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Posted by Jon Bartow (Member # 241) on 01-24-2002, 12:36 PM:
Someone told me how to check/set the bias on a power amp a long time ago and I'll be darned if I can remember how to correctly. Would someone be kind enough to refresh my memory?
Thanks
Jonathan
Posted by Paul G. Thompson (Member # 655) on 01-24-2002, 01:09 PM:
Jon, more information is needed. Are you asking about power amp tube bias or pre-amp tube bias? I would presume you are asking about power amp bias, so here comes my suggestion:
Normally, once the bias is set properly, there should be no real need to fool with that adjustment. For example, the Ballentyne T-25 amplifiers have the bias pot "sealed" with glyptol.(sp)
Does the amplifier sound funky? If it is not broke, don't fix it. The need for re-adjusting the bias would probably suggest something else went wrong, and that is the problem that should be addressed.
What type of tube baising are you using? Class "A", "AB", "AB-1", "AB-2", or Class "B"? Fixed bias or cathode bias? What type of output tubes are you using? Better yet, what type of amplifier is it?
Some amplifiers have a bias adjustment pot. If you are not totally sure about what bias is or does, I would recommend it be left alone. Improper bias could severly damage the tubes, as well as causing distortion in the audio.
If you provide us with the type of amplifier, and where we can see a schematic, there are several of us that can walk you through to get the proper setting. However, you will need a meter.
Posted by Greg Mueller (Member # 17) on 01-24-2002, 01:39 PM:
On Dynaco Stereo 70s It's 1.56 vdc off of pin 8 on the octal socket on the front. And that's what I know about bias 
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Greg Mueller
Amateur Astronomer, Machinist, Filmnut
http://www.muellersatomics.com/
Posted by Bob Koch (Member # 1062) on 01-24-2002, 02:19 PM:
From what I remember,don`t hold me to it, RCA and Simplex amps with cathode bias usually were 19-22 volts; Altec amps with EL 34`s were about the same ,I think 1569-68 series Altecs were adjustable. the The Ampex 120 watt amplifier with 4 807`s were adjustable I forget the value but you could tell if you were close by the plate current meter readings.Frazer 100 watt amps with the tv sweep tubes[6gt5] were adjustable and were about 35 volts,if I remember.The Altec 1570B with the 2 811`s as finals was 35volts at the midpoint of the 2 100k resistors to each grid.
Only to a degree do I agree with the "don`t fool with it'; I used to check these,without fail,every service call.They are a very important indicator of amplifier performance. If you don`t know a capacitor from a resistor, then, you`d better stay away, as you`ve been advised.
Posted by Jon Bartow (Member # 241) on 01-24-2002, 02:48 PM:
Actually I'm more interested in non-tube amps. (I.e. "solid state" or "mosfet" etc.) I remember it involved hooking up an o-scope and a signal generator but I don't remember specifics.
(Case in point: I have several repaired amplifier modules that have come back from the factory (Ashly FTX-2001 
) and I would like to verify that the amplifiers are all working within specs.)
Jonathan
Posted by Mark Gulbrandsen (Member # 72) on 01-24-2002, 04:30 PM:
It depends on the actual amp design. Some are biased higher than others are to get the lowest distortion while with some amps too high of bias will actually increase the distortion level and make it sound worse, or worse than that make it thermally unstable too. The only thing to do here is to call the manufacturer and get the proper procedure for doing this. There is no other safe way.
Verifying any amps specs is easy with the right equipment. Large heatsinked load resistors, a good distortion analyzer(both I.M, and Harmonic), and a true RMS voltmeter are what is needed. You should try to get a hold of a Sound Technology distortion analyzer as it as a residual of way less than .001% THD at all frequencies. The typical H-P surplus harmonic distortion analyzer is ok but only goes down to .01% THD. A Watt meter is also handy so you can verify line current draw at idle.
Greg, The Dynaco's have the finest audio output transformers ever made! No joke here check out this web site. http://www.curcioaudio.com/st7des_3.htm
Mark @ GTS
Posted by Paul G. Thompson (Member # 655) on 01-24-2002, 04:36 PM:
Jon, I agree with Mark.
Posted by Don Sneed (Member # 955) on 01-24-2002, 08:36 PM:
I serviced a drive-in theatre in Houston a few years ago, the 6L6 output tubes on Altec amplifers, the one set of the amp's tubes was glowing bright red been like this at lease 3 hours, sound distorted, no spare tubes, I checked the bias voltage & it was in the postive voltage range, no manule on site to see what the bias voltage was working at, I adjusted the pot until the sound was normal & the plates were glowing normal now, believe it or not no damage to the tube, I think I measure a -15 to -20 vdc I think the working voltage was around there somewhere...the amp worked great for a week, I went back to replaced the tubes & set the bias at the correct working voltage....
Posted by Ray Derrick (Member # 1001) on 01-24-2002, 08:53 PM:
The bias control usually affects both crossover distortion and quiescent current. Obviously the best way is to follow the manufacturer's recommendations because they do vary. However, if you cannot lay your hands on the alignment data for the model that you are working on, you can use the following generic method which works for most amplifiers:
1. Connect a 15-20amp ammeter in series with the amplifier's power lead.
2. Connect a suitable 8ohm dummy load, distortion analyser and one channel of a dual trace CRO to the output of the amp.
3. Note the current on the ammeter with no signal (it will usually be less than 1 amp).
3. If the distortion anaylser has a CRT display skip the next step.
4. Connect the second probe of the CRO to the distortion analyser output.
5. Apply a 1kHz sine wave and set the level for about 90% of maximum output (about 10% below clipping).
6. Observe the Analyser's CRT (or the second channel of the CRO) and adjust the bias control for minimum zero-crossing spikes on the distortion display. This should also correspond to a minimum distortion reading.
7. Remove the 1kHz signal and again note the reading on the ammeter. It should be the same or less than before. If it is higher, re-adjust the bias slightly until it is equal to the reading you took at the beginning. If it takes more than a "slight" re-adjustment, there may be a problem with the amplifier.
------------------
Ray Derrick
President/Chief Engineer
Panalogic Corporation Sydney, Australia
Phone: 61 (0)2 9894 6655 Fax: 61 (0)2 9894 6935
Posted by Paul G. Thompson (Member # 655) on 01-24-2002, 09:50 PM:
Don, it sounds like someone was using the "knobgetwitzen" technique if the bias voltage was positive on the grids. Although you were able to save the day, I'll bet those tubes were pooped out.
Posted by Paul G. Thompson (Member # 655) on 01-25-2002, 12:55 AM:
For the Ballentyne T-25 dudes......bias is -39.8 volts (fixed) 380V on the plates, and 365 on the screens. Looks like almost Class "B" bias to me....what do you think? If I read the transfer curves correctly, (damn out-dated tri-focals and a projector lense for a magnifying glass) it looks like 10ma of plate idle current. Seems to me with a high-power bottle, that is going to be as close to "B" as you could expect.
Posted by Mark Gulbrandsen (Member # 72) on 01-25-2002, 02:51 AM:
Paul,
I think you are right about it being Class B. They probably did that to give the finals longer life. Most amps I've worked on with 6L6's use 30 to 40 ma per tube.
Mark @ HYS