Faint crackling, motorboating?

[ashok]

Hi everyone,

I built an ST 35 (Revision C) in early 2004, and it has served me very well since then.

Just last night, when I went to turn off the amp, I noticed a very faint crackling sound from one of the speakers. Did not do anything else, except to verify that it was the amp that was the source of the crackle, and not anything upstream of it. The crackle was only from one channel.

Today, I investigated a little bit more:

- re-adjusted bias to the recommended 35mA, no change.
- replaced the tubes one after the other starting with the driver

The crackle persists, plus I began to notice a very faint motorboating type of sound. I do not have a better word to describe the sound. Now, it is from both channels, but very faint. I have to go right up against the speakers to hear it. (So don't go right up to the speakers, right?)

There does not seem to be anything obviously wrong with the amp. I will be taking it apart and re-wiring it this weekend. Will check all joints at that time.

One of the filter capacitors is a little bit bulged at the top, but it has been that way since the time I built the amp. Shannon, I think, clarified that there is nothing wrong with them.

I do operate in triode mode. The feedback capacitors have been not fully removed. I have a short (2 inch) set of flying leads off the board. One leg of the capacitor is soldered to one of the leads. The other leg of the capacitor, and the other lead are taped off.

Tube complement is 5751, 12BH7 and set of 6P14P-EV.

I hope I am on the right track here with some of the things I am going to try. If you can suggest other things I might look for, I would be most appreciative.

Best regards,

Ashok

[Geek]

Hi,

Sounds like time for a total recap

Cheers!