B52 AT100 high voltage arcing on tube board

[Tone Meister]

I've poked around looking at other threads about this amp with this same problem, so I am gonna run this by the pros in hopes to gain from your past experiences. Amp came to me with a complaint that it was "blowing the F4 1A fuse."

Pulled ALL tubes and tested, finding three dead 12AX7s and two bad 5881 power tubes. Then checked all fuses, opened up the chassis and the 1.6A filament fuses (F2, F3) were intact as was the 5A mains fuse. (schematic shows 1.5A for F2, F3 but the PCB is labeled 1.6A, so apparently this amp is newer than the schem). Replaced the F4 1A fuse.

Installed just the preamp tubes, including three new 12AX7s. With NO POWER TUBES installed, powered up on LBL and noticed a significant HV arc near the V8 power tube. A closer look revealed some smoked up places near where the OT blue secondary wire connects to the PCB. The brown wire is labeled P9 but the blue wire has enough damage around the lug that I cannot make out the label - the wire connections are not labeled on the schematic.

POWER OFF
measured ~38 ohms between brown/blue OT secondary wires.
Blue OT sec to gnd measured >1.5Mohms
Brown secondary to gnd measured >1.5Mohms

POWER ON, connected to LBL
with B+2 wire disconnected from PCB, B+2 wire measures ~240vdc
with B+2 wire connected to PCB - Blue OT secondary measures ~255vdc (not connected), Brown secondary wire measures ~260vdc (not connected)
With B+2, Brown/Blue OT secondaries all connected I measure ~240vdc on both sides of R104, R105, R106, and R107
I can measure the same ~240vdc at pin 3 of each power tube socket
Bias at pin 5 of each power tube socket is -23.5vdc

The B+ seems low to me, even with the amp on the LBL. The schematic doesn't provide any voltages, so I have no clue whether it's in the normal range or if something is pulling down the voltage. The bias seems awfully hot as well, but again, I just don't know.

Besides cleaning up all the smoke and carbon, what advice can you offer with what I have here?

Schematic attached, although I believe the amp is a newer revision.

[glebert]

Seems to me like you aren't going to be able to get good HV and bias voltages while on a LBL (unless you have a really high wattage bulb in there). Assuming your HV is half the expected voltage (or less) then the bias voltage will presumably also double when not limited, which would put it to a more reasonable level. Pulling the preamp tubes would presumably reduce the current draw and reduce the effect of the LBL.

Also, just wondering what setting you are using for rectification. If the tube rectifier don't know how that will react to being on an LBL.

[Tone Meister]

Seems to me like you aren't going to be able to get good HV and bias voltages while on a LBL (unless you have a really high wattage bulb in there). Assuming your HV is half the expected voltage (or less) then the bias voltage will presumably also double when not limited, which would put it to a more reasonable level. Pulling the preamp tubes would presumably reduce the current draw and reduce the effect of the LBL.

Also, just wondering what setting you are using for rectification. If the tube rectifier don't know how that will react to being on an LBL.

I don't expect good HV measurements with the amp on the LBL, but ~240vdc seems too low, don't you think?. I'm using a 150 watt bulb and the solid state rectifier setting.

I'm more concerned with the long term reliability of the repair, given the charred lug, and was hoping for someone to chime in with direct experience with this type issue on this amp.

[Helmholtz]

I don't expect good HV measurements with the amp on the LBL, but ~240vdc seems too low, don't you think?. I'm using a 150 watt bulb and the solid state rectifier setting.

You can use the ratio of nominal mains voltage to measured amp input voltage (across PT primary) on the LBL as correction factor to "correct" the internal voltages, i.e. calculate the voltages for full mains input.

[Tone Meister]

You can use the ratio of nominal mains voltage to measured amp input voltage (across PT primary) on the LBL as correction factor to "correct" the internal voltages, i.e. calculate the voltages for full mains input.

Sure I can, and that info would be very useful if I knew I was measuring a healthy PT. With no reference voltages on the schematic, what do I have to compare these calculated voltages to?

I don't want to get sidetracked from the HV arcing near the V8 power tube and blue OT secondary connection. I'm a novice here, just for the record, so things that are obvious to a pro may not necessarily catch my attention.

THE GOAL: take the correct steps to prevent further HV arcing on the tube PCB, install fresh power tubes and put the amp back together, then give it back to its owner. Today.

Any advice, drawn from actual experience with HV arcing in the B52 AT100, is appreciated.

[Helmholtz]

You could check corrected heater voltages.

It's up to you to look for intermittent contacts or shorts , carbonized board or tube sockets.

[glebert]

If you dig around in other AT100 threads you find expected plate voltages of 475-485 range.

I also get frustrated when they don't put voltages on schematics, and find the logic that "it depends on the mains voltage" to be really weak. Give me a voltage and tell me what the mains voltage assumption is and let me know if I am in the ballpark.

[Enzo]

Not only does it depend on the mains voltage, but it also depends on the tube current draw, as in the bias.

But looking farther than that, in a typical 100 watt amp with four 6L6 or four EL34, I pretty much always expect to see 485-500v. I don't recall ever seeing a 100 watt amp with 385v B+. And while I do see maybe 525v on a print, I won't see 600v or 800v for 6L6s. I might see as much as 800v for EL34s, but then there will either be a large drop for the screens or a separate supply.

Every B+ except for those in SMPS is born of a transformer secondary. Look at the AC voltage and it is a simple arithmetic to find the expected DC.

[Tone Meister]

Look at the AC voltage and it is a simple arithmetic to find the expected DC.

It certainly is, when we have a healthy power transformer. But I have no idea if this power transformer is healthy - didn't think I'd have to say that out loud, but there it is. If as glebert says expected B+ is ~475-485 then my 240vdc measurement to me is a big red flag. I have never seen that much of a drop on the LBL, with or without tubes installed. I'd have hoped the pros would have been appreciative of my caution, instead, I keep getting repeated instructions on calculating expected voltages, which I do appreciate but don't need. And BTW, there are NO power tubes in the amp, only the 7 preamp tubes.

This thread continues to remain fixated on tutoring me on how to calculate expected voltages rather than helping me determine the cause of the smoked OT connection from the HV arcing. If a 245V drop in plate voltage doesn't concern those who are trying to help, then I'm just gonna clean up the board and try the amp with the power tubes installed.

I will try to phrase my questions more clearly in the future.

[Tone Meister]

You could check corrected heater voltages.

It's up to you to look for intermittent contacts or shorts , carbonized board or tube sockets.

On the LBL the heater voltages are ~3.1vac and 3.0vac.

[Helmholtz]

Multiplied by the mains correction factor this makes what?

[Enzo]

The job of the bulb limiter is to prevent full power reaching the circuit in case of a large excess current draw. Unfortunately it also drops the voltage available to the circuit. Thus voltage readings on the bulb are really of little value.

A power transformer with shorted turns will draw current. If we encounter what seems to be low voltage on the transformer, check its draw. Or use RG Keen's transformer tester from geofex.

Yes, +240vDC is a big red flag. Not even a little Champ runs that low a voltage. Most preamps run higher than that. Look at the other part of my post where I pointed out that 100 watt amps almost always have B+ close to 500v. But that is without the bulb. If the current draw from the mains on the bulb is not excessive, then get rid of the bulb. Now your voltage readings will accurately reflect the amp condition. If the bulb were cutting the B+ in half, then all the other voltages like the 6.3vAC would be as well. If I was on a bulb and saw say 385v instead of 245, I would still see that as probably low. But ditch the bulb.

[Enzo]

You gave two heater readings of about 3v. Did you mean each side to ground? What matters is across it, you need to see about 6v, if the 6v has become three, then everything is cut in half.

[Tone Meister]

You gave two heater readings of about 3v. Did you mean each side to ground? What matters is across it, you need to see about 6v, if the 6v has become three, then everything is cut in half.

Yep, around 3 volts each side and 6.2 volts across it, which is not a proportional drop on the LBL as compared to the drop in plate voltage. I'm suspicious the PT secondary may have suffered some damage due to the arcing, possibly a prolonged condition.

My main point is this: aside from a little smoke and a small amount of damage at the OT blue secondary connection, I see no reason why this board would arc high voltage. That is what I'm trying to get to the bottom of here, yet we continue to be focused on teaching me that there will be a voltage drop with the amp on the LBL. I know the voltage will drop, and I usually know about how much it will drop. But to me, this much of a voltage drop is too much and it has me wanting to be cautious.

I don't want to get in a hurry to pull the amp off the LBL and increase the voltages and run the risk of further (unnecessary) damage that I can easily prevent with just a little due caution. I want to be sure I have taken every precaution to prevent more HV arcing before taking it off the LBL. I mean what good will all this do if I blow a big hole in the tube PCB by being in too big of a hurry?

Do my concerns make more sense when I explain it that way?

[Tone Meister]

And I'm sorry if I sound grumpy because I am. It's not you guys, it's me so please accept my apologies and overlook my smartass replies.