Jcm800 2204 / Oscillating Whistle/high Pitch Noise When Turning Power Off

[InfernoChicken]

Hi,

already posted about my amp project, the other (solved) issue is going into another topic, so here, fresh, my newest issue. So I am building a 2204 jcm800, already kinda working. Presence does not work, i dont know if this is related to this issue. The amp makes this strange oscillating and whistling noise when turning the amp off.



Havent found anything about this specific problem. I know there is this issue when the amp makes high pitch noise when a power tube is bad.
But can you guys tell me what this could be?

 

[ampmadscientist]

Hi,

already posted about my amp project, the other (solved) issue is going into another topic, so here, fresh, my newest issue. So I am building a 2204 jcm800, already kinda working. Presence does not work, i dont know if this is related to this issue. The amp makes this strange oscillating and whistling noise when turning the amp off.



Havent found anything about this specific problem. I know there is this issue when the amp makes high pitch noise when a power tube is bad.
But can you guys tell me what this could be?

You are testing this with the bad power tube installed?
Don't do that.
You are running this amp w/ the bad tube installed? don't do that.
Put in good tubes before you run the amp.
Bias the amp properly before you do anything further.

You could damage the output transformer with that tube in there.

Post good detailed close up pictures of the whole inside with plenty of light.
Maybe we can see what's going wrong...

 

[Gunner64]

And you already know the presence control doesn't work. I would figure that out first..retrace your steps, check values..so easy to mistake a 470k resistor for a 470 ohm resistor.. ect...stuff like that is pretty common.

 

[ampmadscientist]

And you already know the presence control doesn't work. I would figure that out first..retrace your steps, check values..so easy to mistake a 470k resistor for a 470 ohm resistor.. ect...stuff like that is pretty common.

Sometimes the presence does work, but you might not hear it working.
The speaker can limit the high frequencies, and make it seem as if the presence is not working.
(believe it or not)
Hooking the amp to a hi fi speaker, and playing music CD for example.
Then you can really hear the presence working, if it does work at all.
Yeah, some guitar speaker can kill the very highest response.
It could fool me.

But true, maybe the presence does not work at all.

 

[Chris-in-LA]

Hi,

already posted about my amp project, the other (solved) issue is going into another topic, so here, fresh, my newest issue. So I am building a 2204 jcm800, already kinda working. Presence does not work, i dont know if this is related to this issue. The amp makes this strange oscillating and whistling noise when turning the amp off.



Havent found anything about this specific problem. I know there is this issue when the amp makes high pitch noise when a power tube is bad.
But can you guys tell me what this could be?

I did not hear you say that you have a bad power tube. But I also did not hear the problem in your video aside from a pulsing noise when you turned off the amp. Is that what you were talking about?

 

[InfernoChicken]

I did not hear you say that you have a bad power tube.

I never said that I had bad power tubes (because I dont know), I was just relating to a similar case I read about. My power tubes are brand new...

But I also did not hear the problem in your video aside from a pulsing noise when you turned off the amp. Is that what you were talking about?

Yes that weird pulsing is what bothers me. Sometimes this high pitched noise also occurs when I turn up the master or preamp (it is static then, not pulsing), hence the fiddling at the knobs. This strengthens my suspicion in the power tube direction.

Could defective power tubes generate this kind of pulsing noise?

 

[Gunner64]

Are there fluorescent lights, or anything in the area that could cause noise?..electric motors ect..also try it in a headbox, I have heard anps make some weird noises out of the headbox, but be dead silent in the box.

 

[Chris-in-LA]

I never said that I had bad power tubes (because I dont know), I was just relating to a similar case I read about.

Right! It seemed like some people here thought you said that you had a bad tube. I was confirming that you did not say that.

 

[J5684]

Sounds like a filter cap whining almost... Does it do it when you turn it off without hitting standby off?

 

[InfernoChicken]

anything in the area that could cause noise?

nope.

unds like a filter cap whining almost... Does it do it when you turn it off without hitting standby off?

the first thing i thought of was that this noise should be related to a cap (rapid charge and discharge being amplified, hence the pulsing?) I will investigate in the behavior more tomorrow.

One other thing that i noticed today: I tried to play guitar a litte (yes i know this is stupid). it really sounds great as long as i dont exceed the master/preamp threshold at which the noise starts, and it also seems to be very sensitive to loudness in general, because if I hit the strings a little harder like for a nice chord, there is this very loud noise again for a splitsecond.

I think all these phenomenons correlate to the same error, as the pitch of the noise is always the same like in the video. Any ideas?

 

[mickeydg5]

Could be a bad connection having high enough resistances to cause regenerative feedback.

 

[J5684]

Just noticed that this was a continuation from your other thread.

Sorry but you need to clean up that wiring some. Believe it or not one simple wire running to close to another can cause this and other weird issues here a good quick article http://www.geofex.com/article_folders/lead_dress/lead_dress_in_tube_amps.htm

Otherwise it seems to me that the board you have is probably defective somewhere hence why it was sold/received on eBay. If it already had a short on it and now this problem. The best thing to do is trace the circuit for the presence control and to be honest, I'd scrap the whole dam thing and start over point-to-point and fix your lead dress.

 

[InfernoChicken]

Post good detailed close up pictures of the whole inside with plenty of light.
Maybe we can see what's going wrong...

See photos attached. Still cant find the error on the presence.

Sounds like a filter cap whining almost... Does it do it when you turn it off without hitting standby off?

I tested a bit until the amp scared the shit out of me. Ok, so I noticed that the pulsing sound after power off is more persistent the shorter the amp is on standby before switching off. So

• if I hit Power off without putting into standy, the noise lasts a few seconds longer and its louder.
• If I hit standby and wait for like 5 seconds before power off, it is a bit shorter and quieter.
• If I hit standby and wait about 10 seconds before power off, no pulsing noise.

And then. If I power off and instantly power back on there comes a brutally loud noise, not high pitch, more like a ship horn. almost crapped my pants. something is fucked up bad.
Anyone see an error on the pics?

 

[J5684]

Ya take a look at your lead dress on V1 and V2 for starters. You'll need to separate all the different wires (HV, signal etc) before going any further in my opinion.

 

[InfernoChicken]

Believe it or not one simple wire running to close to another can cause this and other weird issues

I would have thought that this may lead to some hum or unwanted distortion. I would classify the issues I have into malfunction?

 

[ampmadscientist]

I would have thought that this may lead to some hum or unwanted distortion. I would classify the issues I have into malfunction?

Put good tubes in, then re-test the amp.

Stop running the amp with a bad tube installed.

 

[InfernoChicken]

Stop running the amp with a bad tube installed

Brand new tubes!

 

[ampmadscientist]

Brand new tubes!

Can't see the wiring of the input jacks.
Pull the jacks and take close up pictures with plenty of light.

Must see all the connections clearly on the input jacks.

Possible wiring error on input jacks.

 

[67Mopar]

I ran into something like this on a mission to Talos IV. We were lured there via a false distress signal. A real disaster of a mission! Anyway, I could be wrong, but it sounds to me like the amp is building up an intermittent forced-chamber overload. You need to check the fusion buffer coupling at the dilithium converter / phase-shift nest assembly. NOTE: Be sure to remove the dilithium crystal, prior to disassembly.

 

[ampmadscientist]