bad power tube noise in my tweed champ

[spicoli]

Hi - I just swapped out the JJ 6v6 thats been in my 5f1 (couple years) for a new tung-sol. The amps been fine with the JJ but as soon as the tung-sol gets pushed into clipping, it makes a terrible loud noise - sounds like an arc welder.

This is a pretty low voltage champ - 260-0-260 PT and about 250v on the 6v6 plate.

Put the JJ back in and all is well with everything wide open. I've never heard anything like this before (except when I'm welding....), But it's a dramatic enough noise that I'm sure that one of you knows exactly what it is.

Did I just get a bad Tung-Sol? and if so what is actually happening to make that noise?

btw.. the TS 6v6 sound fine with the guitar volume backed off (amp is pretty much always 3/4 to wide open), but get the guitar volume up around 7-8 and it freaks out.

[MWJB]

Have you rebiased the 6V6 for the very low voltage you are running at? You should have 40-50mA of plate current at least...if you only have 20-30mA this could be a problem. If you have rebiased, then it's a bad tube, return it to the vendor & ask for a replacement.

Further to that...I'm not sure how you have managed to get less dc voltage after the rectifier, than the AC supplied by the PT? You do have a filter cap going to ground at pin 8 of the rectifier, don't you? I would expect 300vdc-ish for a 260-0-260VAC PT.

[spicoli]

Just using the standard 470 ohm cathode resistor. I checked the voltage at pin 8 and got 15 volts. If I divide this by 470 - I come up with 31mA
is that a correct method? If so, then it sounds like I should change out the cathode resistor for something more like 350 ohms?

[spicoli]

Sorry - I didn't see the second half of your post - yes I have 300vdc before the first filter cap. The 250 reading was taken at pin 4 of the 6v6.

[MWJB]

It's close enough. Yes, 330ohms would be the next standard value down.

Pin 4 of the 6V6 is the screen grid and subject to a voltage drop across the 10K power supply resistor.

Pin 3 is the 6V6 plate, but 300v off rectifier pin 8 sounds ball-park.