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JTM60 Bias

4/9/2005 8:58 PM
Kevin StPierre
JTM60 Bias
I am putting new Power tubes in a friends JTM60.It has a bias adjustment pot,do I just adjust pot to read proper bias on pin 5?
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4/10/2005 6:41 PM
Wild Bill

Kevin, the best thing is to go to the Aiken Amps site and read the FAQ on biasing over and over until you think you've got it down pat.  
Modern tubes vary much more in specs than during the Golden Years of 50 years ago. You could adjust the pot to get a spec'd bias voltage on pin 5 (provided you know what that voltage is!) and hopefully you'd be in the ballpark but it's scary how far out the idle current measure will actually be with modern tubes.  
It's not the adjustable voltage that's important. The voltage is merely a control factor to set the proper idle current with the tubes. No one really cares what the bias voltage is - just the idle current the voltage controls. How do you know the proper idle current figure? How do you know how to safely measure it? All the pot will do is adjust the idle current with only a loose relationship to the proper figure.  
Read the FAQ. It says it all much better than I can! :)  
---Wild Bill
4/10/2005 8:35 PM
Kevin StPierre

Thanks WB that is what I thought.I have always used the 1 ohm resistor method but it looks as though getting at this amps tube sockets is a pain and I am new to Marshalls.I just wanted to make sure the same procedure applied.
4/10/2005 9:47 PM

Then use the shunt method. I thought there were bias current points in that model, perhaps I am thinking of another.
4/11/2005 2:46 PM
Kevin St.Pierre

I have never tried the shunt method,but I will reread my references.I will check for bias points, everything on the circut boards is well marked I'm sure they would be too.
4/12/2005 12:00 AM

The mental image I have is of three pins in a row sticking up from the pc board over near the output tubes. I don't recall them being marked. I found them on the schematic adn tracked down their location. It might have been a different Marshall though.
4/12/2005 5:53 PM
Kevin StPierre

Thanks All,Well I didn't find what Enzo mentioned and my friend thought it might be a chance to experiment,So I adjusted the bias to the point that the plate were just starting to glow red,backed off till it disappeared and then adjusted just a bit more toward the stable direction and let it marinate for awhile.He played it and was happy with the sound.Seemed to make sense to me and now we wait and see what happens.I did tell him that these amps are prone to heat problems and keep it away from the wall,he is already having problems with the series FX jacks and has to use a patch chord.

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