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Topic: Setting Bias
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Paul G. Thompson
The Weenie Man
Posts: 4718
From: Mount Vernon WA USA
Registered: Nov 2000
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posted 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.
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Don Sneed
Master Film Handler
Posts: 451
From: Texas City, TX, USA
Registered: Aug 2001
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posted 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....
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Ray Derrick
Master Film Handler
Posts: 310
From: Sydney, Australia
Registered: Sep 2001
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posted 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
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) and I would like to verify that the amplifiers are all working within specs.)
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