Power tubes and failure symptoms

[jojokeo]

I'm looking to hear people's experiences regarding the various symptoms, intermittent behaviors, & other crazy ways which you've had when power tubes begin to short or fail could be listed?


This could be a good topic even without my recent issue but... I've been working on an EL34 based amp and after finding & repairing several wiring and circuit board issues I'm down to the last final issue which I "think" is a power tube failing type of issue? I've tested the tubes in my tester and they actually test "good". However, I'm getting a loud buzz type of noise *BTW - "buzz" was a bad description see below - shorting type of noise almost similar to when the primary OT wires are reversed - that was intermittent at first before taking things apart but now it's every time I chopstick the power tubes, chassis, and circuit board hard enough.

Forgot to add that in this amp the EL34s are mounted horizontally. Not sure if things change or are more sensitive to certain issues when mounted "sideways"?

[pdf64]

The only way to be sure is to try another 'known good' pair of EL34, appropriately biased.
With horizontal mounted EL34, I'd look to limit screen grid dissipation more than if vertically mounted; when they get hot (at high signal levels), the wires will lengthen, lose tension, may contact other electrodes and cause big problems.
So if 1k are what would normally be used, I'd fit 2k2.

[92Volts]

Does the buzz change with volume or other settings? If not, the output circuit including power tubes is a possibility. If it changes with volume etc, it's not the power tubes.

If a push-pull pair of tubes draw unequal current, you lose the PP noise-cancelling benefit. If one draws too much current you overload the power supply and get even more ripple/noise. You can detect either failure mode by monitoring bias... also, too much current is obvious because the tube usually red-plates.

There are many other ways tubes can fail (or gradually degrade and lose performance) but I'm not sure others would cause a noticeable buzz. Certainly not one that comes and goes.

I'd guess a bad solder joint in the power supply, the obvious candidate is a filtering cap. The bad connection could be at the HV or ground side. In general I'd check ground connections. If any part of the circuit is grounded through the chassis plus a wire, or otherwise has multiple ground connections, losing one might cause noise but it would still basically work.

[jojokeo]

The only way to be sure is to try another 'known good' pair of EL34, appropriately biased.
With horizontal mounted EL34, I'd look to limit screen grid dissipation more than if vertically mounted; when they get hot (at high signal levels), the wires will lengthen, lose tension, may contact other electrodes and cause big problems.
So if 1k are what would normally be used, I'd fit 2k2.

You are close to being over the target pdf... This is a Marshall production amp (Haze 40). It gets used by the owner & others a lot and I think the '34s are original TAD's? I have some expensive '34 sets in my quiver but will not use those in any amps that are suspect and being worked on. I'm awaiting some others to throw in at the moment and I can wait a couple of days. In the meanwhile I figured to post this thread as I haven't seen one like it before or it's been so long I can't remember it being posted? Thinking it could "spark" some conversation?

*BTW - "buzz" was a bad description. I should've said it sounded originally like a loud short where the sound level was very high indeed. Very annoying and intermittent.

[jojokeo]

Does the buzz change with volume or other settings? NO, I checked this If not, the output circuit including power tubes is a possibility. If it changes with volume etc, it's not the power tubes. Yes, I know :)

If a push-pull pair of tubes draw unequal current, you lose the PP noise-cancelling benefit. If one draws too much current you overload the power supply and get even more ripple/noise. You can detect either failure mode by monitoring bias... also, too much current is obvious because the tube usually red-plates. Yes, I know :)

There are many other ways tubes can fail (or gradually degrade and lose performance) but I'm not sure others would cause a noticeable buzz. Certainly not one that comes and goes. Yes, I know :) but am looking for other's to share their descriptions of things that they've experienced that are odd and not commonplace.

I'd guess a bad solder joint in the power supply, the obvious candidate is a filtering cap. The bad connection could be at the HV or ground side. In general I'd check ground connections. If any part of the circuit is grounded through the chassis plus a wire, or otherwise has multiple ground connections, losing one might cause noise but it would still basically work. Yes, I know :) with me you are preaching to the choir on this stuff :)

Thanks for your reply and time 92V's

[dude]

From my experience, a weak power tube (tested weak) gives a "static like noise" when idling, noise goes away when played but volume is low, punch is lost. The static noise comes and goes for a few seconds or so, there one second, gone the next. Tapping the tube causes more low level static noise. I have some great RCA older 6L6 black plates that make this noise and they do test on the weak side, don't sound too bad but low on volume. I should toss them but their RCA blackplates, 


al

[jim]

I don't think there is a "begin to short" state with power tubes. Its always during a gig when the amp is cranked.  Death occurs quickly.  There may be a sudden loss of power and increased distortion while the other tubes take up the slack for a time.  Then the fuse blows.  end of story.   There might have been a buzz but you didn't hear it because your ears are already ringing. Tubes age slowly and tone dwindles unnoticed--it is wise to replace then before they fail.  I do mine every 100 hrs of loud playing or 20-25 gigs.  Jim

[Ritchie200]

Jojo,

You've seen my stack of REAL Gold lion KT88's that I've accumulated over the years.  As Jim said, loss of volume, increase in distortion, and I just usually played through it, popped some new ones in during the break, and took back off.  When I got home I would check bias and rarely had to change anything.

As far as your amp issues... Horizontal EL34's?  I know on KT88's they suggest pins 4 and 8 be on the same horizontal line when tubes are horizontal - and that is from an old data sheet.  Not sure if there is a requirement for EL34's, maybe someone can chime in.  However, I cant imagine new manufacture-quality EL34's lasting a long time horizontal before the grids start bouncing around.  You said it's used a lot?  Retube it anyway as it probably needs it regardless.  You may be spending too much time on it.

Jim

[jjasilli]

Ditto.  Also, you can visually check the getter.  Should be shinny silver.  If it's brown, black or powdery white, or turning to any of those colors around the edge, maybe like a halo, the tube is failing.  Poor getter condition means the tube has little life left, even if it tests good.

Actually some eBay sellers can be devious here.  They'll say the tube tests good, but show you a photo of a brown getter.  Buyer beware.


Also I checked a few EL34 tube spec sheets.  There is no mention of horizontal mounting.  This leads me to concur that it's not OK.

[jojokeo]

The amp is a factory stock Marshall Haze 40 1 x 12" combo made with the tubes mounted this way. I've not had hardly any power tubes I can think of that went bad on me in the middle of playing, building, or testing? Except maybe when I blew the grids out on some EL84s once sort of on purpose >:) I've also purposely blown up some e-lytic caps and other things having fun... err giving them a stress test?!    I've re-capped a lot of bad/old amps and chassis amps too but nothing is quite like what I'm experiencing.

The amp sounds completely healthy one minute then BBBWWAAAAAAPPPPP! For several seconds then can go quiet and play normally again. It will continue to repeat but seems to have issues when knocking on it to cause it to keep reoccurring as well. I've found some significant issues that have been dealt with already like loose speaker connections, a bad B+ wire at the spade connector that was arcing... I keep thinking it's fixed then it'll do it again.

I hooked up my bias meter and initially was reading only 17mA per tube, started tweaking the two individual bias pots and went between ~10mA to 20mA. The tubes were vertical at this point. I turned the chassis on it's edge where it would be in normal playing orientation and the bias went to normal ranges!? So then set them for 39mA each and it seems better but - I tap on the chassis, circuit board, tubes, etc and get static sound now from every time I tap it hard enough. Not good especially for a combo amp. Also, I've re-soldered about the half the board too. There really wasn't anything that looked bad like the usual donuts around the leads. But I re-soldered anything and everything that even remotely looked like even a possible future issue. All pots, jacks, tube pins, etc. the usual suspects were also done. I checked all the send/return jacks etc. as well knowing that many times I'll find these dirty and an issue too.

Lastly when I tap the tubes or whatever and get the buzzing to occur I'll watch my bias meter jump around, but I figure this is par for the power tube course with something like this?

All good info guys, keep it coming! I'll check back later but for now going to see a friend and Eric Lindell playing locally in a couple hours.

[shooter]

in PP I agree with the symptoms already mentioned
In SE I have a gold lion 6550 that is a glowing marvel, past normal blue-plate.  It's lost the smooth, happy sound and darkened up to the point, highs don't seem to make it through.  DC spec's are all within normal range.  The other tube, a EH KT88, has just been whooped on, too much current, too much volts, open speaker side while cranked.  It started out as a chimey, sparkly tone, loved  whatever you gave it signalwise, now it's like I lined the tube with felt, lackluster just popped into my head.  Again, DC spec's fine, except a "tic" , quick transient current spike, and a slower DC sag, but it's random in time, and infrequent enough, it's still my primary PA build tube.

[PRR]

> say the tube tests good, but show you a photo of a brown getter.

Getter can be 99% brown and the tube still be good.

It was customary, in the Golden Years, to find a touch of brown on new tubes which went on to serve well.

A well-browned getter is not a wonderful thing, but is not a sure sign of a bad tube.

AFAIK, white getter is always bad. Brown means the getter has absorbed small gas but is still gettering. White always seems to be a major air leak, all gettered-out.

[Ritchie200]

It sure seems like Jojo could be better at diagnosing these amps that seem to be bettering him. However, he is getting old, his comments are a major air leak, and could just be all bettered-out.

Jim

[davidwpack]

I have a Laney that sounds comparibly bad to what it used to sound. It sounds as if the mids are turned all the way up and barely any treble. Also, there is an audible difference in volume. A friend's AC 15 is louder. I suspect tube failure but haven't yet investigated as I seldomly play through it. Preamp tubes are just as old though.

[jojokeo]

It sure seems like Jojo could be better at diagnosing these amps that seem to be bettering him. However, he is getting old, his comments are a major air leak, and could just be all bettered-out.

...ampS as in plural?! Ahem grandpa Jeeem, you’re pushing your threshold again! 


https://youtu.be/8OJJ0PLEVvc

[jojokeo]

Thread update:
Replaced the output tubes and the poltergeist has been exorcised! Even though the originals still tested good the earlier comment seems to have validity "the wires will lengthen, lose tension, may contact other electrodes and cause big problems."???

I could shake the tubes near my ears and hear things move inside like an old incandescent light bulb does when they are removed. The new tubes do not do this. However I have heard similar rattling with tubes that played okay?

[PRR]

> I have heard similar rattling with tubes that played okay?

My car rattles. If it is the muffler heat shield, it can fall off, the car still goes. If the muffler is falling off, I can drive but the cops will hassle me, I'll have to fix it. If it is the gearbox falling out, that will be different.

So it matters what is come loose and where it is hitting.

Never ever trust "tube testers". Sometimes they can confirm a tube IS bad. They never prove a tube is good.

Another example. My oil burner sometimes failed to light-off. I suspected bad spark coil. The oil tech kept using this "coil tester". It only measured the 120V side of the coil. Some faults (dead-short, dead-open) it would read. It didn't read the 30,000V side and many things can go wrong there with little effect on the 120V side. After the third costly visit (he basically shimmed the electrodes to get a spark) I got a new spark-coil at H-D and ZAM the dang burner lit-up instantly every time. Bah on "testers".

[jojokeo]

Classic! Once again your analogies are the best as always and really helps the info "sink in" and remain embedded deep into my mind space.