tube bias question

[dxjxrose]

i just bought a quad of tubes off a foumite here said to be a matched quad... they are 5881 wxt's. my sovtek mig100 is running at 513v so take 25 / by 513= .048 x .7= 34.1... so when i set my bias using a probe it seems i have 2 tubes that are about 8ma or 9ma higher than the other. now ive tested each tube individually and its the tubes not the sockets that are the culprit. i have never had a quad that far off so what do i do. just bias to the hotter tubes or split the dif. should i run the tube like (34-42-35-43) or (34-42-43-35) not quite sure what the best way to go is..

 

[glpg80]

the 70% is for worst case scenario of whichever tube or pair is drawing the most current at idle. Technically you have a bit less because the correct bias is the difference of the plate current and screen grid current when measured across a 1 ohm resistor on the cathode of the tubes.

do the 70% of the highest bias current tubes and keep them matched/together for a class AB amplifier. So inners and outers. This is because each pair is amplifying a separate phase of the sin wave.

 

[dxjxrose]

so (34-25-24-35)

 

[glpg80]

Id do (38-30-30-39) to help get the other pair out of crossover.

Remember your calculations do not take into consideration the difference of screen grid current

 

[dxjxrose]

thats just about what i have them at right now im like 1 ma higher all the way around

 

[glpg80]

If they were 6L6's i would say bring it down some, but with 5881's I would not worry about it at all. In reality you can set it by ear and check it with the meter afterwards to make sure you're within spec.

Personally i prefer them as low as possible as to get out of crossover - anymore and you can hear that the amplifiers tend to get a bit soggy. great for lead playing, but not for rhythm or tube life

 

[Anonymous]

If they were 6L6's i would say bring it down some, but with 5881's I would not worry about it at all. In reality you can set it by ear and check it with the meter afterwards to make sure you're within spec.

Personally i prefer them as low as possible as to get out of crossover - anymore and you can hear that the amplifiers tend to get a bit soggy. great for lead playing, but not for rhythm or tube life

this is good advice.

i'd get the lowest 2 paired with the highest 2...you're not gonna notice any difference....even 10 ma, you probably wont notice....but, back on point....

i'd set the bias so the lowest 2 tubes are 26 or so. that's where 5881's feel best to me in my 5150 at least.

 

[Ventura]

glpg80 is a "tube sensei"...

I mean it. Cat knows his tech like none other around here

 

[dxjxrose]

thanks