Anyone have a good schematic for gain stage for a jcm800?

[Kapo_Polenton]

Thanks for the reply..even simpler than that, I want to know in the image I attached, is the DPDT switch wired correctly? I'm questioning the signal flow and trying to wrap my head around whether or not it would work or if maybe it was drawn incorrectly. I agree though, i will be going with more traditional values..

Start at the jack, with the DPDT switched to the upper lugs, the signal just goes to through the switch and to the 68k resistor of V1. This would be your normal circuit.

Switch the DPDT switched to the lower lugs, the signal now goes to the 68k resistor of the extra tube, out of the that to the pot (which I assume is an added pot), from there back to the DPDT (we're at the middle lug left now). Since it is switched down, that signal goes the lower left lug but that is bridged via a wire to the upper right lug which goes back to the 68k of V1.

I'm with brian, though, I'd do it in such a way you're not making the first stage switchable. It may work fine, idk, but I don't think that is standard. You also wouldn't have to fool with added an extra gain knob....you'd just use your existing one.

Ahhh ok, that pot is added! (that makes a bit more sense)in terms of the wiring.. So on the Hoffman diagram, if I am adding half a tube to my 2204, I can run the input resistor to the DPDT and it to pin 7 of the new tube and I am in business?..

 

[Rogue]

Ahhh ok, that pot is added! (that makes a bit more sense)in terms of the wiring.. So on the Hoffman diagram, if I am adding half a tube to my 2204, I can run the input resistor to the DPDT and it to pin 7 of the new tube and I am in business?..

Here's how I'd do it.

Take the wire coming from the plate of V2, after the .22uf coupling cap but before the 470k/470p dropping resistor/cap. You're going to insert your switch here. Wire from coupling cap to pin 5 of DPDT. Wire from 470k/470p will connect to pin 4 of DPDT. This is your stock circuit.

Pin 1 of DPDT will go to the grid of your new tube*1. The anode will go to pin 6 of DPDT*2. Bridge a wire from pin 5 to pin 3 of DPDT. Pin 2 of DPDT to ground.

1) You're going to want to put a dropping resistor/cap here between the DPDT and grid of the new tube. You're also going to want to put a grid leak resistor from grid to ground as well. Might as well just use standard 2203 values of 470k and 470p. Tweak as to your tastes, of course.

2) You're going to want to put a coupling cap between anode of new tube and DPDT. Might as well use a .22uf.

You'll pick your load resistor and cathode resistor/bypass cap values for the new tube. I'd probably start with values similar to your first stage.

You could also try putting a dropping resistor between grid of v2a and the grid leak. Try 220k and work around from there. This may be needed to tame new newly found gain.

That should work, but it wouldn't hurt to get that checked up by some more experienced folks. This does not speak of current draw and filtering needs of this newly added tube. I would think your stock amp should handle this fine, but I don't know.

Crystal clear?

 

[Kapo_Polenton]

I think it clear! Thank you very much for your time on this..hopefully I can get something reasonably good sounding out of it.