Bandmaster Reverb arcing rectifier AA768

[Mr_bibbles]

EDIT: Correct schematic: https://cdn.shopify.com/s/files/1/23...97210715640943

I've been beating my head against the wall trying to remove the arcing from a GZ34 in a bandmaster reverb; It arcs as soon as it comes off standby.

I've run into this problem before and fixed it but in this case, it's become a no-go. At this point it has all new filter caps, protection diodes on the rectifier, a NEW rectifier, new power tubes (the old ones were arcing as well)

Things I've tried;

1. Replacing the Filter caps (they were leaking, seemed like the obvious choice...)
2. "Yellow sheet mod" 1n4007 on HV nodes of rectifier
3. swapped to a new (expendable) rectifier, still arcing. Hesitant to pop a new one in until things are cleared.
4. Dropping the value of the first cap from 100uf to 47uf, doghouse is now 22/22/22/100/47
5. Checked dropping resistors
6. Checked for bad joints, dirty contacts.

Any ideas for other ways to address this? Tom Bavis on Audiokarma stated that "Arcing occurs when there's too much forward voltage drop, due to too much load or not enough emission from the cathode. Input cap might be too big, transformer winding resistance especially low, or just a weak tube."

So perhaps it's time to do qualitative tests to the transformer??

Thanks

[Helmholtz]

Wrong schematic attached.

Where do you see the arcing?

[Mr_bibbles]

The arcing is internal within the GZ34, tube sockets don't show any sign of dirt or carbon traces from arcing of the pins.

[Helmholtz]

The edited schematic has no GZ34.

What happens if you pull the power tubes?

[Mr_bibbles]

The edited schematic has no GZ34.

What happens if you pull the power tubes?

Eep! Sorry didn't actually paste the link.

If I pull the power tubes the rectifier does not arc.

[Helmholtz]

You might have a problem with power tube bias, maybe a bad socket.

It might make sense to bring up mains voltage slowly using a variac.

[g1]

Do you have a variac? Either with variac or with SS diodes stuffed in the rectifier socket, try to find out if there is a fault making the power tubes run too hot.

[Mr_bibbles]

Great advice, though I do not have a variac unfortunately, just a simple current limiter. I did as a precaution measure bias and the grid / plate resistors, didn't find anything. HOWEVER the other good hint I think we have here is that the pair of 6L6's that was in originally had been arcing as well, so I will go over that circuit with a fine tooth comb.

[g1]

The OT primary voltage and resistance type calculation will get you the idle current on the power tubes. Or bias probes or other method that you may be using to check power tube idle current.
Likely they will be running very hot or redplating to make the rectifier arc like that. Or possibly oscillation.

[Helmholtz]

Total idle current consumption can be measured by connecting the Ammeter probes across the standby switch.
Switch should be closed first, then opened with meter leads clipped on, to protect the meter from surge current.

[Mr_bibbles]

The OT primary voltage and resistance type calculation will get you the idle current on the power tubes. Or bias probes or other method that you may be using to check power tube idle current.
Likely they will be running very hot or redplating to make the rectifier arc like that. Or possibly oscillation.

Tubes are cold biased at 20ma right now, so possibly oscillation? There's no AC coming off the coupling caps before the 6l6's, plate voltage, Plate and grid voltage in spec (455 VDC / -44 VDC).... I've been able to turn it off several times in succession now without an arc, perhaps because the filter caps are holding charge and changing the forward current when standby is flicked???

[Mr_bibbles]

Total idle current consumption can be measured by connecting the Ammeter probes across the standby switch.
Switch should be closed first, then opened with meter leads clipped on, to protect the meter from surge current.

Perhaps I'm not doing this right, but with the clips attached, standby switch in the OFF position, I get 97MA and then when I switch it to ON, i get .04 MA. I'm not sure what to expect here, but I double checked the readings and what I posted here.

[Helmholtz]

..been able to turn it off several times...

Why "turn off"? Standby means that HT is turned off.

[Mr_bibbles]

Why "turn off"? Standby means that HT is turned off.

Sorry, don't think I'm being clear here;

To clarify, I've been able to turn the entire amplifier off and then back on (From no power to standby disengaged) without the rectifier arcing. Basically I'm trying to say that I can flip the standby without it arcing SOME of the time, meaning the arcing is intermittent and doesn't happen every single time I disengage standby"

[Helmholtz]

Perhaps I'm not doing this right, but with the clips attached, standby switch in the OFF position, I get 97MA and then when I switch it to ON, i get .04 MA. I'm not sure what to expect here, but I double checked the readings and what I posted here.

The "off" position is what matters here (though it's confusing). When the switch is closed, the meter will see no current.
Anyway 97mA looks too high.