Diagnosing source of 100Hz hum

[martin manning]

Does your HT center tap go directly to the reservoir cap negative?

[timrobertson100]

SOLVED!

- relocate main rectifier diodes to the far right position- for the test to not destroy the board, can be built in the air, p2p with several new diodes, and mount directly to the capacitor for main Ub+ node

Thanks a million Bepone - your intuition was correct and I would not have got there without your help

I knocked up a quick rectifier board and used the BY255 diodes I had to hand and the amp is *silent* in this mode.

Thanks everyone for looking and helping - really appreciate everyone's comments and advice.

[martin manning]

looks like the B+ from the rectifier (pre filtered) is bundled with the post filtered B voltages. Is there enough ripple voltage to transfer inductively/capacitively between the two?

Try running the B+ and ground leads from the FWB like this.

[bepone]

SOLVED!

Thanks a million Bepone - your intuition was correct and I would not have got there without your help

no problem, enjoy

[timrobertson100]

Not quite finished tidying up, but the rectifier ended up mounted on the side like this.

If it helps anyone stumbling on this thread - there seems to be no issue with these B+ leads bundled but note the wire running from the diodes is no longer in the bundle.

Thanks all - I'm going to finish tidying and then close this up.

(Martin - it looks like a FWB but it's just a FW rectifier. The other pair of diodes are a second FW feeding the bias circuit as per valve wizard)

[martin manning]

I'm betting your problem was bundling the rectifier/reservoir leads in with the others, and that it would have been quiet if you just separated those out, with the
CT going direct to reservoir negative. No denying that making those long runs over to the main board isn't the best idea, though.

It was very difficult to see what you had there with leads bundled and under the boards. I thought FW at first from the diode layout on the board and 2550 scheme, but 2555 (diagram visible in your photo) had a FWB.

[timrobertson100]

Thanks Martin

I had tried splitting out the rectifier lead and then moved all those around with a chopstick, but never tried moving the CT. I think you’re right that it was somehow getting onto b+ for V2 on that run, or under the diodes. I also wondered if changing the diodes may have been a factor.