This one has taken a while. Mostly due to working-for-a-living (highly interruptive), raising a puppy (highly engaging), and doing other good things (mostly outdoor fun). And while it's up and functioning, it's not nearly in its final form.
I completed a design that works perfectly in theory, and in my modeling software.
... and the subsystems of which all passed prototyping trials on my breadboard (see prior post). But the final implementation of the reverb proved too complex for me to pull off... and possibly also the tremolo, but we'll see. The summary:
For the reverb I opted for a widely available solid state IC stereo unit that requires 5 volts DC power, and which draws off part of the guitar signal and adds a reverberated version of it back in further down the line. In order to prevent this unit from sucking up too much of the signal, one must include "buffers" or gates at either end, to allow just the right amount of signal to pass. This is commonly achieved with tube triodes, but I don't have room for another tube so I decided to use common IC Op-Amps, salvaged from the donor amp. For a variety of reasons, these Op-Amps require a 12 volt supply and a 24 volt supply to operate, and while I was able to create separate power supply nodes to produce those voltages, and to successfully test them each individually, I was just unable to reconcile all the various voltages and their grounds, within the amp. So, I've added a passive effects loop and I'll purchase a stereo reverb pedal that will be a permanent part of the amp.
I also chose a solid state approach for the tremolo design, borrowing aspects of the TK Imperial, Fender VibroChamp, and other successful circuits to arrive at a MOSFET driven tremolo... which works, but is just slightly noisy. I'll keep working on it, but so far I haven't been able to solve the noise issue, which probably wouldn't be noticed in live and informal settings, but would not work for recording (which this amp is not designed for anyway). Maybe I keep it, or maybe I redesign it - we'll see.
The amp is still pretty heavy - 40 lbs - so it will be necessary to rebuild the cabinet with lighter materials, and install lighter speakers. This should reduce the weight by around 15 lbs.
The current schematic removes the reverb sub-circuit, doesn't show the effects loop, and includes a variety of minor changes from earlier, but otherwise pretty much reflects the current circuit.
Some notables: the power supply and much of the filtering is in a separate sub-chassis located on the floor of the amp, which gives the amp a lower center of gravity and helps prevent high AC voltage from inducing noise in the circuit. You can see it in the gut shot below, at bottom of frame sitting on the bench under the main chassis. I use an 8-pin tube socket and modified tube base to allow an umbilical chord connection between the two.
The FET input boost sub-circuit is based on one that Dumble installed in a famous amp model of his, but my 24 volt power supply for it is probably better regulated than his fairly big high pass filter and voltage divider at the end of the B+ rail. I'm still figuring out how best to configure the input boost: Maybe I will only ever need it when using the Overdrive circuit, so why not just tie them into the same switch? Or, is it useful as a separate switch for guitars (like the Tele and Strat) with low output pickups.
I'll probably still use the 12 volt supply (reduced to 9 volts) to provide a convenient on-board plug-in for the reverb pedal, so I'm glad it's there even though I didn't use it as planned.
The input stage features a parallel input triode similar to some Matchless amps. For mine, I added an adjustable cathode bypass capacitor control to allow fine tuning of low frequency roll-off.
The way I installed the now unused digital reverb unit (black plastic thing, mid-board below) makes it difficult to remove it without UN-installing the whole circuit board... so, it stays put for now.
Current gut shot:
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