Amp bias question - help please

john b

Member
Messages
1,057
I have a Gries 20 which runs 2 6v6's and should be biased at 25. On the back it has three bias adjustment places for the multi-meter - a center negative and a right and left positive. There is a small bias adjustment pot on the underside of the chasis - one adjustment for both tubes.

I just threw in a pair of GE 6v6GTA tubes. When I bias the left tube to 25, the right reads 37. When I bias the right to 25, the left reads 14.

Which is the correct was to bias the tubes at 25? Is one tube the one to read and let the other one go or, should I average the difference between the two tubes?

Thanks and happy new year!
 

john b

Member
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1,057
Will it be damaging to the amp to run this set if they are so far off from one another?
 

phsyconoodler

Member
Messages
4,301
What I would do is find a close medium and run them for a couple of hours.Then re-check the bias again.You might find it has changed.
A mis-match is ok within reason.You might find that it actually sounds better than a perfectly matched set.YMMV.
Just make sure neither tube is biased too hot and play for a while.
 

WaltC

Member
Messages
2,123
As long as you bias the amp so the hotter of the two is within the limits (25 max?) then you'll be ok. It might not sound as good as if the tubes were more closely matched, but it won't hurt anything.
 

john b

Member
Messages
1,057
Pedro Borbon, Manny Mota...who are these people?

Anyway, back to the subject at hand and many thanks for all of your help.

I bias the amp to 25 on the stronger tube. The weaker one read 15. Just played for over two hours and it sounds great. Nothing blew up and it's a big improvement over the stock EH tubes. So, not too bummed on my part.
 
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WaltC

Member
Messages
2,123
echo.... duh, I don't get it ....it.........it........<G>

Happy New year all!!!
 

deeval

Silver Supporting Member
Messages
2,272
Of Course,you need to check your plate voltage,and just calculate the idle plate dissipation,also seems your tubes are way off match,I run alot of my amps with no more then 5 ma mismatch,I like mismatched tones better for my ear,s tone.
If you have other 6v6 tubes try mixing them around to see if you can get closer match?
 




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