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View Full Version : Marshall JTM 30 Bias Adjustment


kramps
10-11-2006, 02:47 PM
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.

Thanks,
Kramps

wixedmords
10-11-2006, 07:25 PM
There isn't a blue pot viewable when looking at the side of the chassis?

It is a little hard to find, but is there on my JTM 622. It may be different on the 30 watter.

kramps
10-11-2006, 08:39 PM
There isn't a blue pot viewable when looking at the side of the chassis?

It is a little hard to find, but is there on my JTM 622. It may be different on the 30 watter.

Th JTM 60 has a 100k pot but the JTM 30 line only has a 100k resistor that is impossible to reach.

Swarty
10-12-2006, 09:05 AM
As you've probably noticed these amps are a joke (bad joke) to service. Doing any kind of diagnostic that requires the amp to be powered up can consume a ridiculous amount of time. I suggest trying several sets of tubes until you find a set that is close enough.

wixedmords
10-13-2006, 01:55 PM
Read that as don't send it to Swarty's shop, send it to someone else that will work on the JTM 60 and 600 series.

Swarty
10-13-2006, 11:06 PM
Read that as don't send it to Swarty's shop, send it to someone else that will work on the JTM 60 and 600 series.

Ouch!

I thought I was speaking tech to tech and offering solid advice. The original poster stated he had one of these amps on his bench (the clue that he's a tech).