Question about tube bias

seanrr

Well-Known Member
I recently got my first amps that have adjustable bias. They are Antique Sound Labs 1006-845 monoblocks that have had a bunch of mods done to them by previous owners. I have attached some photos, including one of the guts to show the cap replacements, etc. It has also been modded to be driven by 12SX7 tubes rather than 6NS7 tubes. Thus far, they sound very nice indeed, but I do have a question about bias. These have manual bias with digital readouts. As I understand it, the amps should play at about 800 on the meters. While I have them set close to that, the bias seems to bounce around a bit, sometimes going down to 789 or up to 810. Is that normal? Anything else I should know about the importance of the bias, etc.? I am not an engineer - so, bias for dummies would be preferable, and it is even better if there is anybody else with these amps who can tell me about their experience with the bias controls.

 
Bias wandering up and down some % is normal, caused by mains voltage fluctuation, tube temp etc.
Whatever you should do now is talk to previous owner and demand a schematics on the changes made.
 
Bias wandering up and down some % is normal, caused by mains voltage fluctuation, tube temp etc.
Whatever you should do now is talk to previous owner and demand a schematics on the changes made.

Thanks - what percentage is normal? These amps wander about 2%. I will see what schematics I can get - I think the switch from 6SN7 to 12SX7 was merely a change in plate voltage. Other than that, I think other mods have only involved replacing original caps with high-end ones - mostly Jensen - both PIO and electrolytic.
 
Thanks - what percentage is normal? These amps wander about 2%. I will see what schematics I can get - I think the switch from 6SN7 to 12SX7 was merely a change in plate voltage. Other than that, I think other mods have only involved replacing original caps with high-end ones - mostly Jensen - both PIO and electrolytic.

Well, it's hard to say. If the PS isn't fully regulated it'll likely fluctuate about the same percentage as your mains voltage.

Where I live the mains are pretty stable, typically +/- 2% or so.

USA mains nominal voltage is 120V. What is called the service voltage (that delivered to your house) is supposed to be 120V +/-5%. Note that is delivered to your house. That does not necessarily mean what you measure at a receptacle. At the receptacle would be considered utilization voltage and has a wider tolerance under ANSI. 120V -13% + 6%.

Now, whether all, many, most, or some utility companies adhere to ANSI is another matter. :)
 
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As I understand it, the amps should play at about 800 on the meters. While I have them set close to that, the bias seems to bounce around a bit, sometimes going down to 789 or up to 810. Is that normal?
Are you sure your amp is supposed to Play at 800, or is the bias set to 800 while the amp is idling?

Your amp may be different, but it's my understanding that bias is set while an amp is idling, which means connected to a load (speakers) with no music playing. Also known as setting the quiescent bias.

The operating point of a device, also known as bias point, quiescent point, or Q-point, is the steady-state (DC) voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied.

Once the amp starts to play, the operating conditions will change and the bias you measure will fluctuate.

It's not the line voltage that's fluctuating, it's the power (as measured by current drawn) the tubes are dissipating while playing.
 
It's not the line voltage that's fluctuating, it's the power (as measured by current drawn) the tubes are dissipating while playing.

Yeah, if the bias is being checked while playing music I agree. But, variations in line voltage can cause the bias readings to vary as well. Here is mainly I think the point whether it's jumping around, probably playing music, vs. it varies up or down a bit over some amount of time, probably line variations.
 
I've never had an amp that didn't wander at least a little bit when checking bias. If you use an analog meter you'll never see it, but digitals will dance slightly. Nothing to worry about.
 
I've never had an amp that didn't wander at least a little bit when checking bias. If you use an analog meter you'll never see it, but digitals will dance slightly. Nothing to worry about.
Thanks - that was the answer I was looking for....;-)
 
As mentioned - adjustments should be made when there's no input signal. Check the bias a couple minutes after turning it on just to make sure you're in the ballpark, but plan to fine tune the amp after a session when it's been working for a while and is completely warm and stabilized.
 
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