Marshall JTM30 low plate voltage

[pablovera87]

Hi everyone, my Marshall JTM30 sounds extremely low, with the master knob all the way up i can barely hear it, all distorted. I re-tubed the amp, changed all electrolythic capacitors in the poweramp section and powersupply, but i still get low plate voltage (around 140v) when i plug in the 12ax7. If i do not plug in the 12ax7 tubes, and i only plug the 6L6 tubes, plate voltage goes up to 400v. Any ideas?

Thanks in advance

[Randall]

Try a different 12AX7.

[pablovera87]

I already tried three new ones, and another three 12ax7 that i had on another amp, which i know are working fine.

[J M Fahey]

140V plate voltage is NORMAL, so look elsewhere for your problem.

I would be worried if I found 400V on 12AX7 plates.

Please post schematic so we can suggest some signal tracing tests.

[pablovera87]

OK.

This is schhematic for poweramp section.

http://drtube.com/schematics/marshall/jtm3-61-02.pdf

What i mean is when i put in 12ax7 i get 140v on all tubes, even 6L6. I'm guessing that's why i get low output volume. Does that sound correct?

[pablovera87]

OK now i get 380v on B+ with all the tubes ON. But i still get very low output. And 6L6 tubes glow blue when i turn on standby switch. Could it be bias problem?

[Roberto]

Blue is normal for 6L6GC. Over biasing would be red.

[xtian]

Yes, it certainly could be a bias problem--if biased extremely cold, volume will drop towards zero. Did you measure bias?

Agree with ^, blue glow is normal operation.

[Mick Bailey]

Did you check the screen voltages on both tubes? If either R126 or 128 are out you'll get low volume and distortion.

[pablovera87]

Did you check the screen voltages on both tubes? If either R126 or 128 are out you'll get low volume and distortion.

I did Check voltages on those resistors. Voltages are fine.

[Mick Bailey]

OK now i get 380v on B+ with all the tubes ON...

How come you suddenly got a different voltage reading?

BTW, Whenever I get mysterious voltage problems I always make sure the battery in my DDM is OK.

[pablovera87]

OK after days of double checking every solder joint, component and wire, i started to look at the output transformer. I assume this values are not suitable for the amp to work, so i post them here:

OT Primary: 390VDC on every pin (B+ and both 6L6 plates)

I disconnected all wires on OT, measured both B+ to Plate resistance, and i get 7.8ohms on one side and 83ohms on the other side. I hope i was clear.

This can't be right, can it?

Thanks everyone.

[J M Fahey]

Funny, I just answered this very same question at DIY Audio.
Are you posting same questions all over Internet?

[pablovera87]

There might be different people on different forums, who can have different opinions.

[Chuck H]

OK after days of double checking every solder joint, component and wire, i started to look at the output transformer. I assume this values are not suitable for the amp to work, so i post them here:

OT Primary: 390VDC on every pin (B+ and both 6L6 plates)

I disconnected all wires on OT, measured both B+ to Plate resistance, and i get 7.8ohms on one side and 83ohms on the other side. I hope i was clear.

This can't be right, can it?

Thanks everyone.

If you measured it correctly (primary ct to either primary end) and your meter is working properly (no intermittent probe shorts or low battery) your OT is bad.