This is a generic bias video and can be used for reference when biasing almost any adjustable bias amp which uses a single bias pot to adjust a pair, a quad or a sextet of power tubes. At the bottom of the page is a list of a few amps and the plate voltage they have on average. You can use the listed plate voltage or you can measure it as described below.
This video is of a 50 watt Marshall 800 and as the video will show, quite a few of these amps have a lower plate voltage which can sometimes be as low as 360 volts DC. This particular amp came in at about 375 when the bias was set at 70%. I have run across a lot of 800's that ran up to 470 plate volts so if you do not know what voltage your amp has you should measure it with your old tubes installed first. This procedure is described in the last segment of the video showing you how to measure the DC plate volts from pin #3 of any octal power tube socket to ground. The pic below shows a close up of a socket from the bottom side with a tube installed so you can easily identify it. Just make a ground contact with the black lead of your meter anywhere on the chassis and touch off on pin #3 with your red lead with the multimeter set to DC Volts in a 1000 volt scale.
Most Marshall Mark II's and 900's will be between 460 to 480 plate volts and the older Super Lead and Super Bass Marshall's sometimes run as high as 530 plate volts.
So if you're biasing an 800 or a Super Lead, Super Bass or Marshall Major then I highly recommend measuring the plate voltage. If you're biasing a Mark II or 900 then you can use 470 plate volts or you can measure it.
The formula for calculating the bias is the dissipation of the tube divided by the plate voltage which gives you 100% and then you can bias anywhere between 65% up to 85% of this. Most amps will just start to come out of crossover distortion at 65% and so we recommend using 70%. You can of course go up to 85% but you will wear out your tubes quicker and brown out the tone a little.
For example 25 watts dissipation can be used for just about any new production EL34, 6L6, KT77 or KT66 tube and most amps on average will have about 470 plate volts (see the list below). So 25 watts divided by 470 plate volts equals .053 or 53 milliamps. So .052 X .7 = .037 or 37mA.
Just to illustrate how little difference your plate voltage makes lets use 450 volts. So 25 watts divided by 450 plate volts equals .055 or 55 milliamps. So .055 X .7 = .0385 or 38.5mA. You can see that 20 volts only makes a difference of 1.5mA in where you would set the bias.
For KT88's and 6550's use 35 watts dissipation.
Some trim pots sit perpendicular to the main board as shown in the video and are adjusted from the side where others are flat and adjusted from the top. Here are a few examples.
The list below is the average plate voltage that we have found in amps that we have biased. This will of course vary depending on your line voltage so once again here the word is average plate voltage!
Marshall 900's and Mark II's. 470pv - 36 to 42mA.
Ampeg Reissue Reverb O Rocket's and Ampeg J12's. 400pv - 42 to 50mA.
Ampeg VL Series. 460pv - 38 to 44mA.
B52 Amps in A/B Solid state rectifier mode. 460pv - 38 to 44mA. Fixed bias in Class A mode.
Bogner Shiva. 495pv - 34 to 40mA.
Bogner Ecstasy. 510pv - 33 to 38mA.
Bogner Ubershall. 520pv - 32 to 38mA.
Carvin X Amps, Legacy, MTS and V3. 460pv - 38 to 44mA.
Crate VFX. 400pv - 39 to 46mA.
Diezel Herbert, Einstein and VH4. 460pv - 38 to 44mA.
Engl Amps including the Screamer, Powerball, Fireball and Thunder all average about 460pv - 38 to 44mA.
Fender Bassman, Bandmaster, Super Reverb, Vibroverb, Showman and most new Fenders average 460pv - 38 to 44mA.
Fender Silverface amps made between 1969 to 1972 can sometimes come in at 500 to 530pv - 32 to 40mA.
GenzBenz El Diablo. 460pv - 38 to 44mA.
Gibson Goldtone 60. 475pv - 36 to 42mA.
GT Soul-O Series amps average about 480pv - 36 to 42mA
Hiwatt. We have seen older Hiwatt amps come in as high as 530pv so we recommend measuring these!
Hughes and Kettner "Tube" series, Duotone, Switchblades and Triamp all average about 460pv - 38 to 44mA.
Koch Classic, Twin Tone, Multitone and Powertone all average about 460pv - 38 to 44mA.
Krank Revolution averages 475pv - 36 to 42mA.
Krank Krankinstein averages 535pv - 32 to 37mA.
Laney Amps including the AOR, GH and VH series all average about 460pv - 38 to 44mA.
Musicman amps in the high power mode will hit 700pv - 25 to 28mA.
Orange, vintage Orange amps cab be up to 490pv - 34 to 40mA.
Orange Rockerverb 50 is about 400pv and uses 6V6's (14 watts dissipation) which we bias between 24 to 28mA.
Orange Rockerverb 100 is about 450pv - 36 to 42mA.
Peavey 5150 (with bias mod), 5150 II, XXX and JSX. 480pv - 36 to 42mA
Reverend King Snake. 460pv - 38 to 44mA.
Rivera M series, R series, Clubster's, Chubster's, Pubster's, Knuckle Head's, Fandango's and Quiana all average about 440pv - 38 to 44mA.
Soldano Hotrod series, Decatone's, Lucky 13's and Slo's all average about 460pv - 38 to 44mA.
Sovtek Mig 50's, 60's and 100's average about 480pv - 36 to 42mA
SUNN Solarus, 100S, 200S, 1000S, 2000S and model T's using KT88's (35 watts dissipation) all average about 510pv - 48 to 54mA.
Traynor, older adjustable bias Traynor amps average about 475pv - 36 to 42mA.
VHT Classic, Pittbull and CL average about 480pv - 36 to 42mA
VOX, vintage AC50's and 100's average about 460pv - 38 to 44mA.