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In Reply to: RE: 450v plate and screen? posted by Alpha Al on June 08, 2017 at 11:25:37
Is the 24 ma. plate current or cathode current(which includes both plate and screen current)? And what's the screen voltage on the amp?
4 or 5 ma. of screen current might push the total tube dissipation up to or over 12 watts and may cause some discoloration in some cases.
Follow Ups:
That was plate current. I didn't measure screen current, but the screen voltage was just a little less than plate voltage, maybe 5 volts difference.
I was mostly curious why the hot area was so small, instead of the whole plate heating evenly.
"I was mostly curious why the hot area was so small, instead of the whole plate heating evenly."
I think it's always that way with beam power tubes. The beam deflector plates force the electrons to that area of the plate.
Is the bias voltage adjustable?
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Yes, the bias is adjustable. If I reduce plate current down to 12 mA, the hot spots disappear completely, but the power and tone quality suffer.
12ma 450 vdc is only 5.4 watts
Something is wrong with those tubes.
The plate of the 6v6gt should be good for 12 watts without glowing.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Maybe a good idea to check all lines going to and from the 6V6GT. If over 5.4 watts per tube causes the tube anodes to glow red --- something is definitely amiss. I'd check any bypass cap and grid-to-ground resistors, along with other suspects.
Have you tried other know, good 6V6GT tubes?
Good luck!
8^)
Check the screen current and see what you find. That might be quite helpful.
I changed to a pair of JJ 6V6S, they don't have the hot spots.
Normally, I stay away from JJ octals, but in this case, they fixed it.
I think the TungSol was a defective pair.
The screens are drawing 2 mA each with either tube type.
That definitely could be the case. I've had ONE case of the 6V6GTs acting up in all the years I've had them. And many have been used at 425 -450 volts. But it can happen... Hopefully the vendor will stand behind yours!
Al, on many Fender amps... changing the screen resistors to wire wound or metal oxide, 5-watt 1k-ohm usually helps stabilize the output tubes.
My DR runs over 450VDC on the plates. I bias them to about 20mA per tube.
Photo is of my Super Reverb, I put 1k-ohm bricks onto the screen grids.
Good luck!
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