Finished a new JTM45 - High Pitched Treble whistling?

[jrewing]

Hi Guys,
I just finished my newest JTM45 and it sounds great. However, when I am using the bright channel I get a high pitched ring if I have the volume, treble and presence dimed. If I have the volume and presence on 10, it will start making the noise when I go above 3-4 on the treble. It does the same with the mid. I can also have the volume, treble, and mid on 10 as long as I keep the presence lower than 7-8. The normal channel has no issue. I did use a 47k and a 500pf rather than a 56k and 250pf on the tone stack. Any suggestions would be appreciated.

Thanks,
JR

[Haze13]

Some thing is oscillating. Small caps are good? Can you post some HI-RES pictures?

[jrewing]

[img][IMG]http://i302.photobucket.com/albums/nn88 ... G_1680.jpg[/img][/img]

[img][IMG]http://i302.photobucket.com/albums/nn88 ... _16781.jpg[/img][/img]

[img][IMG]http://i302.photobucket.com/albums/nn88 ... G_1676.jpg[/img][/img]


Thanks,
JR

[Haze13]

The thing is that I've had a similar problem with a Plexi type preamp and a SMPS for power supply... At first I though that the problem is in a power supply, since a jump from 12v to 420v is a very big, and there is no transformer (SMPS power supplies are very noisy, a specially when you try to build them and not fully understand what is going on there) BUT, when I used different type of a preamp - JCM800 the 78 version with only 2 gain stages and CF + tone stack - the problem gone away. Then I started thinking, and after some searching on the net, I've come across some people that claim to have a similar problem in a regular build (transformers for B+). The problem was a 470pf cap. Some times, may be because of the grounding problems, bad filtration or some thing else, it starts to behave like an oscillator. You have same cap, well a smaller one (220pf) on a grid, on the channel that does the same thing. If I were you, I would start from there...
By the way, you have a very clean work and I really sorry to hear that you have a problem. There are some very ugly looking insides of a DIY builds and no noise whatsoever!

[Haze13]

One more thing... Number of times I wrote about it, since I had a problem in my JCM800 amp build (no smps).
BUSS WIRE ON THE BACKS OF THE POTS. Chassis - grounded. Pots mounted on this chassis are grounded too, because they are metal and they are touching the chassis with their metal bodies. Then you put on their back a buss wire that's grounded too! Now you have very good looking ground loops. Chassis - Pot - Wire - Pot - Chassis...
I could listen to a local radio station with that thing. After I removed it, the problem was gone.

[DG45]

The buss wire across the back of the pots keeps the pots from spinning if the nut get's a little loose. Mine are done like that, but I have no noise due to it. I'll keep the buss wire in mind if I ever do.

[danman]

Did you use 5.6k grid resistors at pin 5 on each power tube socket. They usually help to squelch any oscillation. Could be a lead dress issue that may require some chopsticking also.

[jrewing]

Ok,
So I was thinking of chop sticking it, however, I cannot recreate the sound unless my guitar is plugged up and the tone knob on the guitar is up. Without the guitar plugged in, it just makes the normal sound an amp would make if it were turned up to 10. I'm gonna try another guitar to see if that's the issue. Should it be making the high pitched noise without a guitar plugged in and turned up? Also, I misspoke earlier. I actually used a 390pf and 47k on the tone stack trying to achieve a sound between a plexi and jtm45. Maybe that was a bad idea. Any thoughts are appreciated.
Thanks,
JR

[Doug H]

is this just standing too close to the amp when it's on the bench?

[axeman]

Hi Guys,
I just finished my newest JTM45 and it sounds great. However, when I am using the bright channel I get a high pitched ring if I have the volume, treble and presence dimed. If I have the volume and presence on 10, it will start making the noise when I go above 3-4 on the treble. It does the same with the mid. I can also have the volume, treble, and mid on 10 as long as I keep the presence lower than 7-8. The normal channel has no issue. I did use a 47k and a 500pf rather than a 56k and 250pf on the tone stack. Any suggestions would be appreciated.

Thanks,
JR

A bad pot, also how about the 470 ohm resistor? is it 470 or 470k. I don't see any swamp resistors.

[jrewing]

Thanks for the help guys. I am still having the issue. I double checked the 470 resistor and installed swamp resistors. The interesting thing is that when I turn the amp over to chop stick, the sound goes away. The high pitched squeal comes when I turn the presence, bright volume and treble to 10 and apply a very small amount of volume to a guitar that is plugged in. There is no issue when a guitar is not plugged in. It is only the top input as well. Also, I can raise the input side of the chassis while this noise is happening and the noise will stop at about 45 degrees! I have double and triple checked everything. The noise gets slightly quieter when I move the amp off of the speaker cabinet. Any input would be greatly appreciated.

Thanks,
JR

[Haze13]

There is a sheet of metal on the bottom of the head box... Try to place a kitchen foil under the chassis. Fold it couple of times and then place maybe a towel then a foil and then a chassis of the amp. This way you can be sure that you covered it very well... You know, the open wah wah pedal starts to pick up so much noise, but when you cover it, it goes away? Same thing here. May be...

[danman]

I was gonna suggest the same thing as Haze, make sure that you have a metal shield of some sort covering the open chassis. The head box should have the shield but if you are testing with the chassis removed you may need to cover it. Have you tried adding the 5.6k swamp resistors to pin 5 of the power tube sockets? They will help to squelch oscillations. It also appears from the pics that the grid lead for v5 wraps around the PT end of the chassis while the one for v4 leaves the board and goes straight to the socket. Is that just an optical illusion or do you have it routed around the back of the PT? The last thing I would try would be to move the wire running to the presence pot away from tone control wiring. Run it out of the end of the turret board and then straight to the presence pot.

[jrewing]

Thanks for the help guys. I did just as you suggested. A folded layer of aluminum foil, a towel, and another layer of aluminum foil. That seemed to do the trick! Anyway, the last head cab I ordered was from Mojo I think. I don't remember it having a metal sheet/plate. Do all or most cabinet makers add this? Does Mojo do this? Any suggestions on where to get the cab?

Thanks,
JR

[Haze13]

Bought mine from TubeDepot, no metal plate... You have add it by your self. Shame, Shame, Shame... It has to be there!
Fender use thick aluminum adhesive foil in their amps. You can do some thing similar...
Just do not use the trick with a kitchen foil in an amp. Use more reliable and thicker material.
Copper tape for shielding a control cavity may be useful.