B52 AT100 high voltage arcing on tube board

[The Dude]

Do you get the same (low) results in all rectifier positions (rear panel switch)?
Edit: What does B+1 measure?

[g1]

I think what you are calling the OT secondary is the primary? That would account for a bit of my confusion about the OT 'secondary' having a HV arc.

[Enzo]

I am not sure I have a clear picture yet. First, lift the two high voltage fuses and see what the HV secondary AC is when it is not connected to any circuit.

As to arcing, I don't see the transformer at fault for arcs, I could be wrong. The blue wire is a plate connection. The most common arc on a power tube is from pin 3 - the plate - to pin 2 next to it. Pin 2 is heater, which is essentially at ground. Ground either through the virtual center tap resistors or through an actual center tap.

What causes it? A bug crawling between pins, some contamination and or humidity. A previous tube failure could have taken out a screen resistor, and some soot from the burnt resistor deposited itself between 2 and 3, just waiting to happen. Or who knows.

Or another possibility is a failing connection at the blue wire. A failing crimp in a push-on, a solder joint that let loose. ANything causing a small gap. The voltage arcs across the gap arcs across and the heat from that allows the arc to grow and intensify.

After arcs on a socket, it is usually a good idea to replace teh socket. Arcs tend to char the surface they are on - -carbonizing it. CArbon conducts, so any socket, PC board, whatever that has an arc playing over it needs to have ALL the carbon removed. I use my Dremel, but you can scrape by hand. In any case, even if it means making a hole all the way through a board, you must remove all that material.

Have you ever seen board mounted octal sockets for 6L6 or similar and a little slot is actually sliced through the board either side of pin 3? That is there to help prevent arcs.

[Tone Meister]

I think what you are calling the OT secondary is the primary? That would account for a bit of my confusion about the OT 'secondary' having a HV arc.

Amazing how I continued to type OT secondary knowing it was incorrect. Bad day yesterday, my dad has latter stages of COPD and his condition has become a big part of my daily load. Yesterday was far beyond a typical bad day for us, and I apologize for being short with some of you.

It appears the power transformer HV winding is failing or has failed. I put in fresh power tubes for a minute and plate voltage dropped to <50vdc. My buddy admitted to feeding the amp fuses for weeks trying to get it to work. Good news is he has a PT from an AT212 which I'm gonna swap in this morning and see how things go.

There was no HV arcing on the V8 tube socket itself, only in that vicinity. The blue plate wire connection was the only visible damage and it did have some residual smoke which I've cleaned - also tightened the spade connector to eliminate that. There are no charred places except the small place at the blue wire.


I'll report back once the PT gets here and I get installed.

[Jazz P Bass]

" It appears the power transformer HV winding is failing or has failed. "
Now that makes sense seeing as the voltages on the limiter had such a low reading.

At first power up with the AT212 PT I would recommend that you: disconnect the OT, no tubes installed, set the rectifier to solid state.

[Enzo]

I still suggest pulling the two HV fuses and seeing what the HV winding puts out. I would also grab a resistor from my bin and try loading the HV winding down to say 100ma to see.

[Tone Meister]

I still suggest pulling the two HV fuses and seeing what the HV winding puts out. I would also grab a resistor from my bin and try loading the HV winding down to say 100ma to see.

Enzo, before swapping out the PT, the HV winding was putting out <25vac and loaded with a 1.5K resistor it dropped to a little over 20vac.

Replaced the PT with an exact unit from an AT212. Cleaned the board around the burned plate connector, cleaned and tightened all tube sockets, made sure all the connectors were tight. Installed new power tubes and adjusted bias per B52 instructions (120mAdc across the power tubes fuse holder) which is ~30mA per tube, plate voltage is 473V with 120V mains. Amp is stable and screams like it should. Gonna burn it in for a few more hours, then test again and monitor for HV arcing.

If it doesn't arc again, then I'm calling it done. BTW, buddy thinks he may have mistakenly installed a 5AT in the F4 fuse holder once before thinking it was the same as the mains fuse, and that could go along way toward explaining the PT winding failure, or not.

[Jazz P Bass]

"120mAdc across the power tubes fuse holder"
I simply love that bias adjustment.
Really.

[Tone Meister]

"120mAdc across the power tubes fuse holder"
I simply love that bias adjustment.
Really.

The reading taken there reconciled almost perfectly with my bias probe, so I reckon it must have some validity. Anyhow, the amp is long gone and on its way to a local show. I was surprised at how good the damn thing sounds. Now if I could figure a way to get it to arc high voltage in time ...