Bugera 333XL Bias Problem

[Drewline]

Hi all,

I'm new here posting from Colorado. I've lurked on this site for a while & appreciate the amount of info shared here.
I was wondering if someone would be kind enough to answer a question about the "infinium" biasing of a Bugera 333xl head. The bias lights on the rear panel randomly flash on & off with 2 different sets of EL34s. One of the original tubes was shorting. There are no obvious clues inside. No burnt parts or traces I can find.
I can't find any info on this anywhere online & am wondering how to proceed.

Thanks,

Drewline

[Jazz P Bass]

Welcome to the forum, drewline.
As you are probably aware, Bugera (Behringer) does not provide schematics to unauthorized repair persons.
This amp, new as it is, did not 'leak out' as of yet.
The Infinium series of amplifiers boasts a "self bias' circuit.
There is probably a quad of opamps with a set reference that the tubes are compared to.
If the circuit senses that the bias is off, the circuit will 'drive' the bias to a set (at the factory) point.
The LED indicators are supposed to illuminate on a tube failure (whatever that implies).
So the first question should be 'are the tubes that you are trying to install known good tubes?'
If they are then you have a sense circuit problem.
In that case, if you can find one, you really should take it to an authorized repair center.

[Drewline]

Jazz P Bass,

Thanks for your answer. That is what I thouhgt was going on. The kid that owns it bought it on ebay. I don't anyone in the area that is authorized since Mars closed last year.

Thanks,

Drewline

[Drewline]

Hi,

I'd like to share the reply I received from Bugera tech support with everyone here:

Random flashing of the INFINIUM biasing lights, say every 20 seconds or so, should be normal. But, if you start to notice a consistent and more solid pattern with the flashing, then that could point towards an issue with the unit which would require servicing.

[ted0]

hey guys' about 8 months ago I bought a Bugera333xl Infinium..it worked great for a while until about 3 months ago it started acting up..after letting the tubes warm up for a couple of minutes you could take it off standby and it would begin too hum pretty loudly without anything plugged into it you could hear the sound coming through the speakers..and another thing I noticed was I could tap on the faceplate of the amp and it was as if I were typing on a microphone..I did some research on the internet and similar incidents suggested that it was a bad preamp tube until tonight..it did something completely different.. After letting the tubes warm up as I normally do I took the amp of standby immediately the two LEDs for the power and standby switches shut off except the tubes kept on glowing and the sound was still coming thru the speakers.. But after about 10 seconds the tubes started to get super hot and bright until I thought they were going to explode so I immediately shut the amp off...I got the same results four more times after the initial incident then the amp failed to come'on at all..I'm at a complete loss I have no idea what could be causing this for there are no clues as'to what could have caused this....has anyone out there had a similar experience?

[Drewline]

First check the screen & control grid resistors. If they are good then my next suggestion would be to replace the power tubes with known good ones & see if the bias system starts working as detailed above.

[ted0]

thank you Drewline" I will definitely will try replacing the tubes..but I also want to inform you that I took the amp out of its chassis to be able to take a look inside...all of the connectors and soccer point's seemed to be fine" there was no burnt smell that would be indicate of something burning up...but what I did find was a small bus fuse"and it looked like it could be burnt out... I'm not a hundred percent on it considering I haven't tested it yet" but I'm going to try to replace that fuse and see what happens.. Although I'm still concerned about the integrity of the tubes considering they have been super heated..but I think it's still worth a try..I'll keep updating and giving a play by play

[mozwell]

Maybe this will help

[Drewline]

THis schematic is for the older version of the amp which doesn't have the infinium self checking bias system which is what ted0 has. But it is great to get any info on these amps.