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BIAS PROBE SETTINGS


 
10/12/1998 6:58 PM
MARK C.
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BIAS PROBE SETTINGS
I AM LOOKING FOR BIAS PROBE READINGS FOR BLACKFACE FENDER AMPS, VIBROLUX, SUPER REVERB ,BASSMAN ETC.
 
10/12/1998 8:28 PM
TMiller
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35 MILLIAMPS.
 
10/13/1998 9:09 AM
Ken Powers
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I have read that the setting should be 32mA to 37mA for 6l6's. What confuses me is that I have seen a table that lists the following:  
 
Amp Model            Tube/#     Nominal   Max
 
----------------------------------------------  
Bandmaster AD1269 6L6/2 25.3 30.3  
Dual Showman AC568 6L6/4 26.9 32.3  
Deluxe 6G3 6V6/2 15.1 18.1  
Pro Reverb AA270 6L6/2 31.5 37.8  
Super Reverb AA763 6L6/2 27.2 32.6  
Vibrolux Rev AB568 6L6/2 29.9 35.9
 
 
I have also seen a chart that shows that my Twin Reverb should be running at 25. I would also like to see some clarification of this data. I have the amp set at 33 and it sounds great.
 
10/13/1998 11:05 AM
Bruce
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Here's my view Ken....  
Most tube amps sound great with the bias current a little closer to Class A then dead center Class AB1.  
IMO...A new 6L6 rated at 30 watts can be biased  
at idle to around 60% to 65% of it's full power rating for a nice tubey sound and not hurt anything.  
Without knowing your plate voltage under load, I still think you are OK at 33ma anyhow in normal Fender amp. 25ma still sounds OK but a bit cool.  
I have many customers who can hear a 5ma diff!  
The BF Twin I'm sending out today has a quartet of Svetlana 6L6s in it now and I set them at 37-38ma each side (OT shunt) and 442v on the plates.  
Using the 1 ohm cathode resistor method the meter would probably read 3-5ma higer then that.  
A little less curent then I normally would do but the player gigs and likes the amp loud, tubey and still clean at moderate voluems so I'll leave it there for now and see how he likes it for a week.  
Those charts you are quoting are probably based on making sure you don't over do it and shorten the tube life. Bad for business if you advised the current level and it takes the tubes out in a month!  
 
Sine the amp is part of the instrumentation, go for what is safe in tube ratings but still sounds the best to you. Treat the tubes as you would anything that wears out.  
If you treat them hard but have fun, then OK!  
 
Bruce  
Mission Amps
 
10/13/1998 12:00 PM
Richie
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I would agree with Bruce,another thing is not only plate voltage,but type or brand of tube you use,some are alot better than others.I have got a few marshall in that were running at 40,[5881s].This bias probe method is at idle,so when you play the amp, it will make a difference,if the tube can handle it or not.I think everyone has there own way they like best to bias,or setting of the tube.I was curious of these charts being for a certian tube brand, or all brands? I think it also depends on the amp and circut,[setting bias] to keep the tubes from overheating.I think this is another factor in amp companies,in how they set the bias,The bias question sure gets alot of different answers,But if its too hot or too cold,I think they sound terrible! [Richie]
 
10/13/1998 12:10 PM
Ken Powers
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The charts that I quoted are from the book: The Complete Guide to Guitar and Amp Maintenance from Hal Leonard publishing. I thought that the charts seemed to be on the conservative side from the information that I have gathered from various web sites. The values do not appear to be for specific tube brands. The second chart in the book seems to be based on amplifier wattage and plate voltage (the one that reads around 25mA for my '65 Fender Twin Reverb. I know that this value seems very conservative. I have tried several different values just for the sake of experimentation (25mA, 30mA, 33mA, 35mA, 40mA, and 47mA). I seems to like the values between 35mA and 40mA the best. I was afraid to raise it to high for fear of having output tube problems. Thanks for all of the good, fast responses!
 
10/13/1998 12:31 PM
Richie
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Ken: I think you will see alot of us agree on 35ma as the best setting,[6L6s]I have seen some amps at 40[marshalls with 5881s]But I would never go above 40!!.[Richie]
 
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