Help with Musicman HD130

[Steelwitch]

12ax7 PI model. There’s some kind of issue that’s alluding me here. There seems to be an issue with the bias supply, I can’t get the voltage positive enough. I’m getting some other weird readings on the heater supply. 7.2vAC on one side and 0 on the other. Tubes are still heating up though. The amp has smoked. I had new JJ EL34’s installed which may have been damaged. It smoked at V2 near the plate. They weren’t red plating though, I’m having a hard time figuring out what’s going on here! Any help is appreciated. Originally came in with the circuit breaker blown internally. Thanks!

[Steelwitch]

I have measured 7.2vAC across the heater supply winding.. it’s a little high but is it within reason?

[J M Fahey]

an issue with the bias supply, I can’t get the voltage positive enough.

You mean negative enough?
Bias at point B which feeds power tube grids through 330k resistors is negative,probably around -38-40V , whateverīs needed to cause 0.5V drop across cathode resistors, so 50mA per tube at idle.
If you canīt, then maybe tubes are extremely worn, dead, or screens are not receiving proper voltage.

[Enzo]

Look at your schematic, in fact post it. Isn't one side of the heater string grounded?

[g1]

Bias at point B which feeds power tube grids through 330k resistors is negative,probably around -38-40V , whateverīs needed to cause 0.5V drop across cathode resistors, so 50mA per tube at idle.

I think each cathode resistor is for a pair of power tubes, so 25mA per tube at idle.

Schematic on pg.3 of attached. Shows one side of heater winding is grounded, like Enzo said.

[Steelwitch]

Oh wow, I didn’t notice that one side of the heater supply is grounded! The schematic is a little hard to read sometimes, I guess I missed it. Maybe I’m looking in all the wrong places for the problem.

All the PT voltages look normal. 380 on the screen grid, 750v on the plates.. I’m definitely able to get -40v from the bias supply. And all the components around the output section are testing just fine. But the output section is failing. Something arced between pin 6 and pin 7 of V2 after the PI. Could it be an issue with the PI? The failure happened when I increased the master volume.

[g1]

What is V2, a power tube? On the power tubes, pin7 is ground (the grounded side of heaters). Pin 6 of an EL34 is not used, so what are they wiring up on the socket pin? Screen voltage?

[Steelwitch]

V2 is a power tube. I actually had the chassis turned around and I gave some inaccurate info here: the arcing happened between pins 2 and 3, not pin 6 and 7. But pin 6 is control grid I believe. 1.5k between 6 and 5.

there is a diode between pins 2 and 3 that I though maybe could have failed but it’s testing just fine on my atlas DCA.. any clue? I checked the OT and there doesn’t seem to be any damage. At least it’s reading normally between windings.

[g1]

2 to 3 arc (heater to plate) makes more sense and is a lot more common. The heater is almost at ground potential and the plate is up near 700V. It could arc from dust or dirt or just arc. Once it arcs the socket may be permanently damaged or may be salvageable.
Post close up pics of the socket there if you can.

[Steelwitch]

I will post some pics after I'm back in the shop. I'm pretty sure the tube socket is damaged. I probably will end up replacing it for good measure anyway. I'm not completely sure if the arcing originated from a problem with the socket. Now that I'm thinking about it, the amp only fails with output tubes installed and also not immediately. The arcing happened on the outside where you insert the tube. The side on the interior of the amp looks normal. I guess the carbon residue could have caused it to arc? I can't find any other reasons for failure. There has been a couple of caps replaced in the PI master volume so I'm wondering if this has been an issue in the past. I've heard these amps fail due to something about the 12ax7 PI... but I've never understood why, or if it's even true.

[Enzo]

Whether the socket was originally at fault or if some other thing caused the arc, the fact remains that it DID arc, and that carbonized the arc route and makes the socket unreliable. Once a socket arcs for any reason, I replace it.

[g1]

A loss of load even temporarily can lead to arcing. Like an iffy speaker cable, or not having the speaker connected, maybe even incorrect load.
Otherwise, these amps are running extra high plate voltages, so arcing will be more likely than with other amps. Any bit of dust or residue on the socket will help the arc jump across.
.
I can't see why anything related to the 12AX7 PI would cause arcing. Make sure that the bias is set no higher than specified, I would not even bias it that hot. I'd lean more toward .4V across the 10 ohm resistor, rather than .5V. However, this will increase the main B+ voltage so you have to check that the main filter caps voltage rating is not exceeded. I don't think bias would change anything to do with the arcing though. Just mentioning out of concern for tube reliability.