Odd sound

[vetmed]

About a week ago I changed to 2A3 and am quite enjoying the sound. However I've encountered one slightly annoying problem that concerns me. One of the amps makes an odd hissy, fizzing sound that starts a few seconds after powering on and lasts 4-5 seconds. Not loud and the amp can play for hours afterwards without any more noise of that nature. FWIW I ran the amps for 4 years using 300Bs with nary a sound like this. My pet theory is that the cathode bypass cap, having been run at the lower cathode voltage for so long, isn't liking the higher voltages its seeing. I should also mention that I also have the soft start installed, in case this makes a difference. TIA for any help.

Robert Lees

[Doc B.]

You changed to a 2A3 and now you hear a new sound. But you are blaming a cap that has been in there for 4 years without making any noise.
I'd sure put my money on the 2A3 as the noise source, and if it stops after 5 seconds I wouldn't worry about it.

[vetmed]

Mea culpa, the noise follows the tube Of course its one of the first things I would have told anyone else to try. And since you don't seem to be worried about it I won't either. However still curious as to what the cause may be. As far as the cap theory goes I have a vague recollection that running electrolytics at low voltages somehow "sets" them at the low voltage, but I've been known to be wrong before Thanks Dan

Robert Lees

[Paul Joppa]

Your memory about electrolytic caps is correct, at least traditionally. I can't say I've heard this about the newer constructions, it's possible they are better. In any case they can be brought back by applying a higher voltage for some time. In extreme cases there must be a current limit to prevent the cap from being damaged by excess leakage current until the oxide film is restored.

I must say I've never run into a need to do this, so I have no experience with it.

[vetmed]

Thanks Paul, that's very reassuring

Robert Lees