... I will be using a 350V 22uF electrolytic. If that does not suffice I can go to 47uF or possibly 68uF. ...
22µF is plenty.
... One other question, what value pots should be used for treble, mid, and bass, as the volume pot is .5 MEG. ...
Depends on which amp's tone stack you're copying.
If you're going for Fender, just use
the stock 250kΩ Audio for Treble & Bass. Mid can be as low as 10kΩ Audio, up to as high as much as a 250kΩ Linear pot. The latter is not only use for more mid-boost than you get with the stock Fender blackface circuit, but isolates the tone stack so much from ground it can result in a gain boost (really, just no-loss in the tone circuit).
If you're going for a Marshall response, the caps at the Treble pot become 500pF & 22nF, and the other tone cap is a 22nF. The Treble pot will be a 250kΩ Linear, the Bass pot a 1MΩ Audio and the Mid pot a 25kΩ Linear (up to as much as the gain-boost value described above). The Marshall usually uses a different slope resistor than the 100kΩ you have drawn, and the Mid control is connected in a subtly-different way.
... So I will be connecting the line in from the treble to the plate immediately after the .02 coupling cap ? ...
There is no coupling cap at the 12AX7 plate output to the Treble pot input, except for the existing 250pF cap. That's because the tone caps also block d.c. from each of the tone pots. So adding a coupling cap would be redundant and change the response of the tone circuit (unless the coupling cap was very much larger, like a 1µF cap).
... These changes are going to make it cathode bias as opposed to grid leak bias simply by reducing the 18 MEG resistor, and cathode resistor to 2.7K correct ? ...
The original input 0.01µF cap and 18MΩ resistor set the stage for grid-leak biasing. If you cathode bias instead, you can usually input much bigger signals before overdriving the grid of that 1st 12AX7.
You could keep the 0.01µF or remove it; if you kept it, it would block d.c. if you had a dodgy pedal running into the amp and would add a bass roll-off. I presume you'll drop the 18MΩ to 1MΩ. With the 0.01µF and 1MΩ resistor, bass would be -3dB at ~16Hz, so it's really no impact to your guitar sound (low E is ~80Hz).
If you switch to cathode bias, you also have the option of cutting stage gain by about half by omitting a cathode bypass cap. Or you could add a bypass cap across the cathode resistor, and have the option of shaping the bass of the stage. Choice of bypass cap is influenced by the cathode resistor value, unless you default to full gain to essentially all frequencies by using a 25µF bypass cap.
A 1.5kΩ cathode resistor would be the standard Fender gain stage, when used with a 100kΩ plate load. But you have the option of going lower or higher; such changes are usually accompanied by a change of plate load resistor to set up the stage for a slightly different gain or voice. But likely anything from 1k-3kΩ will work with a 100kΩ to 270kΩ plate load. There's really a lot of latitude available, though with higher plate and cathode resistors (more gain in the stage), the tone stack may appear as a heavier load (more loss in the tone stack) because of the slightly higher output impedance of the 12AX7 stage.
With the above, you'll be close enough to have a working amp. From there, you can tinker to see what you think sounds the best.
Topic: Tonestack (Read 4375 times)