Troubleshooting tube amp "meltdown" (bugera 333xl)

[MHBuur]

During yesterdays rehearsal my bugera 333xl tube amp started sounding really thin and distorted, so I turned my head and looked at it and saw two of the EL34s (4 in all) starting to glow up increasingly (the other two showed no change). I turned it off immediately.

I guess the increase in tube glow is because the voltage is rising too? The power transformer was extremely hot and from what I can deduce here, something melted together/shorted in the transformer.. Is this correct? What else could be the cause?

(schematics, see page two http://forum.highlyliquid.com/attachment.php?attachmentid=195&d=1319883369)


Regards,

Michael

 

[MHBuur]

Some additional info: I had the opportunity to measure the resistance of the power trafo primary in another bugera 333xl. It measured 76 Ohm and my assumed fried trafo measures 20 Ohm on the primary side... I'd say this also points towards a shorting of the primary... right?

 

[pucho812]

I would suspect a tube going out first before anything related to the power transformer or anything else. Replace all 4 X el34's and try again. If you order them online, while you wait for them to arrive, you can very easily do a visual inspection of the amp for burned resistors, blown caps, etc and fix anything if needed. You can also test it.


try this for a test as well as other things. measure with a DMM the ac voltage on the secondaries of the power transformer on a known working amp with your amp,  use the same power outlet when you do. tell us what you find.  its been my experience that shorts increase the Ω not lower the Ω

 

[PRR]

Before you put different tubes in, check for NEGative 50 Volts (or so) at the EL34 Signal Grids.

Failed bias supply is far more likely than a PT making "too much voltage".

 

[ruffrecords]

When you say you saw a couple of EL84s 'light up' was this the filaments that were or was it the plates glowing red? If it was the filaments getting brighter that does imply a transformer primary problem.

Cheers

Ian

 

[MHBuur]

I'm a bit reluctant to fire it up *if* it is a problem with the PT - it may damage the amp in other areas..
All the tubes are new - matched quartet of JJ EL34.
Two of the tubes (next to each other) lit up simultaneously, so I don't suspect a bad tube since the chance of two failing in the same block is rather unlikely.
I couldn't tell for sure, but it looked like the light were coming from *inside* the tube, behind that plates.

 

[emrr]

Bias supply failure would seem the most common culprit, but it is possible to get bad new tubes. 

 

[MHBuur]

Okay, it turned out I could simply remove the fuses to avoid possible damage of the circuit if the transformer turned out bad.

The secondary voltages actually checks out fine. I compared to the other bugera I have at hand. Same voltages (+/- a few volts). Is it possible that the transformer bails out when it gets hot? Because it was extremely hot when I disconnected the amp after "the lightning incident".

I'm going to check out bias voltages now..

 

[MHBuur]

Voltages checked out fine.

I tried firing up with an old set of EL34s. worked fine. Then tried with the JJs (the ones that were in during the failure). Also worked fine, and sounded ok (low volume). Strange.

Then I noticed the phase inverter tube only glowed in one side. Tried another tube there and it glowed up in both sides... so I assume the original phase inverter is damaged - could that explain the increased glow in two of the EL34 and ugly thin sound at high volume?

 

[CJ]

these new amps use a lot of PC boards and computer type connectors, not as solid as P to P wiring like the old Fenders, add vibration into the equation and you get a stack of bad amps to fix, Marshall has gone the same way,

look for bad wave soldering on the tube sockets and connectors that are not plugged in all the way,

 

[MHBuur]

Yes, I already made several modifications related to bad connectors. The first and almost certain Bugera 333 XL failure is the meltdown of the power molex connector. I doubt the bad connectors have anything to do with the tube issue described here though.

Lets assume one of the triodes stopped working (it doesn't glow, not sure what kind of failure ths could imply) in the phase inverter - could this explain the increase in glow in two of the tubes. Not knowing how these things work in detail I'm thinking that the lack of inversion could mean that the signal is either all positive or negative, causing the one side of the pushpull design to do all the work and the other nothing, and that this somehow influence the perceived increase in glow in two of the tubes. Its pure speculation though, I'm fairly noob regarding tube amp designs.

 

[mikeyB]

Listen to CJ!
Yeah - the build quality of "brand name" amps has gone down the pan.
You get a lot for the price - but it then needs to be made reliable.
Seen this with a MESA single rect head.

If you can  - i'd get the output section off pcb and wired with chassis mount sockets and i'd get rid of power connectors and hard wire them

 

[pucho812]

Voltages checked out fine.

I tried firing up with an old set of EL34s. worked fine. Then tried with the JJs (the ones that were in during the failure). Also worked fine, and sounded ok (low volume). Strange.

Then I noticed the phase inverter tube only glowed in one side. Tried another tube there and it glowed up in both sides... so I assume the original phase inverter is damaged - could that explain the increased glow in two of the EL34 and ugly thin sound at high volume?

From your description, sounds like your phase inverter tube was starting to fail and only of the two triodes in the 12at7 was working.    CJ brings up a great point about amps with PCB's in them. I am wondering i the fault will return if the unit gets hot again.

 

[mikeyB]

Is the phase splitter a jj tube as well.
I have had 3 ECC83 valves fail in chinese valve mics - in 2 cases one side of the heater went open cct and in the 3rd case both heater sections failed.
I've only seen this in the chinese valves, but in a demanding application like guitar amp/speaker vibration - it may be enough to fail any brand.
The MESA single rect i repaired had a JJ 6L6 tube that had failed - from later conversations with the guitarist, he had moved the amp whilst hot and it may(did probably) have a bit of a clunky landing - this was enough to short the valves electrodes!

 

[MHBuur]

The tube in question was a TAD 7025 High Grade 12AX7.

I've been trying out different configurations and that tube as the phase inverter sounded really good. The first gain stage tube is a TAD ECC803S (a long anode tube) and the rest plain JJ 12AX7.

The 7025 probably couldn't take the beating, but it sounded really good though. I'll go with something sturdier (JJ is in currently, but I'd like a smoother one there as the clean tone is not as "full" with the JJ as with the 7025, any suggestion is welcome)

 

[emrr]

The phase inverter itself wouldn't affect the following power tube in such a manner.

If you've put the JJ's back in and they work now, you clearly have an intermittent open or short.  Could still be an intermittent bias supply connection to that side, which measures fine right now.  Did you measure bias at the bias supply outlet, or at each power tube? 

If the filaments went brighter, they would go brighter in every tube, unless this is a real weirdo amp with multiple filament supplies.    This also suggests a bias fault on the one side, and that you were seeing the plates heat up from additional current draw till they glowed. 



I have occasionally seen clients come in with a one-off failure like that which never repeats itself; amp goes back out with no changes and no future failures.  Disconcerting. 

You don't have this problem: anyone remember my famous hot glue intermittent in an EL34 amp?  Ya know, they stick that shit all over the place to hold caps in place, and some got in an EL34 socket, which you couldn't see.  Amp worked great until it got hot enough to melt the glue, then everything went nuts as melted glue made contacts go open.  Turn the amp off, pull the tube out (cleans the contacts), put it back in, glue has solidified, works great again 1 minute later.  For a while.  Consider crazy crap like further down the line. 

 

[MHBuur]

Well one of the triodes in the TAD 7025 tube was definitely bailing out - no glow at all. I tried it out after confirming the PT voltages were ok (to rule out the first PT shorting suspicion). It worked and the powertubes looked normal. The tone was kind of flat though. I exchanged the TAD 7025 with a fresh JJ 12AX7, both triodes glowed here as expected, and the tone changed towards something more positive (not near the tone of the fully working 7025 though).

Actually I do have some heating problems with the amp - after an hours play or so (its metal, high gain, high volume ;-)) the volume starts to drop somewhat and the sound gets mmm less defined, not more fuzzy or anything, its more like less dynamic response. The PT always get really hot.

I've checked the soldering of the sockets - while I clearly see heat spots and excess flux, the actual soldering looks ok.

I have the EL34 current draw around  27-30 mAh (its in this range I have variation btwn the tubes). I only measured the overall bias voltage (there's an RCA for that, sorry I was lazy here :-[ ), its around -43V (I can check the individual voltages at the sockets next time I get to the workshop.)

And yes - this is one of those amps stuffed with glue - I'll remember to keep an eye out for glue in potentially hot areas.

 

[emrr]

Actually I do have some heating problems with the amp - after an hours play or so (its metal, high gain, high volume ;-)) the volume starts to drop somewhat and the sound gets mmm less defined, not more fuzzy or anything, its more like less dynamic response. The PT always get really hot.

When it gets like this, if you are able to switch to a different reasonably similar speaker cabinet, do so and see what you think.  I have seen speakers that have a 'use fatigue' factor that would seem to be an amplifier factor on the face of it. 

Good luck with it, can't be far off now. 

 

[MHBuur]

I stress tested the amp with the new JJ tube as inverter - no problems at all, so for now I conclude that the partly failing inverter tube was the cause of the strange glow and cruel distortion.

Thanks for your help and suggestions