Multi Meter Provides a Path to Ground?

[jewishjay]

Please help diagnose a weird problem with a JCM 800. The low input works (though its really quite low) but the high input crackles out like a tube going into cut off, and sounds like trash. I was probing and checking for DC at the preamp vol pot....and when I touch the meter to the wiper or input side of the pot, everything behaves and sounds great! Like the meter is providing a path to ground, maybe?

I wonder if the problem is in the jacks or the pot? Some kind of grid leak issue? I measure 1M the ground on the high input, and 1.4M to ground on the low input...

[pdf64]

It may have been oscillating, then probing it changed conditions and stopped the oscillation.

Does it go into the fault mode repeatably?

[shooter]

everything behaves and sounds great! Like the meter is providing a path to ground, maybe?

but the high input crackles out

Pretty good indicators that you have a crappy solder connection

[jewishjay]

It may have been oscillating, then probing it changed conditions and stopped the oscillation.

Does it go into the fault mode repeatably?

yes. i tried swapping tubes, some are worse than others. some do it earlier in the volume pot....some later. but it always does it.

[jewishjay]

everything behaves and sounds great! Like the meter is providing a path to ground, maybe?

but the high input crackles out

Pretty good indicators that you have a crappy solder connection

sorry, not a crackle like a bad connection, but like a tube going into cutoff.

[pdf64]

Hard / pointless to go any further without a schematic, layout and photos.

[jewishjay]

yup, youre right.....

[pdf64]

Any photos of it?

You could try a low value cap, eg 100pf, from the 22nF at the input stage anode to 0V common.
eg at the low input jack.

[PRR]

...sorry, not a crackle like a bad connection, but like a tube going into cutoff.

A bad connection can let a tube go into cutoff.

[jewishjay]

You could try a low value cap, eg 100pf, from the 22nF at the input stage anode to 0V common.
eg at the low input jack.

SOLVED with a 100pf from the preamp volume wiper to ground. Thanks PDF, I'm glad its fixed. But....why? No matter what tube I tested in V1 it would oscillate. What causes that?

[pdf64]

Imperfections in layout / lead dress.
To some degree, capacitance couples all conductors in the universe together.
Inside an amp chassis, such coupling is greatly exacerbated.
All that’s required for oscillation to manifest is a gain of 1 and phase alignment.
So given the high gains, polarity flips, tone control phase shifts and high impedances in a valve guitar amp, it hardly takes much capacitive coupling for a positive feedback loop that satisfies the above criteria to be met.
Photos of the build?

[Willabe]

Read this if you haven't yet, it may help you;

Lead dress;

http://www.geofex.com/Article_Folders/lead_dress/lead_dress_in_tube_amps.htm

[jewishjay]

Photos of the build?

[pdf64]

The board mounted grid stopper offers simple / obvious scope for improvement. ie it would be better to site it at the valve socket. With the wire to it, from the input jack, being kept well away from the pin1 anode circuit / wiring.