I’ve put together the first iteration of my circuit. (Wall of text incoming!)
The pre-amp is one channel of a pretty standard dual-triode blackface circuit (there are a few three triode blackfaces like the Super Reverb but I’ve opted for simplicity). It then goes through a long tail inverter into a push/pull 6V6 power circuit. I’m opting for a fixed bias using the RobRob (more on him later) design for pulling a low negative voltage from your main HV secondary.
The power supply has been roughed out but not fully designed, I’ll give it a proper look once I have measured operating voltages on the power transformer. I intend to use a diode rectifier though, even though I have the rectifier tube available.
I have included a few modifications to the standard circuit. Most of these have come from https://robrobinette.com/AB763_Modifications.htm. This site seems to be a go to for amp builders and I’ll be going through a lot of his other articles before I start the build proper. The mods I’m looking at are all small changes that add tonal options and versatility. This is what is currently in the circuit:
Needs no introduction. Inserted after the first preamp stage, tone stack and volume control. The send can also be used as a line out.
A switch that takes the tone stack out of circuit, giving you the raw, unshaped guitar tone. It is also supposed to give a gain boost as you aren’t bleeding to ground through the tone stack.
Three-way Negative Feedback Setting
Add a three-way switch and an alternative resistor to the power amp feedback circuit. This is supposed to give you three distinct flavours in the power section. Normal feedback will be fender like (of course), feedback off will be wild and wooly and Heavy feedback will give you cleaner cleans and tighter overdrive.
I’m looking at the design that replaces the power amp bias resistors with a ganged pot. This will act as a pair of resistor dividers for the input signal and as far as I can tell will only affect your bias voltage slightly due to the low current. This is supposed to be the most transparent master volume control but I’m still not 100% comfortable with it yet so I want to read up on it in greater detail.
Vox Style Power Amp Tone Cut
A pot with a series cap bleeding between the inverter outputs, allowing the higher frequencies from the two to cancel each other out. This is supposed to allow you to cut the ‘icepick’ highs without affecting the harmonic content of your overdrive too much.
I am aware that all of these mods pull the amp away from the simple first build it was intended to be but as each only requires one or two components I’m happy to give them a try and slowly crank up the complexity.
There are a few other mods that I’m considering but I’ll probably leave out of the first iteration, they can always be added later.
Built-in L-pad attenuator
Built-in dummy load
Additional cascaded gain stage for a lead channel
Sag resistor, with bypass switch (actually, this one I might add now… I’ll see)
Solid state/tube rectifier selector
TL;DR… I made a frankencircuit
This is my first attempt at putting together something like this and, while I’m fairly confident in what I’ve got, I’d really appreciate any input or feedback that anyone may have, particularly from those who have done similar things before.