Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by matt1969, Jan 27, 2015.
Can a bad tube red plate under normal conditions/ correct bias?
maybe, but typically, no.
red plating is >95% of the time the result of another problem....
Try this, though: if this is a 2-tube amp, swap your power tubes to each other's socket. Does the problem move with the tube or stay with the socket? That will tell you a lot.
Or you may just have grossly-mismatched tubes, where one is biased in "the right range" while the other....because of the mis-match.....is running way too hot.
I personally have had a bad tube red plate while the amp was perfectly fine. I agree, if the tube is bad the red plating would follow the tube. I also have had tubes that I have bought in bulk that were so far out of match, that one would red plate even thought the other was set to the proper bias.
I've seen the same when testing on-hand 6V6es in my Princeton Reverb. The originals worked fine, cathode current 23/25mA, then replace one tube and it starts out at about 40mA and runs upward, redding its plate (lunge for power switch).
Red plating could be a bad tube, but it could also be a good tube with the wrong bias current.
Just because tubes vary in current draw at idle with the EXACT SAME setting means little to nothing.
Even cathode bias amps are not necessarily "self adjusting" at all.
6V6 can vary widely from the factories in terms of current draw at idle..... as much as 20 ma apart is what I have found testing 100's of 6V6 thru the years.
Why do you think Leo Fender wound up with "adjustable fixed bias" for Blackface amps for crying out loud?
Otoh, if you have a hotter idling tube which you then adjust the current draw at the bias circuit within spec for 6V6 at say 25 to 28 ma of current draw and the tube slowly but surely creeps upward without stopping, this COULD be "runaway current draw".
BU, I have seen brand new in box power tubes that needed to be burned for many hours before they settled down and stabilized in current draw.
This is why vendors often burn in power tubes for 24 to 48 hours.....to stabilize and weed out bad tubes under a full operating load.
I've serviced an amplifier where the design bias was too high and the tubes overheated. A slight reduction in bias current was all that was needed. This seems to be an issue with a few EL84 cathode biased amplifiers.
Thanks for all of the replies!
The EL34 tubes are matched and biased at 60% dissipation.
swap the tubes.....see what happens.
Swapped the tubes. Problem is at the socket not the tube. I measured the voltages at pin 5. With master off it was -43. With the master cranked, -42. Dropped one volt.
might be a bad socket.
Makes no sense at all.
Have you tried re-tensioning the individual pin socket/springs in the tube socket. Turn off the amp, discharge the caps and then use a small Jeweller's screwdriver to reset/bend the little springs in each pin socket to ensure good contact when a tube is inserted. Might be getting poor contact there and losing bias voltage on the tube pin and the tube current runs away and red plates the tube.
^^^ +1. This seems likely, except that you wouldn't get a correct bias reading on that tube...unless your bias probe is making better contact with your socket than your tube is.
Found the problem. I accidentally mis-matched my speaker impedance.....
Sorry if you can't make any sense of this. I suggest you read & learn about how tube amps work & how to bias an amplifier.
Again, thanks for everyone's input!
A mismatch should only cause an overdissipation problem when the amp is pushing out high signal levels; was that the case (if so I missed it!)?
For the record, what impedance was the amp set to (to cause the red-plating), and what impedance is the cab?
I assume that the cab was a lower impedance than the amp was set for, but anything's possible.
Still makes no sense to me. But now that I have all this free time, I guess I'm off to study up on tube amps!
LOL. Yeah, it doesn't make sense unless, POSSIBLY, in a high volume/dissipation situation as per Pete, above.
It always amazes me how newbies come here with very basic questions and leave experts. I guess we're doing a good job.
Well then maybe I miss understood you then? Maybe you meant my problem makes no sense? bias to 60 % of max dissipation of 25 watts per EL34 does.
You're assuming I'm a newbie. Maybe I am compared to some people.
I have my amp set at 8 ohms w/ two tubes pulled. I have a 16 ohm speaker cab plugged into my Marshall Powerbreak. The impedance switch on the Powerbreak was accidentally on the 8 ohm setting when it should have been on the 16 ohm setting.
Now that I switched it back to 16 ohms, Everything is fine.
Thanks for helping out.