Strange noise in my JMP50

[Doc_Brown]

Hi everyone !

I need your help on that point because this noise is running me crazy for mounths...

One of my friends, more exprerimented than me, build for me a '68 JMP50 clone with PPIMV, based on this layout :
http://home.comcast.net/~jbjdav26/1987PPIMVr17.jpg

===================
EDIT : Pic from inside
http://imagesup.org/images12/1397300303-p1030733.jpg
===================
Transformers are from classic tone, I really love the sound.

The amp is a killer the sound is amazing. But there is a strange background noise a kind of buzz while I'm playing. Here what I try, whithout any improvement :

• change tubes
  change jack
  try another power outlet
  check solders
  try with another guitar (of course)

everything seems to be correct. After all those trys I discovered that the noise was comming from vibrations. The head is on a 4*12 cab and vibrate with speakers, causing the rattling noise. There is no tube microphonny. When I tap with fingers on the head it make some noise BUT only if the head is in it's headshell...

see :


===============================
EDIT : precisions on the video :

On the video you can't hear the background noise that I talked. The buzz on the video is due to the fact that I was just in front of the amp at high volume with a strat. A half meter away from the amp there is no grounding noise at all.

In fact when I play, the vibrations are acting like if someone was taping constantly on the headshell, creating a kind of "Krrrrrrrrrrrr"... The noise is here ONLY if I play. If I don't play notes, no noise at all. That's why I'm taping on the headshell on the video.

There is no tube microphony either. I have checked them and try 3 different tube sets, always the same.
===============================

As you can see in the video I tap on the head shell slightly ! The noise seems to be lower as the amp gets hot but never vanished. You have to be on the playing position (not in standby) to hear the noise, with a guitar plugged in with volume on.

Any Idea ?

Thanks !!!

[neikeel]

The background hum sounds like a grounding issue, the tapping effect sounds like a tube being microphonic.

Suggest you check the quality of the internal work for dry joints, the earthing scheme used (all the nuts of the star washer type ground lugs tight. Make sure each section of the amp has a proper ground (see the Larry Ground threads).

Internal pics might help

[Doc_Brown]

Thanks for your reply.

In fact on the video you can't hear the background noise that I talked. The buzz is due to the fact that I was just in front of the amp at high volume with a strat. A half meter away from the amp there is no grounding noise at all.

In fact when I play the vibrations are acting like if someone was taping constantly on the headshell, creating a kind of "Krrrrrrrrrrrr"... The noise is here ONLY if I play. If I don't play notes, no noise at all. That's why I'm taping on the headshell on the video.

There is no tube microphony either. I have checked them and try 3 different full tube sets, always the same. I'll edit my first post to precise those points.

I'll try to post internal pics ASAP.

[neikeel]

Even more likely to be a poor solder joint

[Doc_Brown]

Yep. What is strage is the fact that outside the headshell I dont have that noise... Maybe something is pressed a bit in the headshell making a poor solder joint rattle ?

[Doc_Brown]

I edited my first post with a pic of the inside.

[neikeel]

Those pictures are part way through (near completion?) as there is a certain amount of solder spatter and a bit of bus wire lying on the board.

Looking at the inputs and the bus bar on the pots these joints are a bit blobby - the solder needs to flow into the joints and the earth wires the solder needs to merge with the bus bar. If it were mine I would go through every joint and make sure it flows, then mark the joint with a sharpie or marker,

Keep us posted.

[Doc_Brown]

Thanks. I'll check this tomorrow and try to make new pics. This one was taken during the building of the amp. It's cleaner now.

I'll keep you in touch, thanks again.

[Doc_Brown]

Hi everyone !

I finally found what was causing this noise, thanks to neikeel advices. It was a bad solder join on the #3 pin on the V2. No taping problem anymore.

I still have a little rattling background noise but it's verry light and only at high volume / high vibration level. I think that some screws need to be tight up (especially grounding srews). I don't have the correct tool yet (I dont have the correct size) but I'll check it as soon as possible.

No noise at my usual playing level is at least a victory !

Thanks a lot !