JCM900 Model 4100 100W Head - High Pitch Squeal

[mikeboone]

I have a JCM900 thats making an odd squealing (oscillation) when turned up above say half way, really stupid loud. It does that in the full power (pentode) mode but NOT when its switched to half power (triode) mode.

I'll add that the problem manifests itself on attack at those higher volumes. So, its not making the noise all the time, only when striking a chord.

I tried removing the two inside tubes, then the two outside tubes to potentially narrow it to a tube, tube socket, etc. The problem isn't there with only 2 tubes in it, in either the outside or inside sockets.

The tubes are JJ6L6GC. I also tried all the same steps with a brand new set of tubes.

The amp sounds normal in either of the reduced power scenarios, triode mode or with 2 tubes out.

Any ideas on what would cause this?

I have attached what I believe to be the correct schematic.

[pdf64]

Some versions intended for 5881 look to have 470 ohm screen grid resistors, others intended for EL34 2k2; which does yours have?
http://www.classictubeamps.com/schem..._100w_4100.pdf
Does replacing the phase splitter V3 help?

[mikeboone]

Some versions intended for 5881 look to have 470 ohm screen grid resistors, others intended for EL34 2k2; which does yours have?
http://www.classictubeamps.com/schem..._100w_4100.pdf
Does replacing the phase splitter V3 help?

This amp is setup for 5881. It's currently running JJ 6L6GC.

I have replaced all tubes with new ones systematically across the board.

The pentode//triode switch was very scratchy so I changed it to be safe. I've also re-soldered everything in and around that part of the circuit and anything that appeared suspect.

My focus now seems to be on what's different when it's running at lower power. Either running it with 2 tubes (in either the outside or inside sockets) makes it go away. And running it in triode me de makes it go away. Something load related perhaps.

The filter caps were 20 years old so I replaced them all. Problem remains.

I have split the amp at the effects loop. I can drive an amp with this preamp no problems. Driving the effects return of this amp, problem is there.

[pdf64]

I've not seen inside one of these amps; are the tube sockets mounted directly on the pcb, or the chassis with flying leads between socket terminals and pcb?
If the latter, are the screen grid and control grid resistors mounted on the pcb or socket terminals?

[Jazz P Bass]

There is an offhand chance that the output transformer primaries may be wired backwards.

If you monitor the output signal & then lift the negative feedback resistor(56K), the amplitude should rise slightly.

If it goes down, then you have positive feedback & the OT primaries should be swapped.

jcm900_dualrev_100w_4100 Output Section.pdf

[drewl]

Also check the solder connections under the rear panel of board.

I find alot of bad/cracked solder joints on the switches and jacks.

[mikeboone]

I've not seen inside one of these amps; are the tube sockets mounted directly on the pcb, or the chassis with flying leads between socket terminals and pcb?
If the latter, are the screen grid and control grid resistors mounted on the pcb or socket terminals?

Traditional sockets with flying leads. Grid and screens are on PCB.

[mikeboone]

There is an offhand chance that the output transformer primaries may be wired backwards.

If you monitor the output signal & then lift the negative feedback resistor(56K), the amplitude should rise slightly.

If it goes down, then you have positive feedback & the OT primaries should be swapped.

[ATTACH]36996[/ATTACH]

That's easy enough to check for sure. The only thing that would make me think otherwise is the amp has worked for 20 years like it is.

[mikeboone]

Also check the solder connections under the rear panel of board.

I find alot of bad/cracked solder joints on the switches and jacks.

I've re-soldered a bunch of points thus far. Mostly in and around the pentode/triode switch since thats the main trigger point of the issue.

[pdf64]

Traditional sockets with flying leads. Grid and screens are on PCB.

The 'grid stopper' action will operate to best effect with the resistors mounted as close as possible to the socket terminals.

[nickb]

That's easy enough to check for sure. The only thing that would make me think otherwise is the amp has worked for 20 years like it is.

OK. It was working. No changes were made and it started oscillating one day. This kind of problem can be tricky to fix as you have one part of the circuit feeding back to an earlier part and there are many different ways for that to happen. There are more diagnostics you can do to find out what parts are involved and that will help narrow it down.

You've identified that the pentode/triode affects the problem. In pentode mode the gain is higher so this no real surprise. So I would not be at all surprised to learn that if in triode mode and you turn the gain up further it starts oscillating again. Anyway, what it does tell you is that the feedback is starting after the power tube plates somewhere. It doesn't tell us where it's getting back in. So how about you play with the controls and see what the effects are.

Some things to look for:
Are one or both channels affected?
Does the reverb level affect it?
Do the tone controls affect it?
Can you make it happen by having the gains at zero and raising just the masters or do you have to have to gains up?
Does your guitar have to be plugged in to make it happen?

Answers to these will tell us how much of the system is involved in the problem and hopefully allow us to narrow it down.

[TimmyP1955]

It would seem that some JJs* are sensitive to grid stopper placement, and will get upset if the resistors aren't directly on the sockets.

*6L6s in a Fender ToneMaster, EL84s in a Vox Vietnamese Handwired.

[mikeboone]

Ok guys. Thanks for all the input. I'm back to it today and got a short clip of the issue I'm hearing.

The amp is cranked on the clean channel. The screeching sounds happens when the amp is in pentode mode. During the video I switch to triode mode and it plays normal. It also plays normal with only 2 tubes in pentode mode.

https://m.youtube.com/watch?v=lgd153rLheU

[nickb]

Ok guys. Thanks for all the input. I'm back to it today and got a short clip of the issue I'm hearing.

The amp is cranked on the clean channel. The screeching sounds happens when the amp is in pentode mode. During the video I switch to triode mode and it plays normal. It also plays normal with only 2 tubes in pentode mode.

https://m.youtube.com/watch?v=lgd153rLheU

So, do the tests requested in post #11...

[Justin Thomas]

Sounds to me that there are 2 separate noises? The first is the "farting" around :02, and the other is the ringing sound about halfway through?

Justin