i posted this at harmony central, but didnt get any bites.
im wondering what i should do to the caps for my PPIMV.
Left (red) is original, right (blue) is PPIMV. Now, original has .25uf caps.
If my math is right, if i just use another set of .25uf caps on the mod, then in series that would make only .12uf of capacitance.
So should i double cap values to compensate for the fact that theres more of them? Would this preserve the original tone as much as possible? My brain is so fried right now. I've been at this for a few days straight.
http://i49.photobucket.com/albums/f289/thok_101/PPMIV.jpg
The value of the coupling caps determines below what frequency signal starts to be attenuated (by 6db/octave).
For a 0.25 µF caps and 100 k grid resistor (assuming zero output impedance from the PI!!!) this frequency will be about 7 Hz (actually it will be lower than this due to the output impedance of the PI).
This is well below the limit of human hearing, and even further below the lowest frequency on a bass guitar (40 Hz). Halving this valve will have no audible effect of bass roll off in your amp.
Sadly things aren't always as simple as they seem though........
Post PPIMV has a number of issues to address:
1) You are changing the open loop gain of your amp which will affect the amount of -ve feedback (assuming you are using -ve feedback) around the power stage. The effects of this can be hard to predict and will depend on the circuit, however conventional presence controls will become progressively less effective at the MV is turned down.
2) All PPIMVs (conventional!) introduce resistance in series with the control grids as the MV is turned down. This will interact with the Miller capacitance of the power valves and cause top end roll off.
3) By wiring the MV as in your schematic the impedance seen by C13 and C14 and the series impedance with the control, grids will vary depending on the position of the MV control.
As you have relatively large caps in your circuit this should not effect the low end frequency response of the amp as the MV is used.
It would thus seem sensible to make the couple caps relatively large so that the low end roll off is never an issue.
Unfortunately, very large coupling caps will exacerbate blocking distortion, which occurs when an amp is driven into positive grid conduction be a very large positive signal. Needless to say blocking distortion sounds terrible!
Thus you might want to experiment to find the smallest coupling caps you can use that don't adversely effect your tone.
The circuit shown by Hamfist is a better circuit as by taking the signal from the wiper of the pot the impedance "seen" by the coupling caps is constant, and you only have one time constant in the grid circuit. The only issue with this circuit is that the bias supply reaches the valves via the wiper of the pot so if this contact fails (and it will eventually) you have lost bias supply. Not good.
The 2.2M resistors in the schematic are to prevent this happening, however if you lose wiper contact you will now have a very larger grid resistance, which could cause problems which thermal runaway due to grid current.
An even better method would be to tie the wiper connection to the raw bias via the 2.2M resistor. This will cause the valve to shut off if the wiper contact is lost.
Regardless of the theory, as long as it is safe and you like the sound, then go for it.
