Presonance control - check my values?

[Gainzilla]

Hi All,

I thinking of giving the presonance control a try in the ol' Jet City 20HV (20 watt fixed bias jcm800, el84s, with presence and a switchable fixed resonance... See power amp schem attached). Note: the presence cap is .68, not the .1 shown in the schem.

The question is, do these values look ok? Assume I'll just jumper the existing NFB and resonance r//c on the PCB and wire off-board.

My original in bold:
R1 = R FB = 47k
R2 = R Tail = 4.7k
C Pres = .68

Calculated values in bold:
R3 = 2 * R Tail = 9.4k (is 10k?)
C1 = C Pres *(1 – (R FB / R Tail) /40) = .68 = .51 (.47?)
C2 = C Pres * (R FB / R Tail) /40 = .17 (.22?)
(with the intent that C1 + C2 = C Pres)
RV1 = 2* R Tail to 4 * R Tail , 10% Audio Taper = 10k?

Thoughts? Everything look ok?

[martin manning]

Looks ok to me. Give it a try, but look out for oscillation on the resonance side.

[Gainzilla]

Thanks Martin,

Do you mean mounting off the board might lead to oscillation due to lead dress, or because of the values?

If the former, the NFB board trace leads right up to the current presence pot. Even though the NFB resistor is closer to the back side of the board (resonance r//c is on the effects loop board), my thought is that if I bypass those, the NFB trace will lead right up to the pot resulting in minimal off-board wiring. Thoughts?

Thanks again!

- Bryce

[Gainzilla]

Ok, I just read the entire instability thread. Sounds like the r3 resistor helps but that there are some amps that just don't like it.

Gotcha. At least now I know if it's acting weird it's somewhat expected and won't spin my wheels on it too much.

Cheers!

[Bob S]

I might just be lucky but I've had good success with the Martinizer (presonance) circuit. If you get oscillation at the full bore resonance side just hang a resistor (10-20%) off the pot.

[Gainzilla]

Thanks man, that's good to know. I have a good feeling about this. I've not had any instability issues with this at all (knock wood).

I just need to pick up the pot (assuming this needs to be a 1/2 watt). I'll follow up with my results.

Cheers!