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Vintage threads from the first ten years

6L6 bias adj.

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5/11/1997 12:59 PM
M/J	6L6 bias adj. I installed a 1 ohm resistor at the cathode to adjust the bias in my bassman. Books I have read call for 30 to 35mv but amp techs tell me 25 or 28. Some say use your ears. Well my hearing is shot. Any input would be greatly appreciated. I'm just learning this trade and want to keep doing it. Long live old tube amps. Thanks...M/J

5/12/1997 12:45 AM
R.G. Keen	25 to 28 ma strikes me as being awfully cold. I have some excellent recommendations from John Stokes, who has a LOT of experience at biasing listed in my Tube Amp FAQ, over on http://www.eden.com/~keen, as well as a good deal more info on biasing. Give it a read.

5/12/1997 12:32 PM
Zoltan	I usually bias 6L6's in my amp, which has a plate voltage of 430v,at 25 to 30ma per tube.In my experience you can go as low as 20ma in a typical bassman.If you can't hear the difference between 20ma and 35ma, go with 20ma,the amp will run cooler.

5/29/1997 9:42 PM
Rusty Bretz	After reading every opinion I can find on the web, the bias should be set to where it sounds best, yet runs the coolest possible. Somewhere between cross-over notch and glowing red-hot plates .

5/30/1997 4:30 PM

For typical fixed bias class AB Fender amps running at 430-460 plate volts, I really prefer the warmth and response you get when running at around 35ma per 6L6/5881. I agree with Mr. Keen that 20-25ma sounds cold and sterile. My guess is that a lot of techs bias them in the 20-30ma range is because either they're not serious guitar players themselves and/or they want some extra insurance that the amp is not going to wind up back on the bench a few days later because of a blown power tube (especially if they installed Chinese 6L6's).

If a tube is operating at 450 plate volts and is drawing 35ma current, then the plate dissipation is less than 16 watts. This is well within the max. rated plate dissipation rating (30W for 6L6GC and 23W for 5881).

5/31/1997 7:27 PM

I bought a book by Torres. I found it to be good reading. He had a scale on plate voltages and the correlation between those voltages and the ma of current drawn at the cathode. My schematic called for 425v at the plates but when I measured the voltage it came up 415. Maybe one of you can explain the voltage being out of spec. Anyway Torres called for 28.8 to 34.3 low to high AB class respectively. In the book he recommended that you bias to the cooler spec for rock music and towards the higher end for cleaner tones. I biased at 28.8 and the amp sounds great. I went from the low range to the high range and could not detect much of a difference in the sound. I should also say that my hearing is not the best. The amp runs cooler at the lower setting and breaks over good when cranked up. Jim...you state at the bottom of your thread that the 6L6 and the 5881 put out different watts. Is there any difference in the tone? If so... what? I'm running a matched set of RCA black plate 6L6's in it right now with Telefunken preamp tubes. I did change the Driver tube from a 12at7 to a 12az7 RCA and the second stage preamp from a 12ax7 to a 12ay7 GE. The telefunken seems to push that GE real good and the RCA az driver is not as brittle as the AT. It makes the amp have a warm overdriven sound. I appreciate all the help you guys have been giving me. I'm really having fun messing with this thing. I also want to thank you guys for showing me that the guitar and effects are only half of the equation. M/J

6/2/1997 12:26 AM

Although my personal preference is to bias 6L6's at 35ma, I can see why some players would like the sound of them biased closer to 30ma (slightly cleaner and crisper tone). It's hard for me to see how anyone could like how they sound biased under 25ma however!

It's hard to answer how 5881's sound different than 6L6's because there are differences just within a particular type. The NOS Phillips/Sylvania STR 6L6GC sounds very different from the RCA and GE 6L6GC (it's much harder and cleaner). Also, the NOS Tung-SOL 5881 is a different-sounding tube from the Phillips 5881 (which is really just a small-bottle version ot their 6L6GC STR tube - the plate structure looks identical) and from the popular (and reliable) Russian 5881/6L6WGB.

Those RCA black plates you have are worth big bucks! I hope they last you a good long time.

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