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Marshall fixed bias

2/17/1998 2:02 AM
Marshall fixed bias
Hi, this is my first try with this mailing  
thing, I hope I get through.  
I have a Marshall Super Bass 100W plexi,  
four EL-34 and a fixed bias (I think).  
The problem is that it distorts all the time  
I have measured the bias -90 volts, I guess  
it is a bit to high, on the schematic I have  
available the bias should be inbetween -60  
and -80 volts. Today there is no fixed  
resistor and pot in series to ground, I have  
tried with a 47k and 22k pot and the bias  
dropped to -44 but then the tubes on one  
side almost cooked. The question is what  
values for the fixed and pot and can it be  
more to it than this (maybe a coupling cap)  
Best regards
2/17/1998 3:17 AM
mike saklar

be carefull! i know you already know that, want to stay around -37v to -43v with your bias number. yes, you can go much higher, but to get more a versitile sound, tighter highs and lows, stick to the sugguested settings. you'll want to raise your resistor before the bias pot to slightly higer values until you get in the ballpark you're looking for. one thing i've learned is that going from 6550's to el34's, you need to change that resistor from 150k(or120k) to 220k or so, half watt. maybe this will help...mike
2/17/1998 5:35 AM

Thanks for your quick reply, but I have tried  
two things:  
one, have the original(for me) wiring then I  
get almost -100 volts bias, this way I get  
an ugly distortion almost immediately.  
two, install a 47k fixed resistor in series  
with a 22k pot, then I get -44 volts max,  
this way the tubes in one side get overheated.  
So my question had two parts  
one, the recommended voltage and current for  
the EL-34  
two, can it be that there is something else  
too, a coupling cap maybe  
Thanks again
2/17/1998 6:42 AM
J Fletcher
Could be a leaky cap on the side that gets too hot.Check the dc voltages at the power tube grids.Also check the bias feed resistors(220k from grid to bias supply)Proper bias voltage should be in the -36v to -42v range for EL34's,depending on the plate voltage,tubes,and desired tone.
2/17/1998 7:07 AM

Thanks again for great service,  
I just wonder if you can explain why they  
have stated -60 to -80 volts in the  
schematic, that I have.  
The plate voltage is 560 volts in the  
same schematic and I have measured it  
to 530 volts.  
Is this because they had different tubes  
30 years ago or other criterias.
2/17/1998 8:21 AM
J Fletcher
If the schematic says -60 to -80,then I'm wrong with my estimate.Didn't realize you had such high plate voltages,I was figuring somewhere around 450volts.Does your schematic show 6550 or EL34 tubes?The 6550's require more a more neg voltage than EL34's.That sounds too high of a bias voltage for EL34's to me,but I'm kind of in unfamiliar territory here.I would guess more in the -46 to -56 range,but that's a guess.If one pair of tubes runs way hotter than the other pair though,sounds like a problem with that pair.Did you check the bias voltage at the tube socket?
2/19/1998 10:51 AM
Maybe your tube are not mached close enough per side.

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