Marcel's scratch build AC15 based clone

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Originally Posted by Marcel

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.

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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.

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...

Some more tube theory Gold...

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

https://ia601602.us.archive.org/18/i...dbook_1954.pdf

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-26130539...tscaster-build

I can see it on SC but it's greyed out and won't play, have you got the settings right

I believe so Dave. It plays fine for me. What settings do I need to set right?

I checked out the soundcloud demo and it played fine for me.

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Originally Posted by Marcel

I believe so Dave. It plays fine for me. What settings do I need to set right?

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

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Originally Posted by DrNomis_44

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

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

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

Something that many moons ago I have been so guilty of...

https://www.newyorker.com/humor/dail...a-horror-story

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

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.

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/)

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.

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.

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

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!

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.

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.

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.

Thanks for bumping this thread doc. I had a listen too, sounds great!

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Originally Posted by Andy40

Thanks for bumping this thread doc. I had a listen too, sounds great!

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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?

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.

Hey Marcel,

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

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Originally Posted by Andy40

Hey Marcel,

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

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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"

https://www.ebay.com.au/itm/Marshall...8AAOSwxCxT7pAh

@ Andy40 You can also try having a look on the Evatco website for those pilot lamps.

Thanks guys.