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previous: Trash Hi all, this is my first post here ... -- 9/7/2000 7:52 PM View Thread

Re: Biasing a Marshall Major

9/9/2000 1:09 PM
Randy Jamz
Re: Biasing a Marshall Major
I have had my ass kicked eight ways to Sunday here telling anyone how I like to bias. Having said that, I will tell YOU that;  
1)Svetlana states that their KT88 is rated for 42 watts plate dissipation.  
2)Your 60mA number probably came from the Tom Mitchell video/book.  
3)You are running the tubes right now at 36 watts plate dissipation. I don't know the popular wive's tale of where you should run your tubes compared to maximum dissipation, but I've heard 60% of, and I've heard 70% of. Either way, you are 'safe' according to folklore.  
Now, to get my ass kicked again;  
4)Measure screen grid voltages. If one tube measures a handful of volts less, pull that tube. The rating says 8 watts screen grid dissipation. The last Major I worked on had 600VDC on the screens as well as the plates. This translates to about 13mA for the screen grid. Should the voltage drop to about 585 or something like that, you are drawing too much screen current. The tube won't like that for very long.  
5)Compare bias voltage on pin #5 and at the bias supply. If there is more than 1 or 2 volts difference, you are drawing grid current. Not good.  
6)You could 'scope out the waveform, and see what is going on. Do you have 200 watts output? Is there a crossover notch? Do the tubes run really hot?  
There's more to this than 'what would be an acceptable cathode current range be?' unless you listen to everyone else and cathode current is the panacea for that ailing amplifier.

Ken Gilbert q{unless you listen to everyone els... -- 9/9/2000 1:22 PM
Liam q{I have had my ass kicked eight wa... -- 9/10/2000 2:37 PM
Reid Kneeland q{I have had my ass kicked eight wa... -- 9/11/2000 7:55 PM
Peter S Randy, The "popular wive's... -- 9/11/2000 8:58 PM