Bugera 333XL Infinium Issue

[stevenrb718]

The voltages I tested on the EL34 sockets were with the tubes pulled. I also did pull the fuse to test it and it was fine after running for a bit with no tubes in the output board.

[Enzo]

I don't have the Infineon version either, but I think the 333XL is close enough to discuss. The power tube board has no 6V? That power comes onto the board through two pins on the 10 pin connector, pins 2 and 3. Is there 6v right on those pins? That in turn comes from the main board, also pins 2 and 3. But on the main board, pretty much all it is is a pass through from the yellow wires from the power transformer. The fuse is in one of those lines. SO is there 6v between the yellow transformer wires? Is ther 6v between pins 2 and 3?

[stevenrb718]

I in fact do have 6V across the pins and sockets. My fuse wasn't making solid contact to the terminals as they were a little wonky. I had all the tubes on the output board pulled with 6V to the sockets for a good 5 minutes without the fuse blowing. I dropped in the 4 EL34s and a different known-working 12ax7 on the output board (which I presume to be the PI tube, but please correct me if I'm wrong) and the fuse popped immediately. I then pulled only the 12ax7 and the fuse popped immediately again. Put the 12ax7 back in and pulled the EL34s...no blown fuse and that 12ax7 is now glowing. Obviously something pertaining to the EL34 circuit is shorting out.

[Enzo]

Or one of your EL34s has a short to heater. Plug EL34s back in one at a time.

EMpty tube sockets don't stress fuses. And if the circuit was faulty, it likely wouldn't need tubes in place to show. SO I tend to think you have a tube causing the fuse to blow.

[The Dude]

This is not making any sense.
In post #12 you said, "I also pop the F4 with or without the power tubes in".
In Post #18 you said, "Put the 12ax7 back in and pulled the EL34s...no blown fuse and that 12ax7 is now glowing".

Did you change/fix something in between these posts?

[Enzo]

If blowing comes and goes, check for a loose screw rolling around under the circuit board.

[The Dude]

Yes. I've also found screws stuck in tube sockets that are elevated from the circuit board- between the socket and the board stuck in between the pins. I hope that makes sense. It's kind of hard to describe.

Edit: Adding picture with arrow. Not sure if that amp has this type of socket configuration, but just in case......

[olddawg]

Not sure if a “lightbulb limiter” would work well on this amp because it has an auto bias circuit. It’s never good practice to troubleshoot anything by the blowing fuse/not blowing fuse method. But in some cases it’s a valid choice. As Enzo said, plug in one EL34 at a time. If one blows a fuse and the other doesn’t you have a filament short in that tube. Tubes are mechanical. The filament may have broken and stuck to a different structure internally. Back in the day I had a bunch of fast action circuit breakers with alligator clip leads to do these type of tests. Is the amp under warranty? Bugera tubes are not particularly known for their reliability. They replace them under warranty all of the time. If the amp is a few years old you probably need a new set of output tubes anyway. You don’t want to keep stressing the amp and blowing fuses, especially without a schematic. It’s begining to look like you simply have a defective output tube to me blowing a fuse. Happens all of the time.

[stevenrb718]

The Dude: I didn't do anything specific to it between those times other than unscrewing and rotating the pcb to test diodes. Perhaps there is something physical like a screw or a shard that is bridging something? But why would it only bridge when a tube is in, other than bad sockets.
Enzo: I checked the tubes and they're all good. I only have a couple fuses left so I don't want to start throwing things in, but I did try a couple tubes in one of the sockets (I believe 4) and the fuse popped immediately. I then tried all 4 tubes in I believe socket 2, and the fuse didn't blow and the tube lit up. I then added another tube in (again I believe) socket 3, and pop.

Would there be such a thing as a bad socket that shorts only when a tube is inserted? Or does this start to point towards the auto bias as being a culprit?

[The Dude]

Tubes can also be intermittent. Perhaps you have one that works sometimes and sometimes not. It might be best to try a new set of tubes. If the auto bias was at fault, you would be blowing a B+ fuse and not a filament fuse.

"Would there be such a thing as a bad socket that shorts only when a tube is inserted?"

Earlier you said the fuse blew without the tubes in. That is why Enzo suggested checking for something rolling around in the chassis. (I believe).

[stevenrb718]

Tubes can also be intermittent. Perhaps you have one that works sometimes and sometimes not. It might be best to try a new set of tubes. If the auto bias was at fault, you would be blowing a B+ fuse and not a filament fuse.

I do have an older Mighty Mite tube tester that I previously checked these tubes with when I originally discovered the problem. Unfortunately they all tested ok. Would dropping a tube in that socket where they seemed to work and leaving it on for a bit do damage to anything else, basically to check intermittency? Otherwise time to pause for 4 new EL34s!

[Enzo]

A bulb limiter should work fine for this. We are only going to flip it on for a second to see if the "fuse would have blown". If the auto-bias gets confused for a couple seconds, so what?

Once the amp is operational, a bulb limiter probably would confuse the auto-bias.

I had a collection of breakers with clip wires too. I passed them along to the young tech I have been coaching. He was amused at first I think, but one day later, he told me he remembered them and what do you know, they worked.


BY THE WAY - as long as you are in there, check the AC mains connector board. If this is one of the ones I am thinking of, there will be a handful of thermistors in series next to the IEC connector. They are an inrush limiter. MANY Bugera amps show up with the solder cracked on those. I suspect the lead free solder, but in any case, do yourself a favor and resolder them with good solder. I never had one come back for that again.

[Enzo]

TUbe testers. A tube tester can tell you a tube is bad, but it really can't tell you a tube is good. If my tester says a tube is bad, I toss the tube. If the tester says it is good, I put the tube in a pile waiting to be tested in an actual amp.


yes, anything is possible. We can wiggle a tube around to see if something intermitts.

What is key here is to be systematic. The whole point of troubleshooting is to isolate the problem. We take steps to narrow down where the problem can lie.

[g1]

I'll ask what are the exact type of fuses you are installing.

[stevenrb718]

New EL34s with same result with blowing the fuse when multiple tubes are in. I also see nothing awkward like a screw shorting out a socket. I guess I'll just reflow all the points on the output board, along with pull the components for individual testing. What else would be on that fuse circuit that I need to check? No schematic is not helping. There are also 3 wires that go to the output transformer. I don't suppose a bad transformer would be causing this issue?