Bugera 333XL guitar amp head blowing Phase Inverter

[Soccerman58]

Hi
I noticed a few posts here about Bugera problems, but none seem to address mine, and according to the folks handling Bugera tech support there is no tech bulletin about the phase inverter blowing, but it is sure happening to me.

The first time I knew I had a problem with something I was playing some very  loud metalcore, so my first thought was it was all the heat being generated, but I have also had two blow on me just strumming in the living room, so that seems unlikely. When it first blew we took it to a guitar shop tech who said it was the phase inverter so he put another one in and we bought it home and promptly blew it again. It will play for a few minutes, then the volume just drops out.

I've blown a stock AX7, then I switched to AT7s and blown 3 of those - a new Electro-Harmonix (whose tubes I wouldn't use in V1 at any price) a NOS Tung Sol AT7 and a used Amperex AT7 (which pissed me off even more).

I'm not sure what more I can tell you. I took the chassis to work and some of the octal pins for the power tubes seemed to have iffy soldering, so I redid those. The PI octal looked OK tho, so that probably had nothing to do with it either.

There were a couple of ratty connectors that I cut and hardwired, but I did not see any issues with burned connectors that seems to plague these amps. Didn't see anything burned, come to that.
But, I also did not see any evidence whatever that this tech had done anything at all to it. Had he tested it with any volume at all, I'm sure he would have popped it. Maybe he did, several times - who knows.

I guess the first place to start would be to ask you what would cause a PI tube to pop please? I assume that because there is still some volume that it is only one of the triodes blowing, but which one and why is a deep mystery. That's also about the limit of my understanding of all things tube - that it has more than one triode in it

I am personally much too scared of tube amps to do anything while this thing is turned on, but I have a regular multimeter if you want me to measure something while it's off. And if you guys can give me some tips on what to look for I have plenty of electrical engineers at work and all kinds of scopes and test gear. Maybe they ain't so chicken as I am
If I remember correctly, I also have a link to a schematic for you if it would help, but it is on my other computer. Come to think of it, I believe the link came from a post on this very forum. But reading schematics is well above my pay grade.

So gentlemen - any and all advice is welcome. I really need to fix this - I can't afford another head right now.
Best regards
Phil

 

[PRR]

> took it to a guitar shop tech who said it was the phase inverter so he put another one in and we bought it home and promptly blew it again.

You should have discussed this with the tech right away. His initial diagnosis was incomplete; it happens.

As you are rightfully unwilling to put your hand in the snake-pit, you will probably have to take it to this or another amp tech.

 

[Soccerman58]

Hi
I forgot to mention this when I wrote the post, but we did actually give this guy two cracks at it. The second time he gave it back he told me that it was not possible to diagnose any further because he said the circuit board was black and he could not do any tracing. I accepted this at the time assuming that by "black" he meant that it was fully encased in epoxy to stop folks back-engineering. I've come across that before.
When I opened it up, I found the circuit board was indeed "black" but it was not encased in epoxy or anything else - it just happened to be colored black. I decided - after paying him twice - that he either wasn't interested or wasn't capable.
There are other guys in town - I just think I should try harder before I throw more money away.
Regards
Phil

 

[andyfromdenver]

Hi Phil.

I did have one question, does it fail after idling for a while with no playing?

The self route is dictated by your willingness to test @ high voltages.
If that were the case, a schematic, awesome pics, and some willingness to execute meter readings etc. would be a hopeful proposition.  Though I haven't even seen an exterior pic of a Brugera Amp, so not sure how mega busy the amp is.

tbc!

Andy

 

[trobbins]

In addition to Andy's comments, you may be able to 'check' the valve glass temperature with the amp in idle, and compare with other valves of that type in the amp?

Did the tech tell you it was the valve that does the phase splitting?

That valve has 2 triodes.  Some circuits use both triodes for the phase splitting function, while other circuits use just one of the triodes.

The fault is likely to be a faulty or failing part on a PCB.  So a schematic and a PCB layout or an unpopulated or second amp PCB is likely to be a big benefit to further fault finding.  Vive la tag boards!

 

[CJ]

how do you know the tubes are blown, do you have a tester? what does it say is wrong with them?

never seen an amp pop a phase inverter, 

may as well check the PI tube voltages, life is short, 400 volts DC won't kill you, more likely to die from under cooked Jumbo Jack w. Cheese,

change plate resistors to 100 K (R19, R24)  change C1 to orange drop or Nichicon/Panasonic  .047/400

might be a pain getting the board out, that amp is built for low price, not technicians,

check DC volts on pins 1,3 and 6 and 8,  do the heaters look like they are burning too hot?

 

[Soccerman58]

Hi Guys
Thanks for the replies. I'll get some pix and anything else I can put together and post em.

The reason I know it's the PI tube that is blowing is because that's what the tech told me was the issue. I replace the tube, it blows again. I replace the tube...etc. Blown 3 of em so far.

The board with the power and PI tubes is mounted upside down, so to look at or change any components on it I will have to remove it. Not too difficult a task, except there are two connectors that have to be removed. I'm not sure if I can gently pry them off the board with a blade or if they are somehow held in place by a clip of some sort. Replacement boards are not available so I need to be careful. I can only attach one pic per post - this one is a 6-pin connector with thicker wire that I assume is power. It looks like there might be a clip or something holding it (arrowed). It's not easy to get to so it might be easier to pull the connector on the other end.
Actually, it would be much easier to just cut the wire and reconnect them - maybe I'll do that if I encounter much resistance from the connector

Thanks very much - I appreciate the help.
Phil

 

[CJ]

do not throw the tubes out, they are not bad, to check, plug them into a preamp socket,

worked on one of those, ended up ripping the board out, using the pwr and output transformer, chassis, wired in a simple Marshall 50 watt circuit point to point with good caps and never looked back, might be a good way to get into DIY electronics,

 

[alexc]

What is 'blowing' a tube ?  Does it literally explode or meltdown internally etc?

I've never seen a tube do that - especially a miniature type like you would find a guitar amp phase splitter.

As for going thru them like fuses -  well I've never heard of anything like that.

Intrigued!

 

[mjrippe]

What is 'blowing' a tube ?  Does it literally explode or meltdown internally etc?

I've never seen a tube do that - especially a miniature type like you would find a guitar amp phase splitter.

As for going thru them like fuses -  well I've never heard of anything like that.

Intrigued!

Also curious.  Can only think of two things that would "blow" a tube to the point of failure:

1 - Filament, burned by over-voltage can go open.  Tube would no longer glow if that were the case.

2 - Loss of vacuum, possibly from a damaged tube socket stressing the pins and breaking the seal.  Tube would likely turn white where the silver getter material was.

I agree with CJ - plug in a "blown" tube somewhere else and see if it works.

EDIT: Just saw CJ's mention of DC filaments.  I recently repaired a Marshall JCM2000 where the first two tubes have DC filaments.  After a few minutes it would fade out.  Turned out the bridge rectifier for the filament supply was faulty.

 

[Soccerman58]

Hi again
Thanks for the interest.

When I say "blown" I mean not working - they are not physically exploding. I don't see any evidence of anything white or milky-cloudy in the tube. We kept a brand new Electro Harmonix that failed in the PI slot within a few minutes, and it is crystal clear inside. I did what you suggested and put it in V1 but it did not glow at all.

We have our last 12AX7 in the PI slot right now (it just happens to be one of the original Bugeras that we took out a couple of hours after we got the amp to retube it) and it is glowing gently on both sides, and my kid is playing through it. It's not turned up very loud, but we have had tubes fail at this sort of volume before, so I am expecting it to fail at any time. The glow is steady (not fluctuating as plays - something that I thought might happen) and it doesn't look like anything is overheating though at all. If anything, I would say it's perhaps a tad less bright than the preamp tubes.

Tomorrow we will try cranking it and see if we can put some stress on it. I don't know if this is meaningful, but he says his clean channel is much louder than his insane high-gain channel. Seems to me that it should be the other way around, but I don't know if the discrepancy would have anything to do with this tube situation - does anyone have an opinion on that please?

I'm inclined to agree that it's probably a failed/failing component on the board, and I haven't ruled out lifting the board and flipping it over. If I'm going to do that, it would make sense to replace the components you suggest and I can also post some pictures. Removing the board does not look at all difficult - except for that big power connector. I posted the pic of that - I can't tell if it would lift right off the board or if it somehow locked in place.
To be honest, it would be a lot easier to clip the wires and splice em back together later, but I don't think my kid was too enthusiastic about that. ;D ;D
I'll keep you guys posted soon.
Thanks
Phil