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

[Enzo]

If you think it is shorted, simply remove it. at this point we are chasing unwanted DC voltage, we are not concerned with sound. SO who cares if removing the cap kills the sound? If removing it gets rid of the 300v on the grid, then we are onto something. If not, then it is a red herring.

[DrGonz78]

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!

Well C34 is a filter cap for the +/-15vDC power supply rails. That cap, judging by your pics, does look like it is bulging up on the top a bit, but pics always deceive me. So you have HV B+ on the pin2 grid? I don't think that C34 has anything to do with the problem at hand. You need to not throw parts at this repair, but rather troubleshoot to find the fault. Did you follow advice as mentioned and measure resistance from pin2 to pins 7 & 9? That needs confirmation foremost. Best regards.

[Danohss]

Soccer Dad this afternoon and evening, won't be able to get back to this until late tonite or sometime tomorrow. Will check the pin 2 to 7 and 9 resistance and report back as soon as I can. Thanks for all the help!

[Danohss]

Success! After reading the last couple helpful posts I took a step back to think about the issue logically. I did put a meter between pin sockets 2 and 7/9, did not find any conductivity between them, BUT, turns out that when R33 failed it made itself a nice little conductive trace to the ribbon cable termination for pin 2 of V4. I had replaced that resistor but didn't look closely enough at the damage it caused when it burned (nice little carbon bridge between the ribbon cable end and the R33 trace. Scraped the carbon trace off of the CB, problem solved. In close examination, I see that some of the other ribbon cable ends are bent over as well, VERY close to the nearby trace which leads me to believe it wasn't a resistor component failure but rather a short between the end of the ribbon cable (bent over) and the close by trace. Before I button it back up I will try to clear some space between all the ends and the traces where they look too close. I will also replace the remaining radial mount electrolytics since they are 15 years old now and I ordered them anyhow. Can't hurt. Thanks for all the help, you guys were great! Now on to do some mods on a muddy sounding Bugera V5...