2204 6v6 build bias problem

[Auke Jolman]

My last build was a 2204 with 6v6 as powertubes. For the trannies I used the trannies from an 18 W kit I bought a couple of years ago.

The voltage at pin 3 reads 375 V and I set the bias current at 14 mA at idle.

Normaly I don't play loud, but today my wife was out so I took the volume up After hitting a dirt pedal I saw one of the power tubes glow red... After brining down the volume, the red glow disapeared.

When I measured the bias while playing the amp louder, the bias spiked at over 50 mA. Does anybody can explain to me how that is possible and what I can do about it?

[shooter]

was the idle still at 14?  Are all your B+ voltages still where they were?   I'm not very good at fixed bias - yet, but it seems if you have too big a drive signal one tube would be completely cutoff and the other would start drawing grid current, which is not good, but guessing it wouldn't red-plate, just die?.
 
you could swap tubes and see if the red-plate follows the tube.

[PRR]

> bias current at 14 mA at idle.
> louder, the bias spiked at over 50 mA.

A "fixed"-bias amp, it is normal to have low current at idle and much higher current when you *push* it.

Think of your car. Idling it may burn a gallon an hour. Take it out on the superhighway and it drinks 3 gallons an hour.

Interestingly the heat IN the tube plates may fall-off when pushed LOUD. So red-plating when pushed is something to be concerned about. OTOH, they are only tubes, they are tougher than they look, and they are very easy to replace.

[Auke Jolman]

I swapped the tubes and as a result the both start glowing more or less at the same time when pushed hard. As soon as when I stop playing the glowing disapears.

Could it be that the tubes are being pushed to hard and if so, how can I change this, besides not playing that loud. I would like to play the amp safe regardless of the level the master volume.

[shooter]

Does it red-plate without the pedal?  Are your original values still the same, 375v 14mA? 

does it sound good?  Maybe try re-biasing cooler as an experiment, or adding a resistor and drop maybe 25v of the B+ as an experiment. 

[PRR]

Re-check your load impedance.

However this may just be the way it is with over-volted 6V6 amps. (375V is well above any spec-sheet's Suggested Conditions for 6V6.)

[Auke Jolman]

Thanks for the replies sofar.

The tubes redplate also without a pedal, although I have to turn it on real load and it takes a bit longer.

The load impedance is matched. I tried biassing it real cold to about 3 mA, but that did not chance a lot.

So, I would like to get the voltage down to specs of the 6V6 datasheet.

What would be the best way to get the voltage down so the 6v6's operate in a save voltage level. I would like to keep the preamp voltages as they are.

[sluckey]

6V6s should work just fine at 375V in a fixed bias amp. Have you tried another pair of tubes? Maybe some JJs?

[Auke Jolman]

I put in a pair of JJ's and the amp sounded fine even at high volumes. No redplating whatsoever.

I guess the JJ's can handle the voltage better than the set of Tung Sol tubes I had in the amp.

It seems that my problem is hereby solved. Thanks for all the kind replies and support.

[Willabe]

I put in a pair of JJ's and the amp sounded fine even at high volumes. No redplating whatsoever.

Interesting.