Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Fresh_Start on October 18, 2010, 10:49:32 pm
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How long does it normally take for the power tubes bias to settle to a steady idle level?
Working on the old Princeton Reverb & trying some new Tung Sol 6V6 power tubes. I'd never kept a bias checker on while playing an amp before. Also had plate voltage on a separate, analog meter.
The idle cathode current kept going up over at least 10 minutes of noodling on the guitar. Kept adjusting the bias voltage upward to keep idle cathode current at about 25 ma (plates @ 440 VDC). Yes, I know that's on the hot side. It's where the old tubes were biased before I touched anything in the amp. Anyway, the idle current did seem to stabilize after a while but I don't think it ever took this long before.
Turned off the amp, let the tubes cool a bit, then switched my bias checker to the other 6V6. On power up, the idle cathode current was only 20 ma... what's up with that? Thought the "matched pair" wasn't until I switched quickly over to the other 6V6. That was only 21 ma at first, but it appeared to continue climbing very slowly.
BTW with the volume at 6-7, it was fun to watch the plate voltage sag on my analog FET meter while playing loud chords. Dropped down almost to 300 VDC at times. I had no idea the voltage dropped that much. Oh, and the tone with the Tung Sols is very sweet to my ears. I think that the old power tubes are on their last legs - haven't been replaced in well over 10 years and the owner plays this amp a lot (although mostly clean).
One other side note, turning Speed to "10" and Intensity as low as possible helps get useful bias readings for an amp with bias-vary tremolo. Even better is shorting out the tremolo circuit the same way the footswitch does.
Sorry for rambling,
Chip
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How long does it normally take for the power tubes bias to settle to a steady idle level?
How much use/burn-in have these tubes had?
All tubes drift. And according to some books I've read (I'm thinking "Getting the Most Out of Vacuum Tubes" by Tomer), all tubes tend to have the most change in their characteristics early in their life span. Things should settle down after the first hundred hours or so and become more consistent.
But tubes can and do drift up and down somewhat just about always.
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I certainly find I have to keep adjusting the bias on my fixed-bias amps every few months. As you note Chip, you can tell on the BFPR (and other fixed amps with bias-vary trem) when the trem starts to lose some of its mojo.