Eh...what just happened?

[Diapason]

Short version:
Flash of light out of the corner of my eye and an unholy pop through the speaker. No visible damage to anything. I'm a bit traumatised.

Long version:
In recent weeks, one amp was creating popping and crackling sounds through the speaker as it warmed up. The associated driver excursion was mildly worrying, so it was on my list of things to get sorted. I thought changing all the old input valves might fix it but the problem persisted, so it was either output valves (just over a year old) or, as seems to be a problem with these amps due to the ridiculous heat the valves generate, the caps.

This evening, I was turning on everything after a week away, and just after I switched the power amps on I remembered I now own a preamp too. I turned off the power amps (maybe 30 seconds after switch on, not ideal, I know I know) and very soon afterwards I powered up the pre (again I know, a bit impatient definitely). As I switched on the pre or very soon afterwards, I saw a flash of light out of the corner of my eye from the direction of the left amp/area and heard a massive pop through the left speaker. I turned off the pre (and everything else at that stage) and went to assess the damage. Thing is, I can't see any. All valves look normal --I haven't taken them out yet, but there's no obvious discolouration or anything -- and the Kharma ceramic driver is, as far as I can tell, also intact. They're notorious for turning into expensive dust, so at least that hasn't happened.

Having disconnected everything, I decided I'd switch the amp on again to see if I could at least identify a faulty valve. They are all glowing normally from what I can tell, although I'm not connecting the speakers to find out if the amp is still working. When they cool I'll take the valves out for a closer look, but for now there's nothing obvious. I didn't see any smoke, there's no smell of burning, the curtains remain flame-free.

What's happened here? Can a cap fail spectacularly in this way? Obviously the amps will be going to Cloney/Bernard for examination and possible surgery, but would anyone like to speculate what I might be facing? I've certainly read online about front cap replacement being necessary every so often due to their proximity to the insanely hot 6C33C valves, and early failure due to this heat. I've no idea what that might cost, or whether they're likely to have taken out something else on the way.

Gah, I could really do without this.

[Fran]

You probably won't see anything wrong with the valves when you look at them - unless they are totally melted out inside.

Its more likely that either a cap or resistor or both inside are at fault.

Having said that, the caps should all be rated at 105degC. At a guess (and no, I wouldn't risk the kharmas, but test with some other speakers) I'd say there was something wrong on startup. Valve amps can be a bit funny like that sometimes. Are you sure the power amps were off when you turned on the pre? Often a valve pre can put out odd stuff until they warm up a bit (ie 30 secs to a minute). If the power amp was still on when the pre did this, that could explain the pops.

Another thing that can cause those symptoms is a bad valve socket, you get fairly alarming crackling noises and the like, and often it can settle down as the amp warms up.

Fran

[Steve]

Oh No.... Not the hi-fi welcome home that was needed... I'm sure the electronics pro's out there will have much more valued advice. But my own (probably spurious) thoughts would be that it was simply a fuse that went...
Based on number of experiences, when a big output valve goes, there's quite a whiff of burnt stuff. Which you don't mention... I've had couple of fuses go without a show. But just recently one did go with quite a flash and bang.. Duly replaced and all singing again...
Very much hope same scene for you...

[Diapason]

Thank you gentlemen. I may steel myself for closer examination, but it won't be tonight!

Were the powers off? Yes, but not for long, and the valves would still have been glowing for example. Fading, but still glowing. The pre seems to have a 5 second buffer before it turns on properly, and I can't be sure whether the problem occurred before or after this 5 second period. I know I was still looking at the pre when the flash happened, so I hadn't got as far as standing up or returning to the seat.

Hopefully whatever happened won't be too expensive. I don't have access to any fuses (not like your amps Steve) apart from the fuse at the power inlet. It's still powering on, so that fuse would seem to be intact. Feckin' nuisance, cos I was really looking forward to getting into a good listening session and some time with the new pre.

[Adrian]

Some interesting stuff here on diyaudio...

http://www.diyaudio.com/forums/tubes-va ... -help.html

this is for a GM 20... which I think you have Si.

One of the guys made up a breadboard with fans to allow cooling of the amplifier.

Here's an image here of a GM 20 I think...



However one thing I notice.. and that is there is a large number of valves up to 10 (I think) fitted into Ceramic sockets which are soldered into a Printed circuit board, PCB.

Here's a fairly good undershot view of a GM 20



As for your problem Si... it could be anything... my suggestion is to get it properly checked out before connecting it to anything.

[tony]

Emm! I just see a Devialet on the horizon I can't see Simon going with the breadboards and fans.

[cybot]

Happened me once with a Musical Fidelity A100 despite the presence of fans, heat sinks etc. Flame shoots out through the vents. Big bang. One woofer fried on my beloved MC 2's. Christmas lights still flashing away. Brain fried. "Waaaaaaaaaaah!" Took me a full year of demos to find a suitable replacement. Enter Croft! Haven't look back since. But......despite the fact I've owned Croft stuff with a lot of valves, I suddenly saw the light and decided to steer well clear of amps overloaded with glassware. That's why I went down the transvalve route with the new Croft gear.
But enough of that. I sincerely hope the Graaf gets fixed quickly and the repair fee is sensible. Best of luck....

[tony]

When my Prima luna smoked a few weeks ago Fran identified a few resistors that had given up the ghost.See thread for details but the initial appearance
of a puff of smoke felt like the end had come for the amp. Turned out to be a simple enough repair(for a skilled techie).
It would sound like something like that or the other suggestions above are the likely cause.
If you look at the resistors the colour indicated they were the problem i.e dull and slight discolouration.
You had posted recently about issues so wonder was this on the way regardless of what you did on the night.
Again on reflection mine didn't sound right the sound got distorted when turned up.Intiially I thought the gain was just too high
but it was the resistors giving up.

Hope it all gets sorted easily and painlessly.

[Diapason]

Thanks all.

Adrian, thanks for the pics. I happened upon that site before and was mulling over one of the mods to allow biasing without the need to open the case. There's a lot of good knowledge there, I think, although as Tony said, I'm NEVER doing the breadboard with fan!!

[bod]

Nightmare (and a bloody dangerous one at that!). Bernard via Cloney should be able to sort it though.