of each of the pdfs.


Note that the RCA pdf has lots of good valve theory in it, good reading material if anyone is interested in learning about how valves operate.

Marcel
11-10-2017, 09:53 AM
No Doc, I haven't. However I have considered putting a +ve DC offset on the entire heater circuit to reduce the potential failure of V2b.

In this and most other designs where V2b is a cathode follower the voltage on the cathode is often up in the high hundreds with 190VDC being typical. The limit specified by most ECC83/12AX7 manufacturers is 200V maximum between heater and cathode, so most amp designs are pushing this potential failure point for nil other reason than simplicity of design and reduced component count. Adding a simple voltage divider to the centre tap of the heater windings and raising the DC level to around 30 or 40 volts DC evens up the "stress" on all the ECC83/12AX7 tubes but most notably on V2b where in most normal designs if an 'arc out' were to occur and apart from possibly irrevocably damaging V2 it will also send a rudely large and potentially lethally damaging spike to the speakers... And given the quality of most tubes today it is a cost that I'm not keen on enduring.

A comment on "Quality" - Tubes have never been 'quality' items. Some people are lucky and their tubes can last a life time, others not so much. so you get good ones and bad ones and the brand has little to do with it. In the last 12 months I've purchased near 2 dozen tubes of various types and have had 2 fall short for different reasons. So on average for me that works out at about a rough guesstimate of a 10% failure rate from new, which is far poorer than what the solid state cousins can account for. It is what it is, but if I can save risking $400 or $600 (or AU$800 for a new pair of Alnico Blue's) worth of speakers by doing what costs me a $1 modification then I'll spend the $1.

DrNomis_44
11-10-2017, 10:08 AM
No Doc, I haven't. However I have considered putting a +ve DC offset on the entire heater circuit to reduce the potential failure of V2b.

In this and most other designs where V2b is a cathode follower the voltage on the cathode is often up in the high hundreds with 190VDC being typical. The limit specified by most ECC83/12AX7 manufacturers is 200V maximum between heater and cathode, so most amp designs are pushing this potential failure point for nil other reason than simplicity of design and reduced component count. Adding a simple voltage divider to the centre tap of the heater windings and raising the DC level to around 30 or 40 volts DC evens up the "stress" on all the ECC83/12AX7 tubes but most notably on V2b where in most normal designs if an 'arc out' were to occur and apart from possibly irrevocably damaging V2 it will also send a rudely large and potentially lethally damaging spike to the speakers... And given the quality of most tubes today it is a cost that I'm not keen on enduring.

A comment on "Quality" - Tubes have never been 'quality' items. Some people are lucky and their tubes can last a life time, others not so much. so you get good ones and bad ones and the brand has little to do with it. In the last 12 months I've purchased near 2 dozen tubes of various types and have had 2 fall short for different reasons. So on average for me that works out at about a rough guesstimate of a 10% failure rate from new, which is far poorer than what the solid state cousins can account for. It is what it is, but if I can save risking $400 or $600 (or AU$800 for a new pair of Alnico Blue's) worth of speakers by doing what costs me a $1 modification then I'll spend the $1.


I recently had a few 12AX7 valves fail on me, two of them were a pair of Chinese 12AX7B valves that were the original preamp valves in my 5 Watt Legacy amp, I noticed that they were a bit weak in terms of gain and then they simply stopped working altogether even though their heaters did light up, the getter inside the glass envelopes still looked normal though, and then there were two of the original Marshall ECC83 preamp valves from my Marshall MA100C amp, they were both very weak in gain and then the getter in them started turning white, indicating that gas had gotten into the glass envelopes, the three Genelec Gold Lion ECC83 valves I used as replacements for the original preamp valves seem to be holding up pretty well, I was surprised at the difference in tone that the Gold Lions made compared to the originals, they really brought the Marshall amp to life, will definitely buy more of them from Evatco in future.

Marcel
11-10-2017, 11:52 AM
I've not encountered many Genelec Gold Lion's, but the few I have they seem to be quite good in terms of gain and microphonics. Never tried the cheap Chinese tubes though I do have four of them in my stock. I do like the Telefunken tubes though one that I have could almost pretend to be a Shure SM58, and in the dim dark past the old Groove Tube was a popular choice though these days they seem to be out of favour.

These days my typical purchases for myself are JJ's. I figure if they are good enough for the likes of Angus Young then they will do me okay, and they are usually quite well priced.

Worst faults I typically encounter with tubes is low emission and/or low gain... and then heater flash...

FYI - Heater flash - A tube reliability issue - If there is this construction fault with the tubes heaters and when power is first applied to that tube's heaters there can be a bright flash of light from near the base of the tube. Happening only at power on, and for only a few seconds, the exposed section of heater at the base of the tube will light up like a light globe until the remainder of the heater that is surrounded by the cathode comes up to temperature. This 'flash' does not affect the operation of the tube as once warmed up the tube will work as normal, however it is a weak point in the heater that becomes more fragile with age. With repeated power on/off cycles the heater is likely to break at this increasingly fragile flash point and thus usually at a moment of least convenience rendering of the tube to a useless failed/dead state with nil 'Red glowy bits' in that tube and nil noises from the connected speakers for your listening pleasure.

Useless fact - I have an infrared temperature meter which I have used to measure the temperature of all sorts of things including various amplifier parts during operation.... On an average 30C day here at my tropical location and after 1hr of moderate use of this amp build I measured the following - Power transformer @44C, Output transformer @ 42C, ECC83/12AX7 RFI protective aluminium shields @67C, EL84 plates (cathode biased to 11.7W dissipation) @157C.

DrNomis_44
11-10-2017, 01:17 PM
I've not encountered many Genelec Gold Lion's, but the few I have they seem to be quite good in terms of gain and microphonics. Never tried the cheap Chinese tubes though I do have four of them in my stock. I do like the Telefunken tubes though one that I have could almost pretend to be a Shure SM58, and in the dim dark past the old Groove Tube was a popular choice though these days they seem to be out of favour.

These days my typical purchases for myself are JJ's. I figure if they are good enough for the likes of Angus Young then they will do me okay, and they are usually quite well priced.

Worst faults I typically encounter with tubes is low emission and/or low gain... and then heater flash...

FYI - Heater flash - A tube reliability issue - If there is this construction fault with the tubes heaters and when power is first applied to that tube's heaters there can be a bright flash of light from near the base of the tube. Happening only at power on, and for only a few seconds, the exposed section of heater at the base of the tube will light up like a light globe until the remainder of the heater that is surrounded by the cathode comes up to temperature. This 'flash' does not affect the operation of the tube as once warmed up the tube will work as normal, however it is a weak point in the heater that becomes more fragile with age. With repeated power on/off cycles the heater is likely to break at this increasingly fragile flash point and thus usually at a moment of least convenience rendering of the tube to a useless failed/dead state with nil 'Red glowy bits' in that tube and nil noises from the connected speakers for your listening pleasure.

Useless fact - I have an infrared temperature meter which I have used to measure the temperature of all sorts of things including various amplifier parts during operation.... On an average 30C day here at my tropical location and after 1hr of moderate use of this amp build I measured the following - Power transformer @44C, Output transformer @ 42C, ECC83/12AX7 RFI protective aluminium shields @67C, EL84 plates (cathode biased to 11.7W dissipation) @157C.


Those temperature readings look pretty normal to me, especially the 157C on the EL84 valves, I seem to remember reading somewhere that it's normal to bias output valves so that they're dissipating about 70% of their maximum plate dissipation.

Marcel
11-10-2017, 05:35 PM
For cathode biased power tube up to 100% is permissible, though 95% is preferred. Some amp designs with certain tube brands and/or tube tolerances can default to almost 120% and is one big reason why trained techs should do power tube changes.

For fixed bias amps the bias is manually adjustable to taste. Typically 65% being cool biased, 70% being moderate bias, and 75% being hot biased. There are many videos on YouTube where they say "set your EL34's to 30mA" or "set the 6L6 to 50mA" which in certain amps may work as a generic setting, however the gain of the specific tubes being used and variances between chassis may result in a way too cool or way too hot a setting. For my 2204 build with EL34's and to get 70% dissipation the setting worked out to be 44mA, yet in my JVM210c which also uses EL34's the same 70% dissipation setting came in at 35mA, so tube type alone does not dictate the idle current through the power tubes.

Either way a cooler setting adds to the longevity of the tube. Hotter settings burn through the tubes quicker.... case in point - Angus Young likes his amps biased as hot as possible at 75 to 80% or more... His power tubes on all his dozen or so Marshall heads get changed after 3 gigs, that's if they last that long...

Marcel
12-10-2017, 04:32 PM
Some more tube theory Gold...

1523 pages of heavy going tube and basic electronics theory from circa 1954...

https://ia601602.us.archive.org/18/items/bitsavers_rcaRadiotr1954_94958503/Radiotron_Designers_Handbook_1954.pdf

Marcel
28-10-2017, 07:57 AM
Success at last... I have got my DAW setup working and have made some files that I can upload to soundcloud.

I do like my crunch, and Seymour Duncan rails pick ups in this partscaster Alder Strat build into the hi gain input of the amp do make the first stage JJ 12AX7 work pretty hard. Nil pedals and "All the gain !!!" is just the ticket to hide my horrible playing really well (I am more a tech than a musician after all).

The vented cab is loaded with 16 ohm versions of a V30 and a G12H75 wired in parallel to give an 8 ohm load to the amp. The cab itself is made from 17mm form ply from the big Green shed and is a HiFi ported bass reflex design modified for guitar use which is the reason for the massive bass roll off on the amp. One SM57 into the Behringer USB interface to the i5 laptop running Reaper 5.6 ...

I intend to play with microphone options latter but just getting one track down of something was the first objective...

https://soundcloud.com/user-261305392-406972551/noodling-on-a-strat-sd-hr-017013-partscaster-build

dave.king1
28-10-2017, 09:54 AM
I can see it on SC but it's greyed out and won't play, have you got the settings right

Marcel
28-10-2017, 11:03 AM
I believe so Dave. It plays fine for me. What settings do I need to set right?

DrNomis_44
28-10-2017, 12:29 PM
I checked out the soundcloud demo and it played fine for me.

DrNomis_44
28-10-2017, 12:30 PM
I believe so Dave. It plays fine for me. What settings do I need to set right?


You could check to see if it is set to grant public access.

Marcel
28-10-2017, 01:07 PM
You could check to see if it is set to grant public access.

Don't know how or what it was set to before but SC is now public everything...
Done...

dave.king1
28-10-2017, 07:52 PM
It's now visible Marcel but it's late and the www seems to be slow ( one handed typists late at night bogging it down I guess ) will check it out in the morning and also do a screen shot of the settings I use which do have a lot of limitations for copywrite purposes which obviously don't apply here

Marcel
31-10-2017, 07:24 PM
Something that many moons ago I have been so guilty of...

https://www.newyorker.com/humor/daily-shouts/the-boyfriend-who-noodled-on-guitar-a-horror-story

And, yeah, I'm still guilty of it... ;)

Marcel
01-11-2017, 08:32 PM
For the few of you still following this thread I thought I'd share the circuit I used as a guide to build this animal.

I used the AC15-60-02 and AC15-62-02 diagrams from 1996 for the build. I have made some changes to the high voltage distribution due to using a non-VOX power and output transformers, and not used the reverb circuit of V2 and V3, but the guts of the audio chain is unchanged. The EF86 (6267) addition will be lifted from the 1959 era NU.V-1-5 drawing with some additional switching to be able to select between the EF86 and the V5 ECC83/12AX7. Hopefully I'll somehow work out how to be able to make the tube changeover switching a noiseless remote function on the same hard wired foot pedal box as the tremolo on/off.

Ik7
06-11-2017, 04:50 AM
I built myself an AC15 styled amp with the EF86 preamp, however, I added a parallel ECC83 input feeding into the EF86 and used two 6V6GTA instead of the EL84s. I must admit, I love the sound of an EF86, but trying to implement it into other designs is a headache.

Have you read any books by Merlin Blencowe? His website is also really helpful (http://www.valvewizard.co.uk/)

Marcel
06-11-2017, 06:26 AM
Hi IK7,

No, haven't read any of Merlin's books, and have only just read some of his web site of which I'd agree 100% with. It doesn't matter if it is tube or solid state there is always a lot to consider when building amps. I find for myself that tube is easier to understand probably due to when I learned electronics in the '70's it was taught better by those who knew nothing else.

What I did find interesting was a few of Merlin's comments which sadly (or it could be a good thing) will nag at me in the back of my head on any future amp build. On my previous builds the topic of grounding has been a big issue, and now after a quick read of Merlin's site this EF86 topic has developed a few new dimensions that I hadn't considered previously. Maybe I will not simply copy the AC15 input on this build upgrade, but be a little more inventive and adventurous.

Ik7
08-11-2017, 05:11 AM
If you can get a copy of his book(s) they are well worth the money.

The other thing I wanted to try of Merlin's design was his Morph control, which basically has a small signal pentode (EF86) and connects the screen and cathode together via a pot, so you can go from 100% pentode to 100% triode and everything in the middle. Having another valve feed into this could have some interesting results, especially if it's in cascade system.

Marcel
13-03-2018, 01:23 PM
For those that are interested a quick tally of the parts that have gone into building this amp comes to AU$690.85 which is a lot more than one would expect for what amounts to only a 15W amp.

A case, the two transformers, $18.50 for each 12AX7 and $45 per EL84, sockets, nuts and bolts, pots and knobs, light and switches, electros and other caps and resistors. a IEC power receptacle and a few other minor bits like rubber feet etcetera. Then you can add to that about 20hrs of my labour to drill/cut/mount/solder & test, which at 'best mates rates' starts at $45/hr.... so, for about AU$1600 I can build you one 'head only' unit and I'll come out about even....

BUT... it is hand made, a unique 'one of design'... and ... all that sorta good blurb....

Somehow I don't think I'll sell many....lol

Simon Barden
14-03-2018, 04:53 PM
I don't know. Put it in a nice looking case, improve the looks, get some well known guitarist to use one and you're halfway there. Then just go and live in China where you can live on $4 an hour and get cheap parts, and you might just crack it!

Marcel
08-04-2018, 11:30 PM
Or you could just go out and for a bit less than AU$900 buy a Fender Blues Jr. IV and get essentially the same amp with a spring reverb and with a nice 12" speaker all done in a quite pretty and very portable box... with Warranty...

I think it's all a bit too competitive out there for a small guy like me in the far flung from the big smoke Aussie bush to start up an Geetar amp making business... I'll just stick to modifying my own and fixing for others.

Speaking of modifying.... I pulled the EL34's from my 2204 build and inserted a pair of old USA made Sylvania 6L6 tubes. Biased the buggers up and had a play into the Celestion loaded 2x12 cab using my SGM-1 build as a noise source.

I got interesting results considering the compromises I originally made to accommodate the EL34's. The transformers were wound in the late '60's to suit the Playmaster 116 guitar amp (Electronics Australia magazine. June 1967) using a 12AU7, a 6BL8 and a pair of 6DQ6's, and switching to 6L6's was an easy substitution, but I had to make quite a few changes to get the transformers to work well with EL34's. Those changes haven't been undone so the feedback and presence circuit changes that make the EL34's stable at full power have some pretty weird side effects when you suddenly jump back to 6L6's... First thing of note is the speaker impedance setting is back where it should be instead of being rotated one position, which is a good thing. Second is the 6L6's now seriously don't like the presence control wound up too high and any setting above 6 and with any substantial volume level induces a modulation effect that is truly worrisome. Back the presence control off and things quickly return to 'normal'.

Other than that there is plenty of grunt still in those old USA made Sylvania 6L6 tubes that have been sitting loose in a ratty old cardboard box for the past 30 years. I'd need a second opinion on the cleans as overall due to the 2204 circuit fundamentals in play I still think there is still a level of 'dirt' present, but in cascaded V1 mode with the gain up and the master pushing 7's the SGM-1 build started to show some very nice tonal colours.... of which I'm not complaining.. (yet my neighbours soon might).... ;) ;) ;)

Now to push on. I never used this 2204 build much and it is so easy to work on so I've decided to give it the V1=EF86 treatment instead of changing the AC18W build. The EL34 to 6L6 change was sort of a easy first step. Still not sure of what to do with V2 and the V3 PI stage. I'm leaning towards keeping the PI V3 as is and possibly transitioning the V2 stage to a more Fender'esk Blackface inspired design though the tone stack will probably force the final outcome to its needs than any lofty dreams I may have.

Marcel
18-01-2020, 08:54 PM
Near 2 years later and I am still reluctant to tear down and rebuild either the AC18W or the 2204/Playmaster 136 into a EF86 commanded monster. As I like both amps the way they are, particularly the 2204/PM136 now that it has 6L6GC's in the output it has become a very unique sounding animal... So with that as an excuse I have done the next best thing, and bought all the bits to recreate a EF86 loaded AC15 amp in a head configuration similar to the AC18W build.

This AC8615 build will mostly follow the VOX way of doing things, however there will be a few changes. The biggest of these changes will be it will have a solid state rectifier stage and a choke in the HT filtering. Unsure if I will include a tremolo, and it may yet get a 2nd 12AX7 channel.... Just need to see how things pan out.

DrNomis_44
18-01-2020, 11:06 PM
I had another listen to that audio demo of the amp you posted, and I think it's a great sounding amp, lots of the natural tube overdrive/distortion happening there.

Andy40
19-01-2020, 03:48 PM
Thanks for bumping this thread doc. I had a listen too, sounds great!

DrNomis_44
19-01-2020, 10:38 PM
Thanks for bumping this thread doc. I had a listen too, sounds great!

Ah no worries at all mate, hopefully this year I'll be able to get my Sloclone 50 tube amp head built and working so I can post some updates in the thread I started for it, and maybe some sound demos too.

How are things going with your Tweed 5E3 build?

Marcel
13-09-2020, 10:13 AM
So illness has kept me out of the shed working on my build(s).... hence I've dragged some stuff into my lounge room to help pass this lock down time. .... Yup, the better half left me half way thru last year, so the lounge is mine now to do as I will.

Hmmm.. this amp is now over 3y old. as is the cab I built to go with it... and apart from numerous tests and short runs I haven't given this rig a real "lets see what it can do" run.... and today that all changed...

To reiterate, the amp is basically a VOX AC15 circuit with Marshall 18W power and output transformers, mounted in a Jaycar steel enclosure. The cab is a 2x12" bass reflex box 30x60x80cm made with 17mm form ply from the big green shed, loaded with a V30 and a G12H75 Creamback. I chose form ply cos I did not want to have to paint it black and form ply has a seriously durable surface, and it was cheap.

The amp on its own nothing plugged in fully wound open on 10 is near silent. Cleans are super nice. The gain can be wound up to a medium 12AX7 crunch. Flat out being the master Vol on 10 the headroom flattens down to quite a soft feeling compression, in that everything past 7 only adds compression not more volume. The tremolo and remote pedal works exactly as it should. In a tiny bit of a steam punk move I put a wire cage over the tubes to keep prying fingers away from the hot tubes, and so you can still see the glow of the tubes from any angle which I feel is a cool thing.

In the photos you can observe a curious thing.... the bass control is fully wound back. The cab generates a LOT of bass, which means the amp does not need to push the bass, which leaves a shit tonne more energy for the upper frequencies.... For only 15/18W Damn can this rig get loud, and add the RAT it has sustain for days....lol ... more than plenty for a practice/rehearsal room rig.

Andy40
13-09-2020, 12:24 PM
Hey Marcel,

Where did you get the pilot light? I am considering an 18W TMB Build later this year......

Marcel
13-09-2020, 06:28 PM
Hey Marcel,

Where did you get the pilot light? I am considering an 18W TMB Build later this year......

Hey Andy,
All I can offer you is it came from a Aussie seller on Ebay who is in Western Victoria. Has a range of stuff suitable for building Tube amps... Search Ebay for seller "vicmazz64"

Marcel
13-09-2020, 06:36 PM
https://www.ebay.com.au/itm/Marshall-JTM45-Red-Neon-Pilot-Light/293007656371?hash=item44389e05b3:g:eQ8AAOSwxCxT7pA h

DrNomis_44
13-09-2020, 10:38 PM
@ Andy40 You can also try having a look on the Evatco website for those pilot lamps.

Andy40
14-09-2020, 07:41 AM
Thanks guys.