Mystery cap question

[Drewline]

I've attached a photo of a Twin Reverb I'm servicing which had been recapped a few years back. I have circled a .02 cap across the (2) .1 caps which is something I haven't seen before. Can someone be kind enough to clue me in on the purpose of this cap.

Please see below. The cap value is wrong. It is actually 82pf/500v.

[52 Bill]

I've attached a photo of a Twin Reverb I'm servicing which had been recapped a few years back. I have circled a .02 cap across the (2) .1 caps which is something I haven't seen before. Can someone be kind enough to clue me in on the purpose of this cap.

It's used to reduce high frequencies and sometimes to cure oscillation problems. The signals at the output of the phase inverter are 180 degrees out of phase with each other. Placing a cap across the two end causes the high frequencies to cancel each other out. The old tweed Fenders used this a lot.

The typical value is 47pf.

[J M Fahey]

The typical value is 47pf.

Agree, and even 100 or 120pF but .02 is madness,will start cutting above 100Hz, go figure.
Muddiest darkest Twin in the whole planet.

Plus it looks like a dogbone mica cap, those usually come in low values; are you sure itīs a .02uF?

Any chance itīs 220pF instead?

If in doubt post a closeup showing its labels.

[Drewline]

My mistake, I checked it with a magnifier & it is an 82pf @ 500V silver mica. The amp was converted to run KT66 output tubes & the owner loves the sound. The 2000pf supression caps were removed from the output tube sockets so maybe the 82pf cap was added to supress oscillation?
Had not seen this on a BF or SF & was curious. Thanks for the replies.

[Leo_Gnardo]

What 52 Bill said ^^. I've had to use up to 470 pF to tame parasitic oscillations in some Fenders & similar amps. 20,000 pF = 0.02 uF way overkill - unless the owner/tech was looking for a very mellow tone.

Best bet, remove that cap and observe the amp's output on a scope while driving speakers - not a load resistor - and sweep low frequencies say 50 to 300 Hz. If you see blips of high frequency parasitics on the waveform, then tack in small value caps where that one is, work up from 47 pF to whatever it takes to stop the parasitic. Final check, keep watching on the scope while you play low notes on a guitar. Sometimes double-stops bring out the parasitics, especially fifths.

[Drewline]

Leo,

Thanks for the info. As I said above I misread the value. 82pf, owner loves the sound with KT66s. I'll leave well enough alone. I was just wondering what the caps function was having not seen one in a BF/SF amp.

[J M Fahey]

Marshalls always had them, being treble boosted and intended to be used always on 10: