JCM800 No Sound

[MadMax64]

I think I may have found an issue with the V2 socket. I had fired the amp up and saw glow in V1 and V3 but nothing in V2. I left the room for a minute and when I came back V2 was glowing brighter than the other two. I put the tube shields on and checked the voltage at pin 3 and still got no reading. Confused as to how that could happen since there was power being supplied, I tried again. This time with V2 showing signs of life, I simulated placing the shield over the tube by moving it slightly as I applied a downward force and immediately saw the tube go dead. I have another socket to replace it with.

V2 showing signs of life

IMG_3593.jpg

[MadMax64]

Measuring the speaker output jacks using the speaker cable tip/sleeve I read 0 ohms.

That's about what you should read.

The voltages you listed for V2 are messed up and would account for no sound. V3 pin 1 is wrong also. It should read about the same as pin 6. Recheck the voltages for V2 and V3.

Thanks, those readings were with the original tubes. I replaced the V2 tube and got different voltages on V3. Still have the issue with losing voltage on pin 3 of V2.

1: 271
6: 273

[MadMax64]

With all 3 valves showing signs of life, I plugged in a guitar and got sound. I could hear the guitar very faintly with some crackle and pop from time to time. All controls were dimed. All 3 of the preamp tubes seem very loose but V2 for some reason doesn't maintain a connection. However, when it does there is sound passing through the amp. I'm guessing with a 36 year old amp the capacitors would need to be replaced but how likely is that the OT is also bad?

[Stevem]

It’s pretty easy with just your eyes and ears to tell of you have a OT that’s shorted.

Get the room dark and play the amp cranked while you look at the output tubes.
If the preamp section is working well you will see the output tubes flash up brite on the inside.

If you do see that then the output tubes are being driven as they should and they are trying to drive the OT but can’t because is bad.

[MadMax64]

It’s pretty easy with just your eyes and ears to tell of you have a OT that’s shorted.

Get the room dark and play the amp cranked while you look at the output tubes.
If the preamp section is working well you will see the output tubes flash up brite on the inside.

If you do see that then the output tubes are being driven as they should and they are trying to drive the OT but can’t because is bad.

Perfect, thanks. I did notice the power tubes flashing when I finally got the signal to pass through V2. I have new tube sockets (all 3 preamp are loose), caps, and OT on the way. I appreciate everyone's help. I will update after install.

[MadMax64]

I replaced the output transformer, preamp tube sockets, filter caps, and bias caps. I used a current limiter on start up and discovered a short that turned out to be a shorted diode in the rectifier, so I replaced all the diodes in the rectifier and bias circuit. Voltages look much better but still no speaker sounds - I can hear the amp respond when I turn the pot on the PPIMV that looks to have been added a some point. By respond, I mean I can hear the guitar signal "in the amp" but not out of the speaker.

I've double and triple checked the OT connections at the output selector. Swapped tubes for a known good set, same result. Given that I just replaced the OT, I don't want to continue troubleshooting without some guidance.

Screenshot 2024-12-16 at 10.32.26 PM.png

[Lauri]

You learned a very valuable lesson. Now you know it's beyond retarded if someone tells you that you can tell just with your eyes and ears if an OT is shorted.

Is there any hum or buzz coming out of the speaker? Is there anything coming out of the speaker when you measure voltages on power tube control grids or anodes?

[Lauri]

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[MadMax64]

Is there any hum or buzz coming out of the speaker?

Before the changes, I literally heard nothing from the amp or speakers until I realized that V2 was not getting warm and noticed that the tube was really loose in the socket. Once I tightened it up a some, I could hear a signal but not from the speaker. It sounded like it was "in the amp". Very low volume and fizzy sounding, like crappy distortion. That's when Stevem mentioned being able to use your eyes and ears in a dark room to tell if the OT was bad. I had already noticed the flashing of the power tubes and confirmed it after his observation. I had never heard of that before, but after changing the OT, I now have a much stronger, consistent sound but still low and in the amp circuit not the speaker. I have checked more than once and do NOT see the power tubes flash since change the OT.

Someone had pointed out in an earlier post that a PPIMV had been installed in the amp using the D.I. jack hole as a mounting place for the pot. I had noticed on initial inspection that a ground wire that extended past the jack towards the pot and doubled back to the output jack. The ground was not connected to the output jack and had a cloth shield on it. See photo. I have since grounded the pot and still no change.

IMG_3586.jpg

Is there anything coming out of the speaker when you measure voltages on power tube control grids or anodes?

Nothing coming out of the speakers when measuring signal voltages at any of the tubes. I did check voltages through the amp to the power tubes using a guitar E-string. Found a little over 1V at V1, 15-16V at V2 and 30V+ at the output tubes.

[sluckey]

The guitar sound you are hearing is the output transformer vibrating. You should closely examine all the circuit between the OT secondary, impedance selector switch, and output jacks. Check the resistance between the selected secondary wire and the speaker jack. Verify that the OT secondary common wire is connected to chassis ground.

[MadMax64]

To all that provided feedback, thank you!. Amp is up and running and sounding great. Should last another 36 years.

[johnnyreece]

To all that provided feedback, thank you!. Amp is up and running and sounding great. Should last another 36 years.

So what was the culprit?