Amp blowing fuses

[daz]

This is a continuation from a issue i mentioned in another thread that was not related, so i decided to start a new one. If you happened to have read the thread about my amp changing tone when warm you'll recall the issue turned out to be a bad tube which i found when i tapped one and it arced and the fuse blew. So i put another in leaving the good one and the amp was fine. I have dual bias so no problem with tubes that didn't come together matched. Anyways, i played it several times since then with no issues, but last nite i was playing it for maybe 10-15 minutes and the sound went totally silent for about 5 seconds, then the fuse popped. I wondered if it might be the same side, maybe something on that socket, so i put yet another in there. (all winged C's) It did it again right away. I then wondered maybe it was the other tube and not the one i just changed. Or maybe something in the amp. So i inspected both sockets closely for possible short and read both 1k screen resistors which read fine. I only had one fuse left so i never replaced the one in the other socket. So i stuck a set of EH in and it ran fine for about 10 minutes but i didn't like the tone. So i pulled them and put 2 more winged C's in, ones that were never in it when all this fuse blowing happened and it ran the rest of the nite just fine.

I kept thinking how people have been saying winged C's were not QC'd well, but it would be a heck of a coincidence for this to happen twice in a short time when i have a drawer full of winged C's i have used for years and never one incident like this. So my question is, where should i look for possible reasons for this in the amp? It's basically a 2204 output circuit and it's been dead reliable for a few years, but i recently rebuilt it with pretty much all new everything cept trannys/pots/switches.

[km6xz]

Overall, Svetlana "C" tubes are probably the most reliable tubes available and I have never had one die without a good cause in the first few months of use.
Why is a tube failing? Good question, is it in the same socket that both tubes failed? A loose control grid pin receiver on the socket could result in intermittent removal of bias and high current pulled. What does the fuse look like, did it blacken as a explosion inside or did the fuse wire melt and sag open? I would look for any signs of arching around the socket.

[daz]

Overall, Svetlana "C" tubes are probably the most reliable tubes available and I have never had one die without a good cause in the first few months of use.
Why is a tube failing? Good question, is it in the same socket that both tubes failed? A loose control grid pin receiver on the socket could result in intermittent removal of bias and high current pulled. What does the fuse look like, did it blacken as a explosion inside or did the fuse wire melt and sag open? I would look for any signs of arching around the socket.

The fuses had blackened residue inside. As far as the sockets, i saw no evidence of arching. They are also brand new beltons. Not sure if it's the same socket, but i think it may not be because when it happened for the 1st time last nite (second time counting last week when it 1st did it) i assumed it might be the same socket and replaced that tube only to have it pop immediately again. so i don't know. Doesn't seem to be the socket unless a miracle of 3 C's went bad in a row. I would suspect it's more likely that either it was the tube in the other socket that caused it twice within minutes because i had assumed it was the other socket and replaced what may have been the good tube leaving the bad one in to blow thru 2 fuses one right after the other. Or something in the amp. But then it ran great on the EH tubes and then another pair of winged the rest of the night so that seems to eliminate a problem in the amp leaving the tubes being bad as the reason. But that would seem to be a hellofa coincidence. Like you said, never a issue with them, then this? seems odd. So i was hoping for some ideas on what to check.

[MWJB]

My experience with Winged C 6L6GC is the exact oposite of KM6XZ's. Every single one I have bought has failed in rediculously short time (sometimes immediately out of the box). In fixed bias amps I have kept them to 30mA/13W dissipation or less & still they failed in tens of hours, rather than hundreds or thousands. A well known UK amp builder used them in a SE practice amp, running 400vdc (less cathode voltage) @ ~60ma. For light, home practice use I'd expect a 6L6 to last a decade in this environment, the Winged C lasted 8 months.

Someone I know disagreed with me, quoting the 30W data sheet rating and biased his to 27W in a low voltage SE amp, it lasted mere minutes of light use. In the old Electar Tube 10 they were simply a non starter, likewise in an early 50's tweed Pro, rebiased with a 330ohm cathode resistor (early 50's mA wise, 430vdc less cathode, so early 20's W dissipation).

In all these amps, regular Sovtek, JJ & EH tubes worked flawlessly, with expected tube life. In fact, tube failures (under normal & sensible use) for the other current production 6L6/5881 tubes appear extremely rare, even for the old ST bottle Chinese 6L6s.

I lost confidence in them & gave up buying the Winged C 6L6GC about 5 years ago. I was surprised because these tubes were recommended to me by a very reputable source, specifically because of reliability, so I don't doubt that things may have been different once upon a time. This apparent down turn in reliability predates the rebranding from Svetlana to Winged C.

Daz, if your amps works reliably with other tubes then the amp is not likely the issue. Liking the tone is another matter.

[daz]

Still, regardless of who's experience with reliability or lack of is more typical, the fact is i have about 10 EL34 winged C's that have all been used to some degree, many for a year or more. Yet in the 5 years or so i've been using winged C's i never had a single failure till now, then 2 in a row, possibly 3 if that last time i pulled the tube that was indeed the cause. I just think thats one heck of a coincidence, but on the other hand i have no idea what to look for in the amp that i haven't already. Also, i'm talking EL34's here and you 6L6. Maybe the winged C 6L6 are less reliable.

Daz, if your amps works reliably with other tubes then the amp is not likely the issue. Liking the tone is another matter.

Yeah, but it worked with the C's for days after the first blown fuse then did it again after a week. So i only used the EH a few minutes. Who's to say they won't do it after a time. And i removed the EH and put yet another C pair in when i wasn't digging the EH and those i played the rest of the nite. So nothing really points to the C's as being the reason. On the other hand nothing says they couldn't be. That the problem....there is no clear indication or even a hint towards what is causing it.