Variable Capacitor in '60s Airline Amp

[Neutronarmy]

I was working on a restoration of an old Airline amp and noticed a small variable capacitor (C6 in the schematic). I was curious if anyone had any ideas as to what its purpose might be - variable capacitors aren't something one stumbles into very often in instrument amplifiers.

Thanks in advance!
Chris

[Bruce / Mission Amps]

I was working on a restoration of an old Airline amp and noticed a small variable capacitor (C6 in the schematic). I was curious if anyone had any ideas as to what its purpose might be - variable capacitors aren't something one stumbles into very often in instrument amplifiers.

Thanks in advance!
Chris

Looks like some kind of frequency dependent, variable negative feedback. Like a presence control.

[Neutronarmy]

Looks like some kind of frequency dependent, variable negative feedback. Like a presence control.

That makes a lot of sense. I think the addition of the variable capacitance rather than variable resistance threw me off. Thanks!

[nickb]

I was working on a restoration of an old Airline amp and noticed a small variable capacitor (C6 in the schematic). I was curious if anyone had any ideas as to what its purpose might be - variable capacitors aren't something one stumbles into very often in instrument amplifiers.

Thanks in advance!
Chris

We talking about C6, right? It looks to me like a humbucker. By feeding back a bit of hum in antiphase you cancel the hum on the output and so can use a less expensive power supply design. It's used to modulate the screen grid. Quite clever, really.

[g1]

So is it mounted with the other controls, like a presence, or is it a trim pot somewhere, like a hum balance?