Loud crackling noise and no signal

[Helmholtz]

There is a clause. If your amp does not have a three prong power cord or is improperly wired for chassis ground there's a small possibility that even the chassis could be live. In that case you would want to avoid touching both the innards and the chassis with bare hands until a zero voltage reading is taken from the chassis itself. That would be done by measuring for DC between the chassis and your home ground using the red probe on the chassis. the ground hole on any AC outlet. Use an extension cord if needed.

I think you mean measuring AC not DC.

[Chuck H]

Good call. But I actually did intend this as a DC measurement. If the chassis lacks a ground there would be a danger. AND there would be a danger for AC as well. Best to test for both in this case. Though I would think that a high AC on the chassis would present loud, unmistakable hum.

[Helmholtz]

But I actually did intend this as a DC measurement.

Please help me to understand how a DCV could get between chassis and home ground.
What kind of fault are you thinking of?

[Chuck H]

Well I may have failed to consider some things. I had thought without a "safety ground" that any component after rectification that shorted to the chassis could do that.

[Helmholtz]

Well I may have failed to consider some things. I had thought without a "safety ground" that any component after rectification that shorted to the chassis could do that.

When 2 points inside an amp are shorted, the voltage between these points becomes zero..
The DC potential of the chassis won't change and the internal short can't produce a DCV between chassis and a disconnected home ground.
I order to get a DCV between home ground and chassis, a DCV between neutral and chassis would be needed.
I've no idea how this could happen.

[Chuck H]

Thanks Helmholtz. I'm still wrapping my head around around it but I did see that when I examined the basic Champ schematic in this thread. I modified it to represent my scenario (first filter cap shorted for the example) and I think I'm seeing what you're explaining. Red line represents the chassis. Everything is at the same "potential" if that makes sense. However, What if the user were to contact both the chassis and a 0V contact? Please undertand that I'm clever but not scholarly

[Helmholtz]

However, What if the user were to contact both the chassis and a 0V contact?

Shorting B+ to chassis kills the DCV. The chassis DC potential relative to something connected to home ground does not change, as there's no DCV source between chassis and mains wiring.

But if the death cap is leaky and connects to hot/line, there will be an AC fault voltage between chassis and neutral.
As neutral is connected to home ground, they share the same potential and AC voltage appears between chassis and home ground.

[Chuck H]

Ok. I think I get it. I have no electronics education but my mechanical aptitude is alright. As an analogy I'm picturing a battery. If you short the terminals with a wire the potential across on that wire is zero. And if connect that shorted wire or only only one terminal of that battery to ground you still get zero. Thank you Helmholtz.

[Helmholtz]

It is sometimes overlooked that a single point or conductor may have an arbitrary potential but it can't have a voltage.
A voltage only exists between 2 points having different potentials.

Also a current between 2 different potentials can only flow if there's a return path. So if one of the potentials is floating/isolated, no (load) current is possible.

[nosaj]

It is sometimes overlooked that a single point or conductor may have an arbitrary potential but it can't have a voltage.
A voltage only exists between 2 points having different potentials.

Also a current between 2 different potentials can only flow if there's a return path. So if one of the potentials is floating/isolated, no (load) current is possible.

Which is why you don't attach to buildings together with any kind of wire. The ground potentials are different an increases a chance of lightning using that avenue. See it a lot around here in the early 90s with bank tellers windows that were separated from the main bank . Fiber optics cured that issue.
nosaj

[g1]

Is there no fault scenario that can disconnect circuit ground from chassis and put DC on the chassis? (assuming also that chassis not connected to 3rd prong home ground)

[Helmholtz]

Is there no fault scenario that can disconnect circuit ground from chassis and put DC on the chassis? (assuming also that chassis not connected to 3rd prong home ground)

Not a realistic scenario I could imagine.
It would require a DCV getting on the primary side of the PT.

[Chuck H]

With Helmholtz help I saw the reasoning, but...

I ran some simulations anyway. I can't find a way to create a DCV on the chassis with any typical build typology that is a likely scenario. That said...

I did (do) have a particular fear of electrocution. I made a mistake in my greener days that has stayed with me. I won't go long but some parts of the tale are remarkable. Like I was blown over a chair that was left standing, I had crushed two frets on my guitar down to the level of the fingerboard and that had cut through two wound strings breaking them. And my fingers weren't even cut! There was a pea sized nugget charred in the inside of my left index finger which is completely healed now. I conveyed this tale to the guys at Guitar Showcase in San Jose, California and earned the nick name "Sizzlin' Chuck" . I wish it had been for my playing instead.

[g1]

I wasn't thinking of any likely situations, purely academic. I thought I recalled something about DC faults getting 'out' of the amp but maybe it was through the hot of the input jack.

[Mick Bailey]

We still don't have the power tube voltages.

The other thing that I'd do is to ground the grid of the 6V6 (pin 5) with an insulated clip lead to fully isolate the preamp & coupling cap. Safest way is to do this with the amp powered off and switch it on with the clip lead in place to see if the noise goes. If the noise is still there then its either the power supply or related to the power tube circuit.