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In Reply to: RE: A tube bias question (or maybe not?) posted by ejman on June 20, 2014 at 17:01:02
That's a very small drift but maybe that tube is nearing the end of it's useful life.
Tre'
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Thanks Tre, I'll just keep an eye on it. At what level would it become a concern?
All power tubes eventual fail. When you can no longer bring the bias back up to 500mv it is probably time to replace the tube
Alan
It sounds like you are setting DC bias. Tubes have two characteristics that hve to do with matching: emission and transconductance. It sounds like your amp lets you set DC bias, which is related to emission. Transconductance is a separate characteristic.
Ideally the two tubes in a channel should either be matched for transconductance or the amp should have an AC balance control.
Some or all Scott amps allow both DC and AC bias. The downside is that setting bias is a pain in the ass, as the settings interact, so you approach proper bias as an iterative limit. The upside is that any healthy tubes should work well.
It sounds like the tubes in you amp should be transconductance matched. However, your amp probably has enough negative feedback to make even reasonably unmatched tubes work well.
For the reasons alluded to, it's important to know what needs matched in any amp. Tubes sold as matched often means little or nothing. You have to ask exactly what's matched. Most reputable dealers use transconductance testers, so buying a matched pair for your amp should be ok, and don't worry if they don't match well for emission.
Thanks Alan. When it comes time to replace can I replace just the one tube (the amp has bias adjusters for each tube) or should it be in matched pairs?
with the other one in the channel to verify that it's the tube (not the pot or something else).
When you can individually bias one tube you are fine with replacing just the one tube. I would do matched pairs if you got a pricing deal by buying a pair vs just one tube but replacing one tube is fine
Alan
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