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Tube amplifier bias current question...

My amp is lovely, older Audio Research VT-60 - running a pair of KT88’s for each channel - into ProAcs.

About 8 months ago I had a tech look over the amp (it checked out fine) and he installed new input/driver tubes as well as a new set of Gold Lion reissue KT88s (2 matched pair). I ran an 6550’s before that...

Amp is working great, sounding great.

Today, partly out of boredom given the CV shutdown, I decided to check the power tube bias current/voltage. (The VT-60 has user adjustable bias using a multimeter across a bias resistor, with target bias voltage @ 65 mv DC across the resistor). Amp was properly warmed up and connected to speaker load. What I saw was one channel (L) the voltage had crept up to 70 mv, and other (R) was 82 mv. (I of course trimmed the voltage back down to the target 65 mv number.)

1. It is normal for bias voltages to creep up as power tubes age? i.e. the power tubes suck more bias current as they operate over time, all else equal?

2. Should I be alarmed that the creep up in bias current through one pair of the tubes is so much higher than the other? The amp has been running like this for months, are the tubes on the higher current side likely to burn out much faster?

Thanks in advance for any thoughts.




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lowrider57's avatar
lowrider57
5,417 posts
 
Typically bias should remain relatively steady. It will fluctuate due changes to line voltage. ie, listening daytime vs. late at night.
 Since your amp was serviced, I think there would be some drift due to break-in of the power tubes, resistors, and whatever was replaced.

To address tuberist's answer, I have an old Push-Pull amp that has drift in the power tube bias. All are within range from day to day except for one tube which must be manually biased.

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atmasphere's avatar
atmasphere
11,807 posts
 
I suspect that the bias was not tested after the tubes had sufficient warm-up time when installed. I usually set the bias low initially and then set it again after an hour of operation.


If the power supply isn't regulated (and in this case I mean the power supply to the output section, which includes the filament circuit) if the AC line voltage is higher, the bias will read higher too.


So you have two explanations of the higher bias. At any rate I would not be alarmed.


As tubes age the bias level tends to drop rather than increase.
oddiofyl's avatar
oddiofyl
3,836 posts
 
I agree ,  I always watch it for a few minutes setting it just below target. Let it warm up for 15 min while always keeping an eye on it for a run away tube , adj , then check again .   Then check an hour later. ... I'm not OCD or anything....
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