Waving hand stops squeal?

[JohnnyCrash]

OK, still hammering away at an issue... ran into some speedbumps, but now I'm back on the case.

With Vol and MV both dimed the amp squeals. As long as I'm touching the guitar strings and/or the amp's chassis AND then I wave my hand over the TMB channel's Input/Volume/Treble area (approximately a 3 inch area) the squeal stops.

Will throwing some shielding under this thing's chassis resolve this?

Nearly everything on the TMB channels Vols and Tonestack is shielded wire.

[rbruss82]

How have you grounded the channels? I once built an 18 watt that squeeled terribly when the tone pot was dimed. I then read on this forum that others had this same problem, and the solution was to have dedicated grounds for both the tone pot and the volume pot. I tried it and it fixed the problem.

[tbguitar55]

I've also heard/seen some strange grounding problems that can be fixed with dedicated grounding vs. star grounding methods.

[kd]

Try the shielding, it may fix problem and easily reversible if it doesn't work.


Ken

[rjgtr]

Pics?

[clobo]

Hi,

Mine is now dead quiet at all settings (36W-RP with both channels feeding V2).

Each of the following steps improved the s/n performance:
- Normal preamp has its own star ground point near input jacks
- TMB preamp have uses traditional bus ground ending on closest PT bolt
- PI stage has its own ground point at closest OT bolt
- Input jacks are isolated from chassis
- Shielded cables are grounded at only one end (signal feeding side)
- Cabinet bottom is shielded with aluminium tape

Claude

[JohnnyCrash]

How have you grounded the channels? I once built an 18 watt that squeeled terribly when the tone pot was dimed. I then read on this forum that others had this same problem, and the solution was to have dedicated grounds for both the tone pot and the volume pot. I tried it and it fixed the problem.

My TMB channel is grounded at the same place my Normal channel's tone pot is grounded.

I will change this tonight and see how it goes.

The PI is also grounded to the TMB channel... may have to seperate it from both channels.

I know its a grounding issue, so tracking it down is now the task at hand.

[JohnnyCrash]

Put the PI on its own ground spot. Took the TMB channel to its own ground location as well. Finally added a shielded wire from Treble pot to Bass pot.

Still squeal, but a tiny bit less.

Went through and checked the Input/MV/Vol wiring directly to tube sockets... the 470k resistors to Pin2 may be the issue.

So then I made them shorter from the shielded cable to the sockets. Tried it out and it started motorboating and/or squealing depending on if I hit a note or moved the wires.

Going to make sure the solders there are all solid tomorrow... sounds like its the issue here. Finally getting it all traced to the problem!

[clobo]

Hi JohnnyCrash,

While fine tuning mine, I played a lot with cable layout, and was only getting close... Then I found out the official 36W-RP layout was only missing one critical shielded cable.

Using a shielded cable from V1 output coupling cap, to the TMB vol input, completely stopped those oscillations.

I also found out that the shield braid needed to be connected on the board side only, I used closest turret (V1 TMB cathode ground circuit).

Also important, this shield must be left unconnected on the pot side.

Hope this helps

Claude

[Xayth]

Johnny I don't want to sound like a jack-a$$ but it was my problem. I am used to playing a 100watt turned down quite a bit. I had a lot of squealing on my 18watt until I backed off from the amp. I need to stay atleast 10 feet away. I am sorry if this is stupid and totaly wrong but it helped me.h

[rjgtr]

Johnny I don't want to sound like a jack-a$$ but it was my problem. I am used to playing a 100watt turned down quite a bit. I had a lot of squealing on my 18watt until I backed off from the amp. I need to stay atleast 10 feet away. I am sorry if this is stupid and totaly wrong but it helped me.h

That sounds like a pickup feedback issue, probably at very high frequencies. A 47p cap across the PI plates could help that or it might be a microphonic tube just at the edge of being microphonic.

[retailer]

I had a similar problem in a dual channel I built a while back, nothing seemed to make sense, chopsticks, waving hands shielded wires and different grounding all affected it but did not stop it. Finally nailed it when I earthed the secondary of the OPT, can't recall it it was the common or one of the taps.