Marcel's scratch build AC15 based clone

[Marcel]

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...

[Marcel]

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]

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]

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]

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]

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]

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]

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]

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]

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.