DC on guitar

[Reeltarded]

Haven't metered it, but I can feel it. I can also hear it scratchy on the guitar volume. When I turn up or down, the hiss changes. Wide open guitar, clear amp. Guitar off, raging hiss. Gain 2 goes "woooo" from 7-10, increasing pitch a little.

It pretends to be bad lead dress in first stages, or floating reference from 1M at input.

I know where to go with this. Coupler on V1a after swapping 1st tube if no change. Right?

I killed another mustard cap, didn't I? Darn.

Make bets. I might get to this later.

[Colossal]

How is the first stage biased? super high gain? If so, try a fat cap in series with the grid on V1a (0.1uF or larger). End of DC on volume pot.

[Reeltarded]

Nah, 2.7k. It acts like a bunch of different problems depending on the moment.

I can feel tiny bites when any small edge of hardware touches soft parts, like inside wrist, edge of palm.

It sounds like rf garbage then you turn the guitar down to zero and hiss turns to viagra falls. Whhhhshhhhhhitty. lol

[Colossal]

I can feel tiny bites when any small edge of hardware touches soft parts, like inside wrist, edge of palm.

Cripes dude, if you're getting zapped, that's a bit worrisome. Sounds more than a little grid current making the pot scratchy. Don't git yerself kilt!

[John_P_WI]

Dude, you don't have the "hot shield mod" on the input shielded cable do you?

Any how do the fat cap trick that Colossal suggested.

[thejaf]

In all seriousness, make sure to check power cord. I had this happen to me once on on old Marshall 50-watter. Someone in the distant past changed the power chord plug end, and didn't make a good connection on the neutral line. Thus the chassis itself was live, with current returning through the ground wire.

Same symptoms you describe, which I dismissed as minor until I plugged into an outlet that wasn't properly grounded and saw lots of sparks... that's when I found out the root problem.

[tubeswell]

What thejaf said. You most likely have a bad/floating chassis ground (and its hitting every part of your geetar that's connected to the chassis)

[Reeltarded]

No shield trick, just shield to ground at jack.

This is the skydiving amp. I worry about throwing a cap at the input because it was fine before the fall, I just need to keep looking. Hmm..

[jodie]

Try another V1, I had this happen with some of my RFT ECC 83.

Best

Jodie

[Colossal]

I worry about throwing a cap at the input because it was fine before the fall, I just need to keep looking. Hmm..

Well, if you are getting DC under your fingernails, that's not just a little grid current. I would get to the root cause of it too before band-aiding it with a cap. Are you getting measurable DC on the chassis? Have you probed the other ground points in the amp? Did your PT get parallelogramed when the amp hit the ground?

[Reeltarded]

I am measuring a tickle at the guitar. lol

I just got home for the second time today. I am gonna remove all the couplers to the tone stack and swap them for sozos I have, and while I am in there I think I'll move all the grounds to the top of the board for later ease of hack-n-chop. Underboard wiring is for fortune tellers.

I swapped the tube at lunch. Wild hiss when the volume is off at guitar, no hiss when volume is even nudged to barely audible. Still tickles.

[tubeswell]

Read my previous post, and Colossals.

The guitar strings/bridge etc are connected to the amp chassis via the guitar cable shield.

If the amp chassis is not properly grounded to an earth ground, then the ground return in the amp (which is connected to the amps chassis) can cause the chassis DC potential to drift to a higher voltage, which you then connect back to ground with a path - through your body - when you touch the geetar strings/bridge metal etc - which causes the 'tickle' (if its a small DC drift, or worse - if the voltage gets higher). So look to the amp's chassis safety ground first.

If you have tried to block the DC-return path between the amps chassis and the guitar cable shield with a cap, you may not have done that properly, because you are obviously still getting DC potential on what should be earth-grounded parts of your guitar.

[Lindz]

Is this the amp that hit the floor?? Dood where's my bias??

Could very well be something jarred loose like neutral as described earlier

[stephenl]

I ran into this same issue on an Express build. I changed the first stage to a cascode. I didn't have any issues with a 12AU7 in that position. I tried a 12AV7, which sounded really good as a cascode by the way, and I had scratchy guitar pots. I was going to try a cap in front, as Colossal suggested, but I haven't got around to it yet.

[Reeltarded]

Done!

It was a cold joint at the jack. 1M was in solder but it could wiggle. (and I changed the first coupler for good measure at the same time)

While I was in there, I wired it as that SIR 34 thing and that made it sound like crap, but that is a whole other (peaky) story. Ewww. With the lights off and the guitar plugged in you could generate microscopic lightning with a screwdriver.

Building an amp is a dood thing. Knowing what the hell is going on in there is a rocket dood thing. Crap!