WEM Dominator Buzz

[Ted]

I've got a WEM Dominator Mk.III here.

With a nasty 50Hz buzz. Not bad enough to make it ungiggable but definitely unrecordable.
Intensity varies with volume setting.

http://www.vintagehofner.co.uk/brita...cs/dommk3.jpeg

Looking at it on the scope there are alternate positive and negative going spikes every 10ms on the output.
The spikes are visible on the output from the PI. But not at its inputs (this may just be my crappy scope),

All valves and electrolytics are new. Lead dress makes a slight difference.

First thought - heater coupling into the preamp.
I tried running the heaters from DC. No difference.

Second thought. Spikey charging current to main electrolytics sharing preamp ground.


Anybody else got any ideas/suggestions?

[Gtr_tech]

Check to see if the center tap wire for the heater winding is not broken off. It should have a good clean gnd. It'll hum like a bastard without it....

[Ted]

Turned out to be too-fast rectifier diodes (Should have mentioned that I'd replaced them and the mains transformer). Slugged them with a few nF and the problem went away.

[Randall]

Turned out to be too-fast rectifier diodes (Should have mentioned that I'd replaced them and the mains transformer). Slugged them with a few nF and the problem went away.

Can someone explain this? Why would too fast rectifier diode cause hum?

[Ted]

Can someone explain this? Why would too fast rectifier diode cause hum?

This was a buzz rather than a hum, visible as alternate positive and negative going spikes.

Fast switching edges give inductive spikes in small inductances. Like ground bounce in digital systems. I'm not sure of the P/N of these diodes but they're for use in SMPS. and I seem to remember them being described as "Ultra Fast" when i bought them.

I'll dig out some 1N4007s see if they make a difference.