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Is it normal for bias to drift as an amp warms up??

4/15/2003 3:39 AM
JR Is it normal for bias to drift as an amp warms up??
Hey guys,  
The bias on my amp seems to be drifting - measuring across the 1 ohm resistors, the bias is around 32ma when I first turn it on (warming up in standby for a couple of minutes first) but after 5-10 minutes, it drifts up to around 38-40ma. Is this normal? If not, what should I check?? The amp is a 100 watter w/EL34s.  
4/15/2003 3:58 AM
Actually, now its up around 48ma. Yikes!!
4/15/2003 6:31 AM
Yes this is normal. The bias should be noted only after it reaches its final value which comes after things warm up. Watch out if it keeps getting higher. There may be a problem.  
One problem may be overheating of something. Components conduct better as they get warm. But if a component(s) gets heated up continuously, it may cause things to go overboard. A good layout helps keep things cool.  
Others will have more specific answers, I recall reading about this somewhere, so it is common knowledge.  
4/15/2003 7:10 AM

Perfectly normal. Keep an eye on it as it warms up or leave the bias really cold until ready. AMps always should be warmed up well before adjusting.
4/15/2003 3:19 PM
Thanks for the replys everyone. The amp is an Ampeg VL1002. I have another one of these heads which doesn't do this which is why I suspect something. None of my Marshall's or homebrews drift this much either. 20mA just seems excessive. What would be some obvious components to check? How bout the bias range resistor? Someone (I don't know if it was factory) soldered another resistor in parallel to the stock one. They both look like 1/2 watt though one may be 1/4.  
The problem was that I let it warm up for a few minutes then started playing and it sounded totally crappy - muddy and harsh. I checked the bias and it was only at 21mA so I increased it to 35mA and it sounded great. But then I watched it over the span of about 20 minutes and it kept going up!  
Anyway, your feedback is greatly appreciated!  
4/15/2003 6:01 PM

JR, I'm sorry to disagree but I think something is not right. There's a reason your other amps don't do this. They are working properly. We have threads now and again on this type of problem. Start with the obvious. Bias caps, resistors, etc. I remember someone saying something about leaking coupling capacitors causing something like this. I'm no expert, but I really think something is wrong and *really* needs to be set right before you lose a set of power tubes or worse. JM2C.  
I used to check my Allen Old Flame all the time since it was so easy to do so. After warming up for just a few minutes it would never drift more than 1mA at idle no matter how long it was played.  
4/15/2003 9:34 PM
Bruce /Mission Amps

I agree with John... something is broken or worn out.  
If the tubes are drawing 20ma in the first 3 or 4 mins, it should not draw very much more then that after 20 mins to an hour or more. Climbing higher tells me something wrong.  
I think you have a set of soft power tubes, your bias supply filter cap(s) is going bad or a bias setting resistor is getting over heated... etc.  

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