Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Other Topics => Topic started by: cmug on May 15, 2026, 03:19:27 pm
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This might be a silly question. But go ahead and ask anyway.
If you look at tone control circuits, there’s always a circuit with a variable resistor (the potentiometer) and a fixed capacitor.
I’m currently converting an old tube radio, and it has a variable capacitor ranging from 0 to 500 pF.
Couldn’t I do something interesting with that cap ? I can’t find a sample circuit diagrams.
(capacitor like this)
(https://upload.wikimedia.org/wikipedia/commons/6/6c/Variable_Capacitor.jpg)
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I love those things!
But they provide capacitance that is too low to use in an audio tone circuit. They are, after all, designed to work in the RF range.
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A couple of options jump out -Variable bright cap across a volume control.
Variable tone stable treble cap.
Downside is that its large physical size will tend to promote coupling between it and with the rest of the amp circuit, leading to unexpected / unintended / reduction of the margin of stability.
Worst case being free running oscillation.
A solution to that would be a screening can around it.
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Thanks for the tips. The capacitor is inside the housing. Removing it would just create a lot of air for nothing. So I’ll just give it a go. There are three synchronous capacitors, so I can adjust the capacitance from 0 to 1500 pF continuously during the experiment.