JCM 800 poweramp not fully functioning

[BeëlzeM]

The poweramp of my modded 800 2205 head isn't working properly. Even with the MV and the channel volume and the gain dimed it's not louder than a moderate bedroom volume. Even when running my guitar straight into an overdrive pedal through my fx-return it's not loud at all.

Both my KT77's glow, though the one in V7 glows a lot brighter than the one in V6 when cranked. You can really see V7 working when playing the guitar, while V6 stays the same brightness. All the voltages on the tubesocket pins are correctly, I've measured those and those look right. Tubes have been replaced about a week ago.

I replaced the screen grid resistors with good new 5watt ones, still nothing. My grid resistors on pins 5 are 5,6k ones. Could these be bad? They measure 5,6k though. Everything is wired correctly to the board for as far as I know.

Any other things that could be bad? Filter caps or whatever? Or a loose tube socket? Eventhough my voltages are correct.

[52 Bill]

Both my KT77's glow, though the one in V7 glows a lot brighter than the one in V6 when cranked. You can really see V7 working when playing the guitar, while V6 stays the same brightness. All the voltages on the tubesocket pins are correctly, I've measured those and those look right. Tubes have been replaced about a week ago.

Have you checked the output transformer? How about the speaker output jacks for shorts?

[BeëlzeM]

Have you checked the output transformer? How about the speaker output jacks for shorts?

I've cleaned my speaker jacks with contact cleaner. The outputjacks are clean. Fx-loop jacks are also clean.

Could it possibly be the OT? I don't know what the symptoms of a bad OT are. The amp right now is as loud as a 50 watt amp on 1, but it's on 10 fully dimed. So there is still sound but it's not as loud as normal, but could it be the output transformer? Any way I could test the OT for faults?

I've also clipped out the reverb signal completely, thought it was maybe out of phase but this didn't help either.

[52 Bill]

Any way I could test the OT for faults?

Test it for shorted turns. R.G. has a simple tester circuit on his site.

Is the reduced output clean or distorted?

[BeëlzeM]

Test it for shorted turns. R.G. has a simple tester circuit on his site.

Is the reduced output clean or distorted?

Any direction you can point me to? I've never tested an OT for shorted turns or whatever before. How should I do this?

But is it possible that an OT gives less output? The fact V7 is working when playing but V6 stays the same and glows less bright than V7 concerns me.

The output is pretty clean. The tone is pretty good actually, it's just not loud at all.

Here's a short clip with the MV, channel V, and gain dimed.
http://www.soundclick.com/bands/defa...songID=7424124

[52 Bill]

Any direction you can point me to? I've never tested an OT for shorted turns or whatever before. How should I do this?

The quickest first test would be to measure the resistance of the windings.

The fact V7 is working when playing but V6 stays the same and glows less bright than V7 concerns me.

Have you checked the phase inverter tube and circuit?

[BeëlzeM]

The quickest first test would be to measure the resistance of the windings.


Have you checked the phase inverter tube and circuit?

PI tube is also a brand new one, pins go where they should I guess. I could recheck again.

This is the poweramp schematic:
http://img6.imageshack.us/img6/1781/...mpowerpsdc.jpg

I should just measure everything in the poweramp? Also, the pcb is totally clean of bad solders or solder touching eachother.

[52 Bill]

I should just measure everything in the poweramp?

Did you check the voltages on the Phase Inverter tube? Your earlier post said you checked the voltages; just the power tubes?

[BeëlzeM]

Did you check the voltages on the Phase Inverter tube? Your earlier post said you checked the voltages; just the power tubes?

Yeah, all sockets, including the preamp sockets and the PI socket. All voltages are as they should be. Can that still mean a bad OT? Honestly, I have no idea how to check a bad OT, have no experience with OT's whatsoever, they never failed on me and I've never worked on those.

Could the tube sockets have loose connections? Eventhough my voltages are right?

How about filter caps? Not sure if they could cause this, I replaced these with brand new JJ's probably 6 months ago, so it would be unlikely these are bad but just checking.

[jrfrond]

Before messing with the OT, go back to your EFX loop jacks and physically jump them with a patch cable. I've seen too many EFX loops gone bad due to dirty jack normals. Bad soldering too.

[BeëlzeM]

Before messing with the OT, go back to your EFX loop jacks and physically jump them with a patch cable. I've seen too many EFX loops gone bad due to dirty jack normals. Bad soldering too.

Yeah I've tried this as one of the first things. Also cleaned them with some contact cleaner. I've had those jacks go dirty on me before as well.

[bnwitt]

You know the first thing I would do is throw some bias probes on those power tubes and observe milliamps per tube to see what you're pulling on each one. I know you said the voltages were fine but what are your voltages on pins 3, 4, and 5 of both power tube sockets?

[BeëlzeM]

You know the first thing I would do is throw some bias probes on those power tubes and observe milliamps per tube to see what you're pulling on each one.

Yeah I probably should. I'll do that. There is just one bias trimpot though for both tubes, and both tubes are glowing differently from eachother. V7 glows brighter and you can actually see it working when dimed. Pin 5 measures -33 volts though on the tube sockets, still pretty hot but not lethal right?

V6:
Pin 3: 439
Pin 4: 438

V7:
Pin 3: 436
Pin 4: 435

Pins 1 and 8 are connected to ground.

[bnwitt]

what about the negative VDC on pin 5

[Enzo]

He said it was -33 on all the sockets, he just didn;t put it in the chart. The chart was to show the one or two volt differences between sockets on pins 3 and 4.

Those couple volts don;t matter, by the way.