Marcel's scratch build AC15 based clone

[Simon Barden]

Sweet. I bet you wished you'd built it in a prettier casing now!

[Marcel]

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]

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]

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 ....

[Marcel]

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]

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]

I am so going to build one of these. one day this thread will make sense to me. Thanks for posting it Marcel

[Marcel]

I'm happy to elaborate on anything that doesn't make sense Andy. Just ask the question...

[Marcel]

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]

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/