Presence cap value vs. circuit impedance

[Chuck H]

Most presence control circuits seem to use a .1uf cap. Like the standard Marshall/Bassman circuit. But I often see circuits with different value resistors in the feedback loop and they all use the standard .1uf cap.

Example: A typical "Marshall" circuit uses a 100k feedback R, a 4.7k pot and a .1uf cap. If I were to change the feedback loop to a 22k feedback R, a 1k pot and keep the .1uf cap, should I expect the same results? or do I need a bigger cap now to effect the same frequencies as when I used the higher values? It's the same feedback "ratio" but the impedance of the circuit has changed.

TIA and happy Holidays

Chuck

[loudthud]

To keep the same frequency break point you would need to increase the cap x5 when scaling the resistors x(0.2). If the amp oscillates when the presence control as all the way up, try a small resistor like 100 ohms in series with the cap.

[Chuck H]

Thanks loudthud

Thats what I thought. I'm having no trouble with instability. I just found it interesting that the .1uf value seems to be standard for most presence control circuits. Even when the feedback resistors are scaled down.

Even the late, great Ken Fischer has a BF Fender mod in print in one of the Gerald Weber books that uses a .1 cap and the stock Fender feedback R values that he says makes BF Fenders sound more like a Marshall.

Ive just never seen this issue discussed here.

Chuck