Shared tube/SS power supply

[Rhodesplyr]

A basket-case amp I'm restoring for a client uses a three-transistor reverb driver circuit, and the designer's quick and dirty solution was to use the output tube cathode voltage as the transistor circuit supply source, dropping it through another resistor and doing some additional filtering.

My client thought the reverb wasn't working, but I found that it was. It was just so faint that you could hardly hear it because it was only being supplied with ~3V. I experimented with running it from an external supply and found it actually sounded pretty good at 7V DC, at which point it was drawing 17.5mA.

So, I'm thinking an improvement over the original would be to drop the B+ (~415V) voltage down to 7V and, maybe regulate/protect it with a zener. This might kill two birds with one stone since the additional current draw through the 6CA4 rectifier would drop the B+ voltage a little, cooling an already hot output stage.

Or, I could increase the value of the cathode resistor and use the cathode supply voltage, but I'm a bit leery about having 1965 transistors in parallel with the cathode resistor in case one shorts.

Are there any drawbacks to powering transistors from the B+ supply rather than from a cathode voltage? I know that Hammond did it in amps like the PR-40, which uses a transistor for reverb recovery, the opposite of what's in this amp (SS drive, tube recovery).

The amp is a Gregory Reverb 1500. I realize there are all sorts of better reverb drivers, but I don' t know that my client wants to pay me to re-engineer it.

The tank is the 1475 Ohm In/2250 Ohm Out type.

It would have been nice if the heater secondary winding had been center-tapped so you could build a low-voltage full-wave rectifier, but it isn't. I don't think I could get +7V from a half-wave circuit.

[Steve Conner]

What's the issue, you have 10V cathode voltage available, and the circuit sounds better at 7V than 3? But 7 is still less than 10, so you can use the cathode voltage. Reduce the 5.6k resistor:

to: R = V/I = (10-7)/0.0175 = 180 ohms

The transistors still have other resistors there to limit the current if one shorts. Well, except for Mr. No Markings, but his current will be limited by the resistance of the reverb tank coil.

Maybe you want to increase the cathode resistor to 180 too, to make up for the draw of the reverb circuit.

[Rhodesplyr]

Thanks Steve!

I think I just have a gut-level aversion to messing with the simplicity of a single cathode resistor as output tube bias control, especially when I can tell that the reverb driver circuit was tacked on after the rest of the amp was built.

The only other known example of this amp has three small signal tubes (this one has two), so my guess is that it used something like a 7247 for gain and reverb drive.