Author Topic: Bias problems VTA-70 amp ?????  (Read 1708 times)

Offline OCCD

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Bias problems VTA-70 amp ?????
« on: December 17, 2009, 04:16:27 PM »
My space heater has me concerned.  When I first got it, the bias was way off. I want online and found out how to properly adjust the bias.After doing that,the sound is fantastic, however, for the last two weeks I have been constantly tweaking the bias on both channels. Is this normal or is there a cap problem?  The tubes are new. :o
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Offline MasterBlaster

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Re: Bias problems VTA-70 amp ?????
« Reply #1 on: December 18, 2009, 07:53:31 PM »
I did a bit of sleuthing on this. (Damn I had a hell of a time getting a reasonable search term.)
Then I zeroed in on Bias "Wander" and I found this little nugget on the Music Man Amps Forum

Hope this helps...

Quote
Bias is measured with no input signal and it should be relatively stabilized before you accept the results. If there is a constant "creep" upwards, the amp would sooner or later eat the output tubes for lunch! It has to stop somewhere.

It is moving either because the tubes are still warming up (although probably not in your case), you just made an adjustment and the circuit hasn't stabilized, or you have a wandering bias condition.

This last condition (wandering) can be due to component aging and worn out output tubes. Just keep in mind that adjusting the bias isn't instantaneous with a tube.

If you change the bias current flowing through the tube, the tube reacts by getting hotter or cooler (depending on the direction of change) in response, which then causes the circuit to draw a little more or less bias current in response to the tubes temperature change... back and forth until the circuit reaches a point of equilibrium and it stays put.

Where it stabilizes is the true bias setting.

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Offline OCCD

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Re: Bias problems VTA-70 amp ?????
« Reply #2 on: December 18, 2009, 07:57:47 PM »
Very helpful MB. 8)
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Offline F1nut

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Re: Bias problems VTA-70 amp ?????
« Reply #3 on: December 19, 2009, 03:28:57 AM »
Other than the tubes being whacked, it's also possible that the bias resistors are going or have gone south. However, the later will result in not being able to set the bias at all.

Generally, one should set the bias right away after installing new power tubes, again after 5 to 10 minutes, again an hour later and once again after 24 hours. If the bias is still drifting after 24 hours, you've got a problem.

It's always a good idea to check the bias at least once a month.
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Offline OCCD

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Re: Bias problems VTA-70 amp ?????
« Reply #4 on: December 19, 2009, 12:58:52 PM »
Other than the tubes being whacked, it's also possible that the bias resistors are going or have gone south. However, the later will result in not being able to set the bias at all.

Generally, one should set the bias right away after installing new power tubes, again after 5 to 10 minutes, again an hour later and once again after 24 hours. If the bias is still drifting after 24 hours, you've got a problem.

It's always a good idea to check the bias at least once a month.

The tubes checked perfect.    I am not chasing it a whole bunch right now. Thanks for the info Jesse. ;D
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