Red plating with a loud shhh

[Colin]

Hello there - I wonder if anyone could help me on this, please. I'm stuck - again!

I have a WEM dominator 25. 2 x EL84 and cathode biased. All the volume and tone controls work as they should with a guitar signal going through it, then as it warms up (no standby) a shhhh noise gets louder and louder and it starts red plating.. so I switch it off. (There's no loud mains induced hum.) Putting a bias probe across it gives me roughly the correct plate voltage (300V), but as it warms up and the shhh noise starts then the bias current just keeps on climbing and the plate voltage falls. I switch off at about 100mA.

I immediately replaced the cathode biasing resistor-capacitor pair thinking that this was obviously at fault.....wrong again! It still does it.

mmmm. Any help greatly appreciated.

Thanks
Colin

[Enzo]

The first obvious thing to change is the tubes, not the resistors.

The more current the tube conducts, the more the B+ voltage will fall. That is natural. The more current through the tube, the greater will be the voltage across the cathode resistor. That's Ohm's Law.

Now the tube has either failed - most likely - or the grid is getting positive voltage from somewhere, a leaky cap from the previous stage. When the tube starts to run away, look at the grid real quick and see if the DC voltage there remains zero. If it is drifting positive, then disconnect the cap from the prior stage to see if that is the cause. If the voltage remains near zero, then the tube is failing.

DO BOTH power tubes red plate? Or just one? If only one, swap places and see if the problem travels with the tube. If so, the tube is bad. Of not, something about the socket is bad.

it is also possible the amp is oscillating at high frequencies. it could be cranking out 50 watts of higher than you can hear. You can see that on a scope. I can usually tell without looking by the nature of the hummy sound. That would affect both tubes.

[Amp Kat]

Also sounds like thermal runnaway but both tubes would red plate. Also check the ohms from cathode to ground and make sure the Cathode Resistor value in true. Then with the power on measure the voltage on the Cathode and make sure it's right. Depending on the value of the Cathode resistor it should be around 25 to 30 vdc. It does sound like a bad tube especially it being Cathode biased.

[Colin]

Thanks Guys.

I should have said that the first thing I did was swap the power tubes for two I had in my spares box - but after what you said I thought I'd pull two tubes from an amp I knew was working. That made a big difference.

Then, following your advice Enzo, I put my probe on the grid of the now much less red-plating tube to read the dc voltage..... and the noise went away! So I tensioned up the sockets... and problem solved.

Thanks a lot for your time and patience. It is, as always, much appreciated.

Colin