Help Troubleshooting unusual problem (for me)

[jlatrace]

First let me start by saying I really appreciate the help I received from this site. Also I enjoy trouble shooting, however, this one has me stumped and I have learned to ask for help before becoming frustrated.

I finished my TW last week and have had it running noticing no real issues until tonight. Tonight I noticed that the amp was building up a small static electricity charge that would discharge when I touched the amp while holding my guitar strings. It would return only after leaving my hands off the for several seconds (not immediately like a poorly grounded guitar). It is noticably stronger when I touch my Trainwreck attenuator while holding my guitar strings (small ouch one time). It occurs regardless of the guitar and my guitars are humbuckers (usa 335 and les paul, guitar is not the issue)

I have no hum, AC polarity is ok and my earth ground is secure. My noise level is very low and my grounding solder joints look solid. Any ideas where to go from here?

Thanks,
Les

[nickt]

Check your earths again!!! you should have continuity from guitar strings to chassis and chassis to mains ground. All exposed metal on the amp (trannie cases, input jacks, tube shields, pot shafts) should have continuity to the chassis. Continuity means very low ohms - like zero on your meter. BTW no offense intended - I don't know what you know.

What you're describing sounds like somethings leaking volts and it's ending up in your guitar - presumably you don't want yourself (or more importantly someone else) to become dead prematurely because of your amp

I wouldn't play or let anyone else play this amp until your problem is fixed - unless of course you don't like them...

[jlatrace]

Thanks I ll recheck earth ground.

Do you or anyone else have any suggestions about the following troubleshooting plan or possible source of the problem?

Since I've already checked voltages, my plan is to start pulling tubes to see if I can determinne where the problem is or is not. Once isolated, check and resolder all connections in that section. After that I wonder if there is something short of gutting the section I could do?

[Ron Worley]

Again as Nick noted, pull out the DVM and measure resistance from your guitar all the way to the ground from the power cord- zero ohms is the ticket.

I'd guess you have some sort of ground loop in the amp, but YMMV...

Ron

[jlatrace]

Thanks Ron. Your reply clears up my misreading of Nick's response. I will check for 0.0 resistance from my guitar to all of the ground points to identify those with resistance and then look for the problem in that part of the circuit.

Nick,
Sorry for missing your earlier advice. I read the post too quickly. Thankyou.
Les

[jlatrace]

Ron & Nick
Thanks so much for the help, both of you helped me find my mistake. Ron, you (and a friend ) properly diagnosed it as a ground loop. Also the input grid tube pin was too close (not touching but almost) to the cathode pin on V1. I suspect the cathode cap was building up and discharging when I would touch the guitar strings and the amp since the issue didn't start until after I added a 33k resistor to address RF noise.

The way the ground loop issue happened is that I wired the grounds some time ago to a buss tied to the PT. Later I experienced RF issues and I read about it here and switched my buss grounding scheme to grounding the preamp and phase inverter at the input jack and the power section and recto at the PT. However, when I made the change I forgot to remove the buss connection to the PT bolt....a mistake that I thought I never would make and so I never saw it (although it was right in front of me). Interestingly though the amp was wisper quiet.

Anyway, thanks for helping me.
Les