Fluttery/rumbly/rubber bandy type sound on JTM50

[rgorke]

I am having an issue with my JTM 50. It is the sustain of a chord that flutters during the sustain of the chord. The presence control seems to have the largest affect on the issue. Turning the presence all the way off is the best. It happens regardless of how loud the amp is and happens whether the volume is on 10 or on 2. One thing I have noticed is that I have 10-12 vdc on the presence control, on the 0.1 NFB input to the Phase Inverter (the side connected to the 10k resistor and on the NFB resistor. I am not sure if there is supposed to be DC voltage on the control itself.


Here is what I have done:

Changed all tubes
Changed Guitars
Changed Cabinets

It happens regardless of these changes.

Here is a clip of the noise...listen to the "instability" in the decay of the chords.


http://soundclick.com/share.cfm?id=13364631

[danman]

It is normal for that type of presence control to have a small amount of DC voltage on it. That is what causes the scratchy sound when turning the pot. The later presence circuit helped to eliminate the problem. As for the fluttery sound, have you been able to take any voltage readings while the sound is occurring?

[rgorke]

Yes, and it appears the plate voltage drops when the chords are struck then recovers. I am sitting at about 410 vdc on the plates, down to 390 or so then recovering to 407-410. It doesn't appear to fluctuate.

Is that what I would be looking for?

[danman]

That seems normal. I really can't think of what might cause that slight warble at the end of the chord. Maybe someone here will have experienced that issue before.

[rgorke]

That seems normal. I really can't think of what might cause that slight warble at the end of the chord. Maybe someone here will have experienced that issue before.

Boy, you're a lot of help....lol

yeah it has been a really frustrating issue. Not sure if I have fixed it or not but turning down the presence really helps. That tells me it is a NFB and/or PI issue.

Also amazing how much lead dress and wire routing affects the stability of the amp. These are pretty close to living things when they are on...

[danman]

That may be a tough one track down. It is hard to hear it properly in the recording though I'm sure it is much more evident if you are in the room with it. My first thought might be something in the filtering...are the caps fairly fresh? Are you running lower than normal filtering? It may be helpful to scope it if you have access to one.

[rgorke]

I am posting here rather than cluttering up Shatki's thread.

Here is a video:

http://www.youtube.com/watch?v=9XwPnWEYLuk


All I fiddled with is the volume on the guitar and the volume a little on the amp. The higher the volume/gain the more the distortion is masked.

[rgorke]

I did some reading and searching here on the Metro Forum now that it is back up. One thread talked about the bias of the power tubes. So, I cranked up the bias to the power tubes and that really seemed to help A LOT!!

When I say cranked, I mean as high as it would go which was 85 mA at 390 V on the plates.

Does this give us any clues?

[shakti]

Got any photos of the amp?

Youtube/iPhone recordings aren't the best to capture these more subtle problems, but I think I can detect a slightly papery/harsh edge/fizz to the notes which does seem somewhat similar to the problem I am experiencing with my 68SL build.

Like danman said, an oscilloscope would definitely be useful. Are you running a tube rectifier?

[rgorke]

I'll have to take some updated pictures.

No tube rectifier. I am using an all in one bridge rectifier.

The tubes are Sylvania 6CA7s.

[rgorke]

Here's an updated video of the ongoing problem. I built the amp about 2 years ago. Sounded great when I built it. This noise started about a month or so ago. In my mind, it seems like something must have burned out/broken and/or died, etc...or could a lead dress issue come up at a later time?

I have checked, switched, and/or replaced just about every component except both transformers, choke, and rectifier.

I haven't checked or changed the swamp resistors. Court they cause a problem like this?

http://youtu.be/D4Y8iOCF244

[Haze13]

Strange... I've had a similar problem, or it may seem like this one, because of the YouTube. My cable was bad The shield was falling apart and that was the cause of the problem. Try to move your cable wile amp is on, if you'll hear any noise than it must be it. Try different cable...

[rgorke]

I would think you were on to something but I can plug straight into a different amp using the same cable and no problems. Here is another video of the dreaded noise....



http://youtu.be/D4Y8iOCF244

[shakti]

Listening intently to your clips again I do think it is quite similar to the issues I am having with my 68SL. I hear a faint, buzzy, harsh slightly crackly/fizzy breakup around the notes, and it feels slightly "congested". I think my own amp is worse, but that could probably also be because of the circuit having more gain and thus exacerbating what I hear. Photos are always a great help, if only to jog the mind looking for areas to troubleshoot.

When you say you have replaced nearly all the components in the amp, can you be a little more precise? Which ones have you replaced, and which ones have remained the same?

[rgorke]

Thanks Shatki's!!

Replaced:

Coupling caps
Preamp filter caps
Screen, mains caps (caveat...I replaced with older caps.) I just picked up 2-50/50uf caps and 2-32/32uf caps to change those out.
I have checked all resistors for proper values
Checked all pots for proper values

I will recheck all grounds and install new filter caps in the next day or so.

I will post pictures...I keep forgetting to do that.