In article , Keith G
writes

"Don Pearce" wrote

The MP3 has virtually identical hum on both channels - it certainly
doesn't have anything like the disparity that the scope picture shows.

http://www.donepearce.plus.com/odds/keith_screen.jpg



Yes and the waveforms are the same height in SoundForge, so I'm suspecting
the scope and/or leads atm. (No big deal, the scopes only a learning tool
atm.)





If U get really stuck run it over to Cambridge Keith ve haf ways of
de-'umming valve amps:-))
--
Tony Sayer

Keith G wrote:

The Bad - It's humming like a good 'un and I suspect it's going to be a
bugger to sort....

Well, I'd expect it to hum.

A single-ended design doesn't have the inherent 'hum rejection' of the push-pull
output stage configuration.

Also, triodes have fairly mediocre anode impedances which make them more prone
to supply ripple effects than tetrodes / pentodes.

I'd suggest an extra stage of LC smoothing on the HT.


Graham

Keith G wrote:


The money's still on an earthing problem atm, but the 100Hz frequency is a
bit mysterious....

Not at all. It's supply ripple.

For reference, 50 Hz would indicate a 'hum loop' type problem, 150 Hz would
indicate pickup from the power transformer @ 3rd harmonic.


Graham