JCM800 2203 late 80s - background crackle noise

[Greg_L]

Also can you check if meter probe at grid still kills the noise?
Maybe some oscillation being manifested as crackle, that the meter capacitance is correcting?

Probing the grid does kill the noise....for the most part. It seems like it does. If I listen really hard it might very faintly still be there under the hum and buzz from probing the grid.

The crackle does change all on it's own. Sometimes it's almost like a static, sometimes it's very faint and just few little crackles. Nothing makes it worse or better. It just does what it wants when it wants. When it's at it's worse I can flick and flip things around with a chopstick and nothing makes it change. And when it's mild I can't make it worse.

[Helmholtz]

You might try to use shielded wire to the grid of V2a. That would also provide some capacitance to ground.

[Greg_L]

You might try to use shielded wire to the grid of V2a. That would also provide some capacitance to ground.

I'll give it a try. Should I "hot shield" it? Where should I connect the shield?
If I remove the grid wire completely from V2a, the noise totally stops.


More fiddling, just throwing hail marys now....
Replaced the two 10k dropping resistors in the B+....no change.
Elevated the ground reference for the heaters....no change.
I've checked and/or replaced every single cap and resistor in this amp.
I've cleaned and reflowed every single solder joint.
I've basically rebuilt everything before the phase inverter in this amp.
IT STILL CRACKLES!

[Helmholtz]

I'll give it a try. Should I "hot shield" it? Where should I connect the shield?

No idea what hot shielding means

Connect the shield at one end to a close ground point.

[Greg_L]

No idea what hot shielding means

Connect the shield at one end to a close ground point.

Hot shielding puts the shield of the shielded cable to B+ thus sending unwanted AC noise to ground through the filter circuit. I think that's the theory anyway. Marshall did this with the grid wire older master vol amps, (I have one) connecting the shield to a plate tube socket pin, I'm guessing out of convenience.

It doesn't seem like a good idea, but hey, Marshall did it....

[Greg_L]

Well the weirdness continues....

I pulled the master vol pot completely off the board and the crackle continues. So with no path from the preamp to the phase inverter I still have crackle. Time to dig into the PI section more.

[Greg_L]

Well I've been through the entire tone stack now and PI section. Tested and/or replaced everything. Still crackles. I've gone this far, might as well redo the power section now...

[Helmholtz]

So the shielded grid wire didn't help?

[Greg_L]

So the shielded grid wire didn't help?

Nope.

[Greg_L]

I removed C9 completely - input coupling cap to the PI. At that point there is nothing connecting the preamp/tone stack to the phase inverter and beyond. The crackle was still there, although very faint. I also removed the coupling caps coming out of the PI going into the output tubes. Crackle is very faintly there under the white noise of the power section. All of the filter caps are brand new and the B+ rail is steady.

It seems like this crackle might be in the power supply and is permeating into literally everything.

[Helmholtz]

Maybe this is useful: http://geofex.co

See >Tube Guitar Amp Tech Pages>Tube Amplifier Debugging Page>Popping Sounds

[g1]

I had one once where one of the grounding lugs had poor solder. Can't remember if it was from the cathode pin of a power tube socket, or the (-) of a filter cap.
There are several mounted by nut & bolt to the chassis I think. As well as bad solder, the also sometimes get corroded where they contact the chassis.

[Helmholtz]

Arcing could be explanation, see the Geofex link.on popping sounds.

[Greg_L]

Maybe this is useful: http://geofex.co

See >Tube Guitar Amp Tech Pages>Tube Amplifier Debugging Page>Popping Sounds

Ha I've actually been through all that already. I was watching it in the dark last night. No arcs.

[Greg_L]

I had one once where one of the grounding lugs had poor solder. Can't remember if it was from the cathode pin of a power tube socket, or the (-) of a filter cap.
There are several mounted by nut & bolt to the chassis I think. As well as bad solder, the also sometimes get corroded where they contact the chassis.

Yeah I hear ya. I recapped it so I've already cleaned, tightened, and fresh soldered all the grounding lugs. I added 1 ohm cathode resistors for easy bias checks at the output tubes and that process also got the cathode ground lugs cleaned up.