Only the tone stack and the tremolo oscillator to go before the real testing starts...
Printable View
Only the tone stack and the tremolo oscillator to go before the real testing starts...
I checked out that link you posted, it's a good read so I've bookmarked it, cheers Marcel.
That loom lacing looks very much like what I learned as a TiT more than half a lifetime ago, nice work
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 ...
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....
And then the mandatory sound clips ( please )
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.
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....
Great work Marcel, I guess building an amp you don't have to wait for the finish to dry & cure
Bravo! it's been interesting and educational watching this come together.
Great work Marcel - it's been fascinating following along.
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....
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.
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 !!
Looks awsome Marcel !!!
Amen to that Dedman.... :)
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
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...
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.
Sweet. I bet you wished you'd built it in a prettier casing now!
Nah Simon....lol
Have always thought about putting a cage over the top, but I like the semi Steam punk look it has now...
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 ....
Was surfing the net and found this...
https://www.farnamstreetblog.com/201...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 ....
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...
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....
I am so going to build one of these. one day this thread will make sense to me. Thanks for posting it Marcel:D
I'm happy to elaborate on anything that doesn't make sense Andy. Just ask the question...
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
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/
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...
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...
Quote:
Originally Posted by Marcel
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.
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
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/...Amplifiers.pdf
And another one ...
http://www.tubebooks.org/tubedata/RC30.pdf
Quote:
Originally Posted by Marcel
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.
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.
Quote:
Originally Posted by Marcel
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.
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.