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I am retubing this amp and the bias, with JJ 6L6 is only 22ma per tube at 500v Va. It's only about 35% of max dissipation. I'm tempted to mod the bias circuit, but any reason these would run so cold? Other than JJs generally running colder? Would you all adjust it or leave as is?
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I'm no expert but I would look for crossover notch at the output on a scope and then decide. Also, as someone is bound to say, does it sound alright?
I find it pretty common to mod bias circuits to accomodate new tubes to get into the 50-70% bias range.
Can anyone comment what the purpose of CR1, 2, 21 and 22 are? I have heard people talk about some sort of diode clamping mechanism in the reissue Sunn Model T that allows you to bias cold and avoid or limit the crossover notch, but I do not understand how this mechanism works. Maybe it could be the same on the PV?
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CR20/21 keep the grid signal from going higher than 700mv on the positive going cycle. Maybe it limits grid current. Thats my 2 cents.
CR1/2 are flyback diodes and protect the tubes/transformer.
It sounds good to me. I am gonna go ahead and clip the 470ohm and put in something higher in value. See if can get bias around 50% dissipation. Been reading these tend to run cold...but 35% seems extreme.Comment
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Extreme what? If it doesn't sound worse, then compared to 70% it's extremely reliable.
Try it at 50%, then go back to 35%. If it does not sound significantly worse, leave it at 35. Why sacrifice tube life just to hit a number?
That being said, I am curious to hear if you notice much sound difference between the various bias points.Originally posted by Enzo
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Most 5150/6505 heads come from the factory at 11-15ma, which is way colder. I agree, what do you mean extreme? That whole 70% thing is just an internet rule of thumb. The stock arrangement works fine, and provides long tube life and reliable operation. What is extreme about that?Education is what you're left with after you have forgotten what you have learned.Comment
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I thought the bias should be up hotter but not too hot...hence the 70%. It's rather hard to hear the difference IMO when it comes to bias settings UNTIL crossover is audible. I had an amp in recently running just a few ma. Sounded AWFUL. I also think it depends on frequency and volume so it's rather relative. Anyway blah blah blah haha... just wanted to see what y'all had to say about it. I'm fine with colder bias and longer tube life if it sounds good. If it sounds good it is good.
This amp isnt meant for pentode saturation or overdrive. I think colder bias generally seems to be more acceptible in such a caseComment
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