I got the power amp drivers set up and tested using two configurations: (1) same guitar signal going to both output channels, and (2) guitar signal going into a stereo effects processor, split into left and right effected signals, with each going to a separate output channel. Sounds great at lower volumes. There's good L/R separation (though not perfect) and the power tubes seem to start to breakup right near the top of the volume range. Here's a schematic of the portion of the circuit I'm talking about:
(homing in on the final power section circuit)
It's been really instructive to be able to take measurements of changes in voltage when the amp is running, and to easily swap components in and out to see what changes. For example, it took me a while to realize that I really needed to separate the cathodes on the two power tubes (which in theory can share a resistor and capacitor with no problems, as in V3 in the picture above) in order to even begin to understand how they work in relation to the plates and screens... and to be able to correlate my readings with spreadsheet calculations, which relies on being able to isolate the power tubes and the current running through them.... and having them separate will be essential to optimizing and balancing their respective biases, or operating parameters, which will vary slightly from tube to tube. Also, after some testing and getting some non-stereo output when I thought I should get stereo, and then using the TINA modeler, I realized I had to place the signal "return" pots (essentially, the volume controls for each channel) after the 220k ohm grid stopper resistors just prior to the V3 triodes, in order to separate the channels. I still may need to increase these resistor values to improve the separation of R/L signals... but that may produce other issues to deal with.
Next, I put together an "ODS" style overdrive preamp (based mainly on the work of a couple of amp builders on the EL34 World forum - "DeGeezer" and "Tubenit"), and ran it through the power amp.
(test board at work)
The final design will have a "clean" preamp section - not designed to produce heavy distortion - that will send its signal either directly to the effects loop, or to another circuit that will push the signal through two additional triodes configured to easily produce overdrive distortion ("ODS"), prior to going on to the effects loop. The mock-up of this idea sort of works as intended, but it's hard to tell for sure without also setting up the "clean" circuit. Because I can't produce a test signal that I'm sure is similar to that of the "clean" circuit in both quality and magnitude, I don't know for sure whether the "ODS" circuit is behaving as it ultimately should. Cranked to full output, the clean preamp as designed will be able to send about 8 volts (peak amplitude) to the "ODS" circuit, which I'm pretty sure is way more than I'll want to send there in practice - it will probably want no more than 3 or 4 volts to run right. Also, while I am able to produce a clean guitar signal using a digital preamp, I know that digitally produced signals and tubes don't play that well together, so the tone quality of test output may be way off... or not, I don't know. In testing, the "ODS" circuit sounds just OK but not great. I've tried using 12AX7 tubes as well as 12AY7 (which provide only half the gain of the 12AX7 and have different tonal characteristics), and from what I can tell I prefer the 12AY7s. There may not be much more major testing that is worth doing unless I build the "clean" preamp circuit as well, but that will require pulling the whole test board apart to make space, and I'm not sure that's worth doing, especially considering that the overall tone quality of the test system will still be lot different than the thing that gets built.
On a different front, I picked up a cheap used solid state amp (Fender Frontman - FM212) that is physically the right vessel for my design - it's got two 12" speakers and plenty of room in the chassis (which I had already removed prior to taking the photo below). I'm playing the 5F6A/JTM though the two 12's - they're OK speakers for now, but based on comparisons with other speakers I have, I'll probably end up swapping them out. I'll see how they sound in the final build and decide then.
(Fryette power station, 5F6A/JTM head, sitting atop a Fender FM212 combo, that will become the home of the new stereo single ended amp)
Here are draft drawings of how I'll organize the amplifier components inside the existing FM212 cabinet. The chassis on the this model amp is a funny shape (see side view below), not a standard box. and will require some cutting, bracing, and reshaping to accommodate tubes and a big transformer. The knobs and jack spaces on FM212 front panel pretty much match what I'll need for my amp, so that's a plus.
