Tube Bias Database

[CJ]

people ask about where to set the bias on their power tubes all the time,

me too!

some of the new tube companies like JJ and Electro-Harmonix seem to prefer different bias points than their early NOS versions,

so if you find a setting you like for a certain tube number and brand, post it here.

this is a site that has info on bias current for different tubes based on plate voltage>

http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm

just set a Marshall Super Lead 100  equipped with JJ 6550 tubes to -52 volts, this comes out to about 120 ma for all 4 tubes,  160 ma sounds a bit better but will probably cut down on tube life,  so you try and find a balance between tone ad tube life,

here is a site with more info and a bias calculator>

http://www.webervst.com/tubes1/calcbias.htm

 

[CJ]

just set the bias on a Music Man 210HD-130

quad set of JJ 6CA7 (EL34)

705 volts on the plates - only needs -30 volts on the grids, they run the screen voltage way under the plate voltage so that the tubes do not melt,

these tubes are pretty sensitive to bias, only takes about -34 volts to starve them down to about 10 ma,

 

[CJ]

here is a chart showing the plate current for a push pull guitar amp using 4 6L6GC tubes,

getting some red plates at  120 ma idle current which is what Egnart wants you to use,  so we are dropping the bias to save the tubes, Fender use to run a low idle current, which gives you more punch and less compression,

edit-dropping the bias did not work, we get red plate at 100 watts, so we adjusting the screen current and now we can bring the bias down to -52 and still crank full shred,

 

[CJ]

here is a chart showing how the screen controls the plate in a pentode,

does a good job, i bet some tricky hamster dude has figured out a way to modulate the tube with the screen instead of the grid,

quad 6L6GC push-pull

this Egnart output trans does not have the Henries to hold back AC plate current, so you set the bias and idle the tubes around 25 ma each, no problem until you crank up the guitar and the AC current turns the plates red,

why rewind the transformer when we can simply use a screen dropping resistor of about 5 or 10 watts to limit screens current and thus plate current, so we bias the tubes at -52 V-dc which lets about 90-100 ma = 25 ma per tube, which is perfectly alright and will give good tone, but when the AC current starts to flow, we limit it with a 5K or 10 k resistor,

so we get to use a warm bias without a melt down when we play the amp on 11,

a 15 K dropping resistor gives us 488>425 on the screens, I-p = 80 ma ad I-s = 3.75 ma, -50 bias,

8.2 k dropping resistor gives 80 ma at -52 bias and 488>445 volts on the screens,

note that this resistor will run cool as a pickle until the music starts,

 

[CJ]

fooling with tubes at max volume can be a hassle, especially at 4 am,

we need a dummy load, have you priced a 200 watt 10 ohm resistor lately?

space heaters are nice in this Arctic vortex going on right now, but what about summer?

we need a cool load, something better than a dead short and a space heater,

how about a WE 111C transformer core wrapped with some junk speaker wire?

seems to work alright, maybe too many milli-henries, but hey, we get to hear the guitar sound come out of the iron at 100 watts so WTF, over?

 

[PRR]

The ratio of plate to screen current is NOT consistent in 6L6.

It is "aligned grid". The screen hides in the shadows of the control grid. This is tricky winding. A little slop and G2 current runs high. A little "too good", and G2 current not only vanishes, it can reverse due to normally negligible secondary emissions now becoming the main thing.

The real, dependable, ratio is G2/G1 voltage. Make either one less positive, less plate current will flow. But G2 voltage changes have to be about 10 (for 6L6) times bigger than G1 changes.

G2 voltage also limits the maximum cathode (mostly plate) current. Like a Constant Current "Source"-- you hold base/grid so there is 10V on a 1K emitter/cathode resistor you get 10V/1K= 10mA maximum. A 6L6 with G2 at 450V will pass up to 400-500mA with G1 barely negative. That's really more current than you ever want. Lower G2 voltage is often a good thing for excessively hot-rodded guitar amps,

Remember the difference between Lotus and Mack. Lotus drives really great for 249 miles. Mack truck aint as fast but will chug a million miles. So there's amps that play great on trial, but can't take a 3-continent tour. I shouldn't generalize about boutique amps, but there's an incentive to make shoppers go WOW! on trial rather than to under-stress for million-chord reliability.

However red-plating in modern 6L6, only on signal peaks, is not a disaster. There's a picture of a 6L6GC glowing red-hot and working fine. Life is shorter but hardly sudden death. (Red-Screen is more dangerous because G2 is small but G2 is hard to see.)