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Mbira

Anybody know proper bias for Marshall JTM60?
« on: November 25, 2006, 02:54:00 PM »
Like the title says.  I usually scope it out, but this amp is having problems, and I'm not even sure what range this is supposed to be at.  I thought it was around -38v.  
The schematic is HERE

After being in for half an hour or so, the bias plummets to -25v and the tubes start redplating.  This amp REALLY SUCKS because the board is vertical and impossible to get into while hooked up.  The bad part seems thermally affected, but I can't figure out which part it is becaus eof the board situation.

 :evil:  :evil:  :evil:
Joel Laviolette

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lydmann

Anybody know proper bias for Marshall JTM60?
« Reply #1 on: November 25, 2006, 03:20:43 PM »
Check C132-133.
C139 and 140 can also be the cause of bias voltage dropping.

To adjust bias,you have to measure the plate current in each output tube.
Then find the power dissipated in the tube by multiplying the plate current with the plate voltage.
In the tube datasheet you can find the max recommended plate dissipation.
Then adjust the bias so the plate current, and thereby dissipation,is within the limits of the tube.

The bias setting will affect the sound of the amplifier,high current usually sounds good,but the tubes run hotter, and too little current makes the amp sound "thin"

Good luck!!

Gus

Anybody know proper bias for Marshall JTM60?
« Reply #2 on: November 25, 2006, 07:00:29 PM »
IC brand caps at C126 and C128?

Red plating alway check tube grid coupling caps.  

  To do it right you need a leakage tester that can test at rated voltage.

12afael

Anybody know proper bias for Marshall JTM60?
« Reply #3 on: November 25, 2006, 07:38:36 PM »
read current on the plate is a little dangerous.  
I put a 1ohm 5W on each cathode and read mV. ohm law say 1mV= 1mA with the 1ohm R. you have the current through the tube reading on the cathode where the voltage is low.
heavy metal is the law!!!

dukasound

Anybody know proper bias for Marshall JTM60?
« Reply #4 on: November 26, 2006, 01:06:22 AM »
Quote from: "12afael"
read current on the plate is a little dangerous.  
I put a 1ohm 5W on each cathode and read mV. ohm law say 1mV= 1mA with the 1ohm R. you have the current through the tube reading on the cathode where the voltage is low.

I agree.
On all my amps I did it.


 

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