ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Marshall Amp Prob - Low Bias Current

9/17/2000 4:23 PM
DHuggins	Marshall Amp Prob - Low Bias Current AMP: Marshal JCM 900, model 2501 (50w combo, single 1x12, no reverb) I pulled out my EL-34 tubes to look at them for condition. On one of them, the center of the socket -which has a column w/flange to correcly justify the pins for reinsertion- came out and I didn't notice until AFTER I turned the power on. Needless to say, my 1 in 8 chance of getting it right didn't come thru. The tube blew, taking out both fuses. I have replaced both power tubes and the 3 preamp tubes (and the fuses of course). In biasing the tubes, I found out I can't get the bias above 21 mA, whereas it should be in the 35-40 mA range for a Class AB EL-34 configuration. For Biasing, I am using an Alessandro Amps Biasing/Matching Unit (which has two probes with meters for each). Any clues as to what is wrong? I know I will prob have to take it to a tech to fix, but wanted to know if there was anything that was obvious for me to look at. I am also just plain and simple, curious. Thanks, Dave

9/17/2000 4:23 PM

DHuggins

Marshall Amp Prob - Low Bias Current

AMP: Marshal JCM 900, model 2501 (50w combo, single 1x12, no reverb)

I pulled out my EL-34 tubes to look at them for condition. On one of them, the center of the socket -which has a column w/flange to correcly justify the pins for reinsertion- came out and I didn't notice until AFTER I turned the power on. Needless to say, my 1 in 8 chance of getting it right didn't come thru.

The tube blew, taking out both fuses. I have replaced both power tubes and the 3 preamp tubes (and the fuses of course). In biasing the tubes, I found out I can't get the bias above 21 mA, whereas it should be in the 35-40 mA range for a Class AB EL-34 configuration. For Biasing, I am using an Alessandro Amps Biasing/Matching Unit (which has two probes with meters for each).

Any clues as to what is wrong? I know I will prob have to take it to a tech to fix, but wanted to know if there was anything that was obvious for me to look at. I am also just plain and simple, curious.

Thanks,

Dave

Replies:

Ray Ivers Dave,I would do a c... -- 9/18/2000 12:38 AM
R.G. Ray's on the right track. You proba... -- 9/22/2000 3:29 PM