dynaco 525 volt 30/20/20/20 replacement capacitor

[Soundmasterg]

The oscillation that you had before wasn't due to the filter cap value btw.

What was the cause? When I changed the caps, it went away.

As Oli said, it was likely a resonance between the values of the parts around there causing the oscillation. Sometimes it can be a cap that is larger, other times it can be one that is smaller...it all depends on the circuit...and often there are hidden circuits going on too that we can't see....certainly that is the case when something starts to oscillate. You can get an LC resonance between the choke (inductor) and cap, or an RC resonance between caps and resistors, or a RL resonance between inductors and resistors, though those last two are more rare. The reason the oscillation stopped was because the cap value was changed and no the resonance was occurring at a different frequency, and whatever that new frequency was, it didn't interact with the surrounding circuitry anymore to cause an oscillation.

Greg

[patlaw]

Sorry, I missed your response. To be more accurate, the turn off noise was more of a major fart than an oscillation. Whatever the final analysis is, putting the correct value caps in the circuit solved the problem. Now that I've read a lot more about this circuit, I probably should have gone with a little more capacitance for the first cap, but it's working well, so I'll just leave it. The amp is not back together yet as I still have to replace the power cord, remove the death cap, replace the power switch, and possibly replace the treble control. It's sweet not being in a hurry and being able to do the work when the mood hits me.

Ever since getting back into repairs after a 30-year hiatus, I've really grown fond of fixing tube-type guitar amps, particularly Sunn and Fender. At the moment, I only have one candidate for a repair. Maybe I'll look around on Craigslist and pawn shops.