Orange Thunderverb 50 bias - PLEASE HELP!!

[Arty]

Hi all I need some urget help regarding bias adjustments....

I have an Orange Thunderverb 50 that has recently had new EL34s fitted by my friend who is an amateur tech. Since I got it back it has sounded like crap! ...really fuzzy and buzzy tone

I have a Maughan bias probe and thought I'd just check the bias for the two EL34s... One reads 14.3mA and the other reads 13.8mA !?!?! Shouldnt they read more like 40-45mA each? ....has my friend totally f**ked up here or does the Thunderverb have a bizarre plate voltage/current setting for the EL34s?

Any help much appreciated

[Gtr_tech]

The current spec means little without the plate voltage. 40ma at 350v is way different from 40ma at 520v.
You need to figure the static dissapation wattage. Plate voltage x plate/cathode current.

So say your plate voltage is 485v, and you want to bias to 60% of max plate wattage (25w x .6 = 15w), you would adjust for 31ma (485 x .031). You can get real picky and figure in screen current as well, but I tend to bias on the conservative side, so I don't worry about it. EL34s tend to draw more scereen current than a beam tube like a 6L6.

[Arty]

Think you just read my mind as Ive just had the chassis out to measure it ...the plate voltage is 395. So doing the bias calc in reverse: 395V / 14mA = 28watts?!! ...so I'm basically frying my EL34s?

So for a plate voltage of 395V, if I want 70% output, I should bias to about 43mA?

[g1]

At 14mA, you are around 5 watts per tube, not 28watts, P=IxE. So likely the crossover distortion is giving you the buzz/fuzz.
43mA will get you in the ballpark of 70%, if that's what you want. You may not need to go that high and still be able to get rid of the buzz and have decent tone. Lower idle current will increase your tube life.

[Arty]

Ah right, thanks for the correction, I wasnt sure if I'd got the calculation the right way round

Is there a way to check the current just using the multimeter but without using the bias probe just so I can be sure I'm not getting any false readings?

[Gtr_tech]

.014, not .14. .14 would be 140ma. You need to use the basic unit of measure...ie volts, amps,ets. Not mv or ma.