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Biasing Marshalls

3/10/1998 7:24 AM
Biasing Marshalls
I'm quite new to this game, so here's a stupid question. Using a function generator and 'scope to set the bias voltage on my 1978 Marshall 50 and 1980 100 MV amplifiers, I've found that the adjusting pot. is really insensitive. However this means that it's impossible to get a great deal of crossover distortion, and also it's difficult to make the HT fuse blow or any of the other groovy fireworks that are mentioned in R.G.'s FAQ page. Is this normal for these amps? Should I just stick some cathode resistors in and set the average to 35ma (EL34 Svets in both amps)?  
Another thing. The 50 Watt head is really phase coherent between input and output, so the crossover distortion is easy to see and remove. The 100 has shitloads of phase lag, so it's hard to see when the crossover distortion has gone, but there's never very much of it. Anyone got a lot of experience of these amps? I'd love some advice.
3/11/1998 4:50 PM
I found the Svets to run very hot as compared to say Sovtek EL-34's. In other words, it takes a lot of negative bias voltage to throttle them back, and this is probably why you dont get any crossover distortion, the tubes are underbiased over most of the range of the adjuster. The range of the adjusting pot is about 12 volts (on my amp anyway), so if you can't get negative voltage in excess of -40 volts on the 50 watt, you may never see crossover distortion to any great extent. That is about the limit of my limited knowledge.  
My personal feeling is that if you like the sound, the tubes are biased correctly, even if they are getting a little red. Others feel that tubes cannot be abused, but I think of them as more or less expendable.  
Good luck,  
3/12/1998 6:31 AM
Thanks Brendan - that starts to explain quite a lot. Weirdly enough the amps both sound great, so I can cope with expendable tubes!  
3/12/1998 1:19 PM
Jim S.

Although I have no objection to "pushing" output tubes a bit in order to get good tone, I strongly feel that running them beyond their max. plate dissipation rating (which is what is happening whenever you see the plates glowing red) is an invitation to disaster.  
Yes, power tubes are expendable, but the ideal time to retube a guitar amp is during a normally scheduled service visit, not on the stage during an unscheduled band emergency time-out.
3/13/1998 6:01 AM

I'd be pretty happy if I could get my Marshalls to either extreme of plate dissipation or crossover distortion, but there just doesn't seem to be a big range of bias adjustment. Maybe this is saving me from destroying things!
3/13/1998 10:04 AM
Jim S.

The first thing you should do is make sure there really is a problem with the bias adjustment pot or circuit. If there is, then you should fix it. Find out what range of negative bias voltages you can achieve. It should span at least 10 or 15 volts (for example -30 to -42 volts). Measure this right at pin 5 of the output tube sockets.  
Once you get the bias adjust issue resolved, you can go back to the task of setting the bias for the set of EL34's you have installed. I STRONGLY recommend that you do it by measuring the amount of current drawn through the cathode, either by installing 1 ohm resistors between cathode and ground or by using a "bias probe" type device which consists of an adapter (or set of adapters) that plugs in between the tube socket and the tube. Even if you ultimately wish to use a scope to help find your ideal bias point, you really need to know how much current is going through those tubes so that you have a reality check. The trouble with using the scope method alone is that you're not measuring anything! Being a tech, I like to measure things - it gives me a warm feeling that I know what's going on with the circuit and the tubes.  
Good luck!
3/13/1998 10:33 AM
I strongly agree!! I use the output transformer shunt method myselfe. I am always concerned ( even more with todays tubes) with the amount of current going through my tubes! As long as it sounds great, and the plates ar not glowing, I am an extreamly happy camper!!!!!!!!!!  

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