Biasing

[rickdagless]

So I just finished up the Mainline build (Woohoo!) and all of the resistances and voltages checked out.  (Quick question: for the fourth voltage reading, is it meant to say "IA on the C side C4S board"?)

I'm adjusting the potentiometers and I'm very fascinated with getting each of the readings to be 145VDC on the dot, so I'm letting the Mainline run and "burn in" as the voltage seems to reach its lower limit.  While I'm sure I'm close enough at this point, I just can't help myself -- is there a reason I shouldn't be doing this?

I haven't been able to listen to it yet as I'm waiting for some connectors in the mail, so till then I guess I'm keeping myself occupied with this fine tuning.

[Paul Birkeland]

I let Josh know about that B/C labeling, but yes, that is the "C" side board.

Technically, the Mainline will make the most power if the plate voltage is dialed to exactly 145V. 

The tubes are going to wander a bit, if you let the amp run for a few minutes, then set to 145V, you should be solid.  Don't be too surprised if you see it wander to 143V or 147V the next time you measure it.

-PB

[rickdagless]

Cool, thanks PB!  Is it something that'll eventually settle after burning in or will it always kind of wander a bit?

[Paul Birkeland]

It's always going to wander just a little bit. 

[Strikkflypilot]

May I ask if one really does this biasing while the amplifier is live?
What do You use, wood chop sticks?

[Paul Birkeland]

Yes, the biasing is done with the amplifier on. 

The trim pot to be adjusted is a little blue brick of plastic, which isn't conductive.  It is adjusted with a small screw driver.

If you have two clip leads, you can secure the probes of your multimeter to the biasing points with the amplifier off, then turn it on and set the bias one channel at a time.

[Strikkflypilot]

Thanks, good to know.

[NightFlight]

I found my one tube dropped about 15v (cold) after the first 100hrs when I re-checked bias. I also noticed the voltage drops slowly (~3-5v) for quite a few minutes until everything comes up to temperature and eventually settles.  Seems to take up to ~20 minutes for me.  I'll bet total warm-up depends on many factors.