Marshall JTM 30 Bias Adjustment

[kramps]

I have a JTM 30 on the bench and there is no adjustable bias. The resistor, R132, which I assume is the one I need to change is burried. Any experience on this model? Any help would be greatly appreciated.

kramps

[tim]

I would replace the power tubes on this one (if that's what you're doing) and check bias first before you change anything. It may be fine where it is. If it needs to be tweeked, it may be advantageous to stick a 250k pot (linear) in the place of that 100k resistor if there's room and make it an adjustable bias...it's been a while since I've had one of those on my bench. I just got done fixing a JTM60 but that has a pot in the bias circuit already.

[oc disorder]

Check Zeners 2 x 15v + 9.1v

Hi
Have seen a couple here one had burnt power transformer other
o/c output transformer.
I think for memory I upped the 9.1 zener in both to 11v as it appeared
to be pushing the tubes too hard.
Schematic here:- http://www.drtube.com/marshall.htm#JTM30.
The trannies were hard to get hold of so I would err on the side of caution.
Just thought I'd add my tuppence worth....

[rebel amps]

I have one that was running at 58 ma. and changed the 9v. diode to a 15v. It lowered the plate current to 45ma. Still a little hot, but much better. Changing R132 and R133 didn't do anything on this amp.

[Valvehead]

edit --( found a zener)

[Valvehead]

forgot to mention the bias is showing 60 and 70 ma at the Ot ! Hope the 15v zener knocks that down....holy moly..

[Jazz P Bass]

forgot to mention the bias is showing 60 and 70 ma at the Ot ! Hope the 15v zener knocks that down....holy moly..

Smokin!!

[rebel amps]

edit --( found a zener)

NTE 145a is the diode I used. They have many different voltages. Should be real handy for getting your Marshall cooled down.

[clintronics]

I put a 1n4746 18V in the ZD101 slot and got 39mA plate current