Fender Blues Junior, bias voltage now positive, fried power tube and resistor

[Danohss]

I have a 2001 Blues JR, did all the Bill M mods on it, LOVE the amp. Yesterday it died with an awful sound, troubled economy shooting I have found the bias voltage which should be around -12VDC is now positive voltage - which explains the fried power tube. Been checking everything out using the service manual (schematics with test points) and can't seem to find a component that shows a problem. Checking resistors and capacitors - all check out OK. No bad diodes, hitting a brick wall on it. Any info would be appreciated.

[DRH1958]

Welcome to the forum! Maybe you could post a schematic of all the mods you have done. Usually it's easiest to get help if the info is readily available so everyone doesn't have to go hunting to find all the mods you did. Like if re-doing the bias supply so it's different from the original schematic was one of the mods.
I went to his site and there is an entry about some of the trimpots being the wrong ones. Maybe that is something to look for. Here's the link. Scroll down about half way to find it.
http://billmaudio.com/wp/

[52 Bill]

Welcome to the place.

I have found the bias voltage which should be around -12VDC is now positive voltage - which explains the fried power tube.

Is the voltage positive only at the tube socket or is it positive at the supply source?

If one of the coupling caps from the driver stage gets leaky and allows positive voltage to pass through, that could cause your problem.

[Jazz P Bass]

Just a thought: the 'bias' voltage should be checked with a known good tube in place.

[Danohss]

Can't bias with a good tube, it red plates and dies as soon as the voltage comes up

Just a thought: the 'bias' voltage should be checked with a known good tube in place.

I have -15 VDC coming out of the power supply section, but as I get down to the power tube bias circuitry I have positive voltages. With no EL84 tubes installed I have 7 volts on pin 2 of one tube, and 356 volts on pin 2 of the other tube. These are where I should have my negative bias voltage. I can't put a new or good tube in, it will get screaming hot, burn out a resistor, and then blow the fuse. I've replaced the 2 (C15 and 16) .022 400V caps, replaced the 220K resistor on one of their power tube circuits (read wrong on the meter). I have checked the diodes, all seem correct with the correct amount of AC on one side, no leakage on the other. I have the service manual from Fender and have checked many of the test points. I have correct voltages at the TP's on the "top" of the layout, the voltages on the "bottom" where the power tube circuitry test points are are wrong. I did put the Bill M bias mod in (tri pot for R52) but that worked fine for the last 2 years. C33 measures correct to ground on one side and voltage on the other, no DC leakage.

[Danohss]

Already went to the Bill M site, saw the isaasue with bad trim pots, confirmed mine is the correct one. I did the tone stack mods which would have no impact on the bias circuitry, also did his power supply "stiffening" mods - both replacement of the 22 ufd filter caps and the addition of another 47 in parallel with the stock one. Like I said, the amp ran great for the last couple years with these mods so I doubt it's any of these. I have tried putting new 12AX7's in, no surprise that's had no effect on the problem. Will take a photo of the PCB and post it if I can figure out how to do it on this forum, and will also try and find a digital version of the schematic and layout.

[nickb]

Unplug CP6 - this is the red wire that goes to the output transformer. With the EL84's out recheck the bias at pin 2 again and report.

[J M Fahey]

Please hand draw a schematic of what you actually did, looking at the amp/board itself.

Do not refer us to "Bill M mod #47" or whatever because what we are trying to catch is any wiring error, not to correct design ones.

Adding a`closeup picture might also help.

[Danohss]

Will do, just spent a few more minutes ion it, the thing that currently seems most odd is R51 has -27 VDC on one side of it, but 7VDC (positive) on the other. Here's the schematic:

[Danohss]

Here's images of the PCB. All orange drops are my work as well as the filter caps. You'll see the small blue tri pot installed for R52. IU also took a close up of the problem areas, I had to replace R33 as it was taken by out when the power tube zapped.

[Danohss]

Bigger image

[nickb]

Please hand draw a schematic of what you actually did, looking at the amp/board itself.

Do not refer us to "Bill M mod #47" or whatever because what we are trying to catch is any wiring error, not to correct design ones.

Adding a`closeup picture might also help.

Juan - in post #1 my understanding is that the mods were done. It was working for some time but then failed in service.

Therefore,

Danohss- I think we are looking for a failure either component or poorly done mods. Also, the voltage is highest at the grid pins, well away from where the mods were. I'd like to eliminate a socket / pcb breakdown from plate to grid by first removing HV from the plates as in post #7. The breakdown may also be from the screen grid to grid, but thing one at a time. If you find by removing the HV the grid is now better, inspect the EL84 sockets for signs of arcing causing carbonization.

[nickb]

Will do, just spent a few more minutes ion it, the thing that currently seems most odd is R51 has -27 VDC on one side of it, but 7VDC (positive) on the other. Here's the schematic:[ATTACH=CONFIG]37360[/ATTACH]

That is consistent with one of the grid pins being 365V. That grid pin is supplying current ( about 365/220 ~= 1.5mA) through the 220K resistor. That current is then split between R52 to ground and R51 to -27V so increasing the voltage drop across them.

[Enzo]

Schematically there is no way for 365v to get on a grid pin, so we must look for some short in the hardware. The problem is at the socket with the 365 on the grid pin, the other 7v socket is just along for the ride. Power off, measure resistance from pin 2 grid over to pins 7 and 9, the plate and screen pins. Does either show shorted to the grid? it could be arcing when powered. Turn off all the room lights while it is powered and see if there are any tiny blue sparks near the socket or ribbon to that socket. Inspect carefully the solder to that socket, if a pin 2 trace comes near a pin 7 or 9 trace or the 2 trace passes between those on the board, there could be a small solder blob or something shorting them, Likewise on the ribbon cable, the wires for the screens and grids are right next to each other. Check both ends of that ribbon for wire ends shorted together.

[Danohss]

No carbonization anywhere on the power tube sockets, BUT, since I had the magnifying glass and a high intensity light in hand, looked around the rest of the board and may have found the suspect component - C34 (220uF 25V) shows some rust colored (slight, but visible) residue the top between the plastic skin and the aluminum, and there also appears to be a bit on the PCB under that cap. Not one I have in my home inventory to drop in tonight, but I'll order that one and a few of the other vertical mount small electrolytics this afternoon, should have them by Wednesday. Looking at the schematic and where C34 sits, very possible that's the bad actor. Stay tuned!