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previous: dale a marshall with 34's at 5ma under t... -- 4/18/2000 5:40 PM View Thread

Re: waddaya think about biasing..........

4/20/2000 12:30 AM
Ray Ivers
Re: waddaya think about biasing..........
This isn't really to the point of your posting, but I wanted to put it out anyway.  
Certain amps - 50 watt, JCM900, 30th Anniversary, DSL and TSL Marshalls, some Ampegs, the Vox AC100, etc. - derive their bias supply from the HV supply. This really sucks, because the bias voltage varies with the high voltage - and since the HV voltage supply in a non-regulated supply will vary with bias voltage, a 'vicious circle' is created - the HV drops, the bias drops, the tubes suck more current, the HV drops more, etc. I ended up having to bias my 30th Anniversary head by the most bullshit method imaginable: enough voltage to ensure that the plates don't glow red after being on for an hour, minus about 10 mils. Older 100 watt Marshalls have a separate bias winding (and a choke) so the bias voltage only varies with line voltage, as it should unless everything is regulated.  
My advice - keep an eye on your plates if you own one of these amps, especially in a place with unfamiliar line voltage.  
You ever see the schematic for the output stage on a Music Man RD100? 15 mils quiescent current (total!), almost no crossover distortion, a really clean sound. I've changed the original Sylvania 6L6GC's in these amps after 5 years of hard use and heard no difference at all - it was like the tubes had 3 months on them.  
Ray Ivers