Guitar or amp?

[surfsup]

Okay I have two strange issues with a Liverpool build.

Guitar=LP I get a hum and no oscillation. HB pickups, I touch the strings or amp and hum goes away. The hum is intermittent. Ground wire in the guitar is soldered to one of the guitar pots. Connection is solid.

Guitar=Gretsch single coil. No hum but oscillation when I hit an A chord. Only the A. No hum at all.

So I assume these are guitar issues. Amp is fine iwth no guitar plugged in. With the LP, I get a hum. With the Gretsch I get an oscillation - no oscillation with the LP.

Could SC pickups cause an oscillation?
What else could cause a grounding hum with the LP?

[Structo]

It's normal for a guitar to hum a little bit if you are not touching any metal on it.
Your body acts as a sort of ground when you touch the strings/ bridge.

As for the single coil, of course those are noisy but that is strange to have it oscillate on the A chord.

I would suggest that you inspect the lead dressing in the amp, especially around V1.
For example, does the amp have the grid stopper resistor for the first input stage at the socket or on the board, or worse on the input jack?

Hmmm, I'm looking at the schematic for the Liverpool and it doesn't appear to have a grid stopper on V1a.

You might try a 22K-68K grid stopper on the input grid.
Solder it right to pin 7 (if that is your input grid)

You may also have a cold or bad solder joint around V1.
Inspect the soldering and touch up anything that looks suspicious.

[surfsup]

33k on the input socket. Shielded run to the socket. the other guitar doesn't oscillate. Only oscillates if I strum the low strings. If I play an A chord "high" its fine.

Lead dress is very nice - I know I know you need a pic... but its good. But, I'll look at it again. I already put all my amp building stuff away today, so it will be next time.

As for the hum with the LP, it happens only sometimes, and when it does it is too loud, not normal hum. I wonder if I should run a wire from the bridge to the ground lead soldered to the pot on one of the volume knobs on the guitar or something...?

[Zippy]

Could SC pickups cause an oscillation?

Are your pickups/pole pieces set high? Try lowering them.

Is this an open A (first position) or for all A chords?

[rdjones]

The guitar is aggravating the situation but I might guess that it's a chassis grounding/shielding issue.
Where is the chassis grounded in relation to the input and V1 ?
Same question regarding the speaker output jack ?

Signal coupled through the chassis by the speaker output can cause this sort of thing.

rd

[surfsup]

Zippy, the open A at 5th.

rdjones, I have a star ground at input with the input, first cap and respective cathodes for first tube and first gain ground. All other caps are stars, going to a second chassis ground near the PT.

The other guitar doesn't do this on the A...

[amplifiednation]

I'm getting the same thing on one of my guitars....on the A...I think there is some fret buzz from the winter weather.

[Phil_S]

Sometimes you get a ghost note on a particular frequency from the tubes. I have a pair of Sovtek EL84's that buzz on the A and it's very weird. It doesn't matter if it's E5 or A0, same frequency. They were cheap tubes. I keep them around for testing. The truth is, I'm not a fan of EL84. I doubt I'll build another for them, but one never can tell. It may be the combo of the tubes and guitar. Who really knows? Change the tubes and see if it goes away.

[surfsup]

Buzz only happens with one guitar, doesn't happen with my other one. So I suspect it would be the guitar? I should perhaps try repotting or rewinding the pickups. Been thinking about doing that...

[selloutrr]

Just throwing it out.. The instrument cable has been tested and is confirmed good?

[keewee]

buzz usually = ground issue, hum is 60hz or line frequency = heater wire or bad rectifier cap. I use an Ipod and fashion a cable to connect to the amp, if a buzz is still present you have a grounding issue (break the loop). No buzz, guitar wiring or cable is suspect ( see guitarnuts.com) touching the strings and stopping the buzz is normal, as is bad guitar wiring nowdays.
oscillation might be purely mechanical - use a damper on v1 v2 to help microphonics if the tube is Known Good. Just some other things to think about.