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previous: HM Could it be the Z of your meter. -- 5/4/1998 9:49 PM

Re: Not your typical bias problem!

5/5/1998 3:36 AM
liam	Re: Not your typical bias problem! I've been having very similar problems with my Marshalls. The 50's seem to run fine on 36mA but my MV100 gets really hot on any more than 32mA. This is a real shame, because I want a better clean sound from the amp. I therefore contacted my friendly neighbourhood ex-Marshall tech. He tells me that this has been a known problem, and Marshall used to bias the amps to low currents to conserve valve life. This is evidently why the amps were renowned for only sounding good dirty. I'm not sure how seriously to take him! Weirdly enough I had to modify the bias circuit to get less than 42.8 Volts bias on the MV100. Marshall obviously didn't intend for me to draw too much bias current. I'm going to get to the bottom of all this eventually, so I'd really appreciate any intelligence on the subject. Another question: Why can't the bias current be checked by measuring the OT resistance, and then checking voltage drop across it? We're only talking about DC current after all. Surely this has got to be much more accurate than shunting current away from it. Thanks for your help. I'm losing count of the number of people that have given me sound advice on this BB.

5/5/1998 3:36 AM

liam

Re: Not your typical bias problem!

I've been having very similar problems with my Marshalls. The 50's seem to run fine on 36mA but my MV100 gets really hot on any more than 32mA. This is a real shame, because I want a better clean sound from the amp. I therefore contacted my friendly neighbourhood ex-Marshall tech. He tells me that this has been a known problem, and Marshall used to bias the amps to low currents to conserve valve life. This is evidently why the amps were renowned for only sounding good dirty. I'm not sure how seriously to take him!

Weirdly enough I had to modify the bias circuit to get less than 42.8 Volts bias on the MV100. Marshall obviously didn't intend for me to draw too much bias current. I'm going to get to the bottom of all this eventually, so I'd really appreciate any intelligence on the subject.

Another question: Why can't the bias current be checked by measuring the OT resistance, and then checking voltage drop across it? We're only talking about DC current after all. Surely this has got to be much more accurate than shunting current away from it.

Thanks for your help. I'm losing count of the number of people that have given me sound advice on this BB.

Replies:

Doc I was recently "tuning up" a 100w 1... -- 5/5/1998 12:30 PM
Steve A. liam:5/12/1998 8:31 PM