Possibly Bad OT Help

[Asusong]

So, I’m new to this. Be kind. I had a friend whose Blues Jr. caught fire. He had it repaired (guy cut out part of the board with a dremel and point to point wired over the hole. Well, it caught fire again due to one of the repair wires shorting a screw. It was toast. I told him no guarantee but I’d give it shot to rebuild. Followed all the instructions and pdfs and read as many posts here as I could. Everything checks out except, I’m not getting any voltage across the 1ohm cathode resistor. And the voltage across two halves of the OT  is in the mV range and different and inconsistent on either side. One side is around 2mV and the other is around 24mV.

That seems to indicate to me that the OT was damaged in the fire…

I’m open to other suggestions and ideas.

Also, voltages from the PT are spot on.

Thanks in advance.

[sluckey]

it caught fire again

I don't see any evidence of a fire? What burned up? Pic of the damaged area please.

Take some voltage measurements (black probe connected to chassis)...

     1.  What are the voltages on all four of the filter caps?

     2.  What are the voltages on all pins of the EL84 tubes?

[Asusong]

Sorry to clarify. The amp in it’s factory state was repaired, then caught fire again. Pictures below.

I replaced everything except the tubes, PT, and OT with hoffman parts.

Caps measure 359V, 347V, 296V, 262V.

[sluckey]

     2.  What are the voltages on all pins of the EL84 tubes?

[Asusong]

Sorry.  Missed that.

V5: 1:-20mV, 2:-17mV, 3:1.5mV, 4:30mV, 5:30mV, 6:40mV??, 7:360V, 8:44mV??, 9:350V
V4: 1:-23mV, 2:-23mV, 3:0mV, 4:30mV, 5:30mV, 6:0mV, 7:365V, 8:0mV, 9:352V

These seem all over the place to me.  No idea what's going on with pin 6 and 8 on V5... I appreciate you patience and assistance.

[PRR]

...the voltage across two halves of the OT  is in the mV range......

OR the power tubes are not conducting.

Of course they "should", with about zero V on pin 2 (grid 1) where should be negative 12V (and mildly high voltage on plate and screen).

Really burnt-up tubes? Wiring mistake?

Answer Sluckey's questions carefully. He's a sharp diagnostician, and he knows the recent Fenders.

[Asusong]

I’m reprinting and double checking the wiring. When I get home I’ll swap the tubes to see if the weird voltages follow the tubes or not. That should help determine where the problem is. I hope. Thanks.

[mresistor]

The EL84 power tubes would probably be the first things to replace .. after the board repair or replace. I would make sure I am using a known good EL84 duet and probably brand new .

[sluckey]

OK, so you had two fires ***BEFORE*** you installed the Hoffman boards. Has the amp ever worked with the Hoffman stuff?

It's very possible that the output tubes and the OT were damaged during the fires. But you have at least one problem that is probably a wiring error, and it must be corrected before you do anything else. You have no bias voltage on pin 2 of the output tubes. Should be a simple straightforward fix. Pull the output tubes and set them aside until the bias problem is fixed.

There should be -12VDC on pin 2 of each EL84 and this voltage should change as you turn the bias trim pot. It's a simple circuit, a secondary winding, diode, two filter caps, a resistor, and the trim pot. Double, triple check that the wiring is correct and the diode and filter caps are installed correctly. Report your findings.

[pdf64]

So have you got heater voltage across terminals 4 and 5 of the EL84 sockets, and were the EL84 getting hot to touch?
No heaty, no conducty!
Which, in the absence of bias voltage, might have saved your bacon here

[Asusong]

Again. I appreciate your help. This has been blast to work on and learning from you guys is great.

Reflowed the bias winding to the diode and now I have adjustable voltage at pin 2 on the el84 sockets. Must have been a bad solder joint. Set it to -12.3v. Triple checked everything else. Installed tubes.

Voltage on pin 2 for V5 is -12.3 with tube installed.  It is -14.18 on V4 with tube installed.

Voltage across 1ohm resistor for V5 is 16mV

Voltage across 1ohm resistor for V4 is   0.2mV

And I just saw on the orignal repair tag that after the 1st fire they replaced the el84s. Since it was essentially the same fire the 2nd time, I’m beginning to think these should be replaced again.

[sluckey]

Voltage across 1ohm resistor for V5 is 16mV

Voltage across 1ohm resistor for V4 is   0.2mV

Swap the tubes. Does the low reading follow the tube?

[Asusong]

Yes. Yes they do.

[dude]

What’s that tell ya..?

[Asusong]

Tubes ordered. Thanks so much again to everyone.