I was thinking a few weeks ago about an offer I got to jam with friends, and what that might look like gear-wise. I have two heads, 1x12 and 2x12 external speakers, and a 2x12 combo, all of which require outboard gear for effects and attenuation if I really want a nice tone. I love my 3 amps - they each have their unique sounds and feels, strengths and weaknesses, and are great for recording, but each is too cumbersome for a casual jam session. The combo ("2SE") comes closest, and paired with a reverb pedal would work fine, but it just weighs too much to be a grab and go rig.
Browsing through an amp forum one day I read about a poster's Vox AC4 project: a small, low powered, great sounding amp, that with some mods would make a perfect casual jam amp. But, I have additional requirements and concerns: the pentode preamp on these amps can be difficult to keep quiet, and the tubes are more expensive than most other preamp tubes and are prone to being microphonic (translating physical vibrations into sounds in the circuit). I also like the stereo sound of the 2SE, so would like to incorporate that if possible... plus, I'd like a decent overdrive sound that is different than what I've already got on the others. Some of these requirements are not specifically about meeting the "jam amp" basic requirements, but if I'm going to bother to build another one, I'd like to be interesting to do and special in some way(s).
I recalled my solid state Fender Princeton Chorus (FPC) from the 1990's and thought that format - a combo with two 10" speakers - would serve nicely, and as luck would have it I noticed a beat-up one on Craig's List for super cheap, so I grabbed it. This model has a wide chassis and built-in reverb tank that I thought might be re-usable in a new amp, and the speakers are in good condition.
I did some research and shared some preliminary findings on an "amp I might build" with a friend, who joked that referring to it as a possibility was perhaps a form of denial, paraphrasing Anton Checkhov's Gun principle to support his confidence that my building of a fourth amp was a foregone conclusion. None the less I thought it provided a good name for my fourth amp, built or not, and as long as I don't sell it, the Vox company will probably not have a problem with it :).
So, I started writing out a circuit based on my requirements: a one-triode input stage (not the pentode used in the Vox's), a simpler two-knob tone control, a two-triode OD circuit, a two-triode driver (one for each channel), and a pair of EL84 single-ended power outputs, each to its own speaker. For the amp to be truly no-nonsense and stand-alone, it will need self-contained reverb, analog versions of which require at least two triodes to run, and a tremolo, which requires a single triode. It's more complicated than the Vox basic two tube amp, but can easily fit in the FPC combo body and modified chassis (see 2SE pics for example of modifications).
So, I modeled the circuit in software, and realized the signal coming from the input triode would just not be strong enough to drive an overdrive circuit well. and recalled a Dumble amp that has one of its input channels boosted by a simple transistor-based amp. I built just that bit (the stuff on the pink circuit board in the middle of the pic below) and gave it whirl - seems to work as expected.
I took inventory, acquired a few parts I needed (though I already have most of what I need), set up my test-board and mocked up the full circuit for testing and tweaking.
Also, this worked more or less as expected, but the tone control and overdrive components needed tweaking to get them where I like them, and the power section here is just just a single tube, so mono not stereo. The boost and OD channel options provide four distinct voices for this design.
Next, was sorting out the reverb tank and getting it to work as a tube-driven circuit - recall the FPC is an all solid state amp, so the reverb tank was selected for that. Alas, I could not make it work - indeed, amp forum members confirmed it would be difficult to make it work, and if it did, it probably wouldn't sound that good. So, I'm abandoning the donor reverb tank, and would opt for an inexpensive digital gadget i can drop into the circuit, which may require no tubes to run it, depending on where it sits in the circuit. I don't like getting too far away from a fully analog design, but oh well.
Here's as far as I got with the schematic for the AC4.
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