Squealing EL84

[gary sanders]

I have my Chump SE amp together and it sounds really good and breaks up like a dream.Its just got like 360 on the plate and according to the datasheet that's too much,right?Using a 5y3 on it but doesn't a 5U4 drop more? I have 220k grid leak,100R on screen,220R on cathode with a 33uf cap, and 1K5 stopper.I copied a few SE amps I found on line.

PROBLEM: I have a master volume on it and past 2:00 it squeals.tried different tubes and still does it.Does the master volume pot going to the EL84 grid throw off the grid bias?

[dartanion]

That's a very high plate voltage for an EL84, especially if SE. However, I think your squeal is probably lead dress induced parasitic oscillation. Have any photos?

[gary sanders]

So whats the practical way to get the plate volts down besides a different PT?

Ill wiggle some wires around then take a pic.I used shielded wire everywhere I could.I shall return....

[ChrisM]

Zener diode to ground on the CT of the HV if you want to drop some volts on the plates. Search for it, lots of info out there.

[M Fowler]

Rectifer then large dropping resistor then your main filter cap and dropping string.

[Cygnus X1]

Rectifer then large dropping resistor then your main filter cap and dropping string.

How much wattage on that resistor?

What about using a choke?

[Cliff Schecht]

Chokes don't drop much voltage usually. I have some 16H 580 Ohm jobs that do drop voltage a bit but in SE amps, I just use a big resistor to knock down the B+ before the first filtering stage. Since the current draw from no signal to full clipping is relatively constant in a SE amp the voltage droop through a dropping resistor isn't so bad. His amp sounds like it has maybe 50mA of current draw (maybe a bit high but easy number to work with) from the B+ line. a 1k, 5W resistor will drop 50V from his B+ and not sag too much more under heavy loading conditions, especially with enough first stage filtering to provide whatever instantaneous current is needed.

[gary sanders]

I had 2 five watt 470R in the drawer and threw them in to see what happened.Dropped the plate down to 330.I bet a 1K would get it where it should be.

[M Fowler]

Like Cliff said 5w should be sufficient.

[gary sanders]

Cool.Thanks guys.

I also didnt notice there was a thread going on with almost the exact discussion......

[gary sanders]

It was the plate wire to EL84.Moved it around and shielded the tone knobs and its all good now!