JCM900 100W Dual Reverb - Low Volume Issue

[g1]

Faulty filter caps where you have high AC ripple can give odd readings depending on the meter. As you have reported low DC on the filter caps, it could be that there is high AC ripple. In that case the B+ may be falling even more when delivering power.
Try measuring for AC on the filter caps, or even better, scope the B+ when amp is delivering full power.

[bw1985]

Here are the schematics of the PCB revs in my amp:

Pre-Amp
https://drtube.com/schematics/marsha...-iss7-0349.pdf

Power Amp
https://drtube.com/schematics/marshall/cd0192-iss10.gif

[bw1985]

The cathodes of the 4 EL34s in the JCM900 power amp are grounded via fuses. So cathode voltages are zero - except if someone installed additional cathode resistors. If so you should see a DC voltage.
The hi-low power doesn't turn off 2 of the EL34s but switches between triode and pentode mode for all 4 of them.



Did you confuse cathode with heater voltage?



Please post appropriate schematic as there seem to be different versions.

Yes, I confused the heater voltage with cathode embarrassingly so I edited it.

I will need to check the PCB revisions again, but the model is the 4100 with 4xEL34s.

[Helmholtz]

All cathode voltages on the EL34s appear to be consistent/healthy as well but I will double check (6-7Vac)

The cathodes of the 4 EL34s in the JCM900 power amp are grounded via fuses. So cathode voltages are zero - except if someone installed additional cathode resistors. If so you should see a DC voltage.
The hi-low power doesn't turn off 2 of the EL34s but switches between triode and pentode mode for all 4 of them.

(6-7Vac)

Did you confuse cathode with heater voltage?



Please post appropriate schematic as there seem to be different versions.

[strato56]

My mistake : JCM900 Reverb only have a Pentode/Triode switch.
30th Anniversary have Pentode/triode and Halfpower switch.

[bw1985]

I've had a half power switch at JCM900 30thAnniversary gone bad. When closed ( full power ) there was still resistance instead of short.
So one pair of EL34's didn't deliver enough.
Compare the cathode voltages from the EL34's.

Thanks for the reply! I believe that I tested the half power switch and it appeared to be functioning as expected. I can double check this evening when I get to the amp again. All cathode voltages on the EL34s appear to be consistent/healthy as well but I will double check.

[strato56]

I've had a half power switch at JCM900 30thAnniversary gone bad. When closed ( full power ) there was still resistance instead of short.
So one pair of EL34's didn't deliver enough.
Compare the cathode voltages from the EL34's.

[bw1985]

Hi all,

I've been working on my Marshall JCM900 100W Dual Reverb (EL34s) and have not been able to solve this problem. The amp is essentially operating at half of its normal volume. The tone sounds great, but I compared it to another and there is clearly something going on.

I've gone through the necessary steps to try to fix this: replaced all power tubes, replaced power tube sockets with Beltons, checked voltages to power tubes, bias voltage, power resistors. So far everything looks healthy. Some voltage on one of the power supply caps was slightly lower than expected (around 320v vs 380v), but not sure this is that significant.

Could anyone recommend other issues to look for to remedy this? It's been super frustrating!

As a side note, I have a similar problem with my Silvertone 1483 head, operating at half volume with great tone! (figure they may be similar problems)

Thank you in advance,
-B