Marshall JTM 30 Very Low Volume

[Pigyboy]

Hi All,
Me again with a low volume problem. This time it is aMarshall JTM 30 with Very Low Volume. I am going to change the power tubes first as they look to be glowing a lttle bit too blue. Am I on the right path?
Thanks,
Pigyboy

[MWJB]

Blue glow is rarely a sign of a problem.

[Pigyboy]

If blue glow is no problem...

If blue glow is no problem where should I start to check? The amp just does not get very loud.
Thanks

[MWJB]

First off, as you suggest, change the tubes. Start with a good known 12AX7/ECC83 in all the preamp positions. It may, or may not, be your power tubes, I was just saying the blue glow is not an indicator of a problem. Be sure to check power tube bias current is within tolerance and that tubes are reasonably matched (+/- 5mA).

Then try a different speaker/cab.

When the plug in/plug out options are exhausted, check voltages (or get a tech to check them) at the tubes.

[killer blues]

I hate to say it but on that amp, it sounds like a blown OT, which is common in those amps. MM sells a bullet proof replacement that will cure the problem.

[Pigyboy]

How do I check the OT(Output Transformer?)?

[Twist]

Check your pre amp tubes filament voltage. They run on DC. Also the bias voltage has a stack of zener diodes which makes adjustment more complicated.
If this is an intermittent problem with volume it may just be a dirty return jack on the effects loop or bad solder joint on the input jack.

[Enzo]

Go over to RG's Geofex web site, I believe he has some transformer testing procedures there.