Bugera 333XL Infinium Issue

[stevenrb718]

Got a Bugera 333XL Infinium that powers up but doesn't produce any sound. After a short time all 4 of the tube indicator LEDs on the back light up. Anybody have this issue?

[The Dude]

Do you have a schematic you could upload?

[stevenrb718]

Unfortunately no. The only schematic of this amp I can find is the 333XL version and not the Infinium version. http://schems.com/bmampscom/bugera/B...20333%20xl.zip The Infinium version apparently uses an automatic bias control for the output tubes.

[The Dude]

I could only find the "non-infinium" myself. Hopefully someone chimes in with it. In the mean time, are all of your filaments lit and do they stay lit? Run a guitar into the effects return and see if it makes sound. Run the send into another amp and see if that makes sound. At least maybe you can narrow down whether it's a preamp or power amp problem.

[Enzo]

That and check voltages. I don't care about the fancy parts yet, but ANY tube amp needs not only ALL the heaters running but also it needs B+ on all the plates. Verify all fuses are good by removing them from clips and testing with a meter. LEDs are low voltage circuits. Just because they light doesn't mean the high voltage is right.

And don't forget to check simple stuff like wires to output jacks and such.

[stevenrb718]

So I powered it up again in a dark room and noticed the EL34s were not glowing. I pulled the fuses and found F4 was blown, but the fuse bracket itself was showing continuity. Upon pulling all the EL34s, the fuse bracket no longer showed a short. I have a tube tester that reads leaks, shorts, quality, etc and all the tubes tested fine.

[The Dude]

The perceived short was you reading across the EL34 filaments, which are just a piece of wire. That's why you no longer read continuity when you removed the tubes.

[stevenrb718]

That's what I figured. The fuse being blown is what confused me. I don't understand the purpose of that fuse if continuity is going to exist with or without a proper fuse as long as a power tube is plugged in.

[The Dude]

There is continuity through the tube filaments, but the connection to the filaments from the heater winding of the PT is broken via the blown fuse, so there is no voltage to the filaments. The filaments aren't a direct short. They have some small resistance, but it's small enough that you may interpret it as a short. The best way to check that fuse is to remove it and test it out of circuit.

[Enzo]

The heater circuit is a big loop. Current flows from one end of the transformer winding, through the fuse, through the heaters, and back to the other end of the transformer winding. The transformer winding has extremely low resistance, and heaters have very low resistance, especially multiples in parallel. The fuse is just a wire, it is either a dead short or it is open. Pull it out and measure it with that in mind.

But if you measure an open fuse still in the clip, or for that matter measure across even an empty clip, you will measure a false path. You THINK you are measuring the fuse, but really you are measuring that path back through the transformer, through the heaters and back to the other side of the fuse clip. It will have a very low resistance.

[stevenrb718]

I gotcha. Well like I mentioned before when I pulled F4 and tested it, it was blown. I'm waiting for replacement fuses but I'm not sure what would have blown the fuse, or if the blown fuse was the original reason for the amp not working. I've tested some of the components on the power amp board and they seem to be fine.

[stevenrb718]

That and check voltages. I don't care about the fancy parts yet, but ANY tube amp needs not only ALL the heaters running but also it needs B+ on all the plates. Verify all fuses are good by removing them from clips and testing with a meter. LEDs are low voltage circuits. Just because they light doesn't mean the high voltage is right.

And don't forget to check simple stuff like wires to output jacks and such.

Well I seem to be getting power to the plates and heaters of the power tubes. 512V and 26v respectively. Of course I'm not sure if these voltages are correct without the schematic. I also pop the F4 with or without the power tubes in, if that makes a difference. The power tubes don't glow under power even prior to the fuse blowing, but the pre-amp tubes do. However, I did plug output from the effects send and I'm not getting any sound there. I checked the diodes on the power board and they seem to be ok. Again I'm having trouble determining shorts or traces without a schematic. I was reading a previous posting on here about a guy who had a similar problem with popping F4, but he actually had sound at some points (although not proper sound) as opposed to mine giving nothing. They also didn't have a solution posted. Umm...

[The Dude]

Check filament voltage across the filament (pins 2 & 7 of output tubes) - not to ground and not DC. It should be ACV. If none of your output tubes are glowing, it's unlikely you have filament voltage there. We don't yet have a schematic, but if memory serves, F4 also fuses the PI filament. So, pull the output tubes and the PI tube and see if the fuse holds. You may have a shorted PI tube.

[stevenrb718]

Check filament voltage across the filament (pins 2 & 7 of output tubes) - not to ground and not DC. It should be ACV. If none of your output tubes are glowing, it's unlikely you have filament voltage there. We don't yet have a schematic, but if memory serves, F4 also fuses the PI filament. So, pull the output tubes and the PI tube and see if the fuse holds. You may have a shorted PI tube.

I'm getting about 280mV on the EL34s. On what I am assuming is the PI tube because it's on the Power amp board, I'm getting about 6mV. I don't have the slightest idea what I *should* be getting across these points if this is bad. I pulled the 34s and the PI tube and the fuse seems to be holding up properly. Would the fuse blow or any other problems occurred if I powered it with the 34s in and the PI tube pulled?

[Enzo]

Pull the power tubes, turn on the amp, set your meter to AC volts. Pick a socket. One probe into pin 2 the other probe into pin 7. Got more or less 6vAC?

Fuse seems to be holding up? Only test for the fuse is to pull it and measure it with an ohm meter while it is out.