Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: evancorylevine on September 11, 2017, 06:36:06 am
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I just broke one of the four power tubes in my 80W AB763. Do I need to buy a matched quad, or can I just buy one loose 6L6GC?
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Nevermind. Figured it out.
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You can buy a loose one, and it will work, but you may not like how it sounds. Sometimes you'll luck out anyway and the tube will be pretty close to the others. It's always good to have a couple spares anyway just in case you're playing live and it craps out, but it's best to use matched pairs/quartets if possible to get more consistent output from them.
~Phil
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Less of a problem in a BF fender than it is in something like a 1959 Superlead 100 with 4 x EL34s idling at over 500V. In the latter case, you are wiser to put in a fresh set that is as closely matched as possible. (And make sure the screen grid resistors are all functioning okay, especially on any socket(s) where a dud tube was).
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For anybody else who might have this question, you need to check the bias. First make sure the new individual tube isn't drawing too much for its own limitations/safety... if it draws much more than the others, you might need to lower the overall bias to keep that tube safe.
Normally any imbalance, even within safe limits, could impact the performance/sound (for a number of reasons). However, in a 4-tube amp that imbalance is a smaller portion of overall current per-side and would be less noticeable.
With that said, if you have to bias the entire amp colder to protect one tube (which is drawing more current than the others) that overall change in bias could impact sound.
With a single PP pair of tubes, all the problems that "should" result from mismatched tubes would be more pronounced.