Laney lc30 destroying valves

bandmaster188

hi folks,
just having a look at my mates laney for him as it stopped working at our jam night the other week.

a brief history.
pub, beer, mantelpiece. Vibration , beer in laney. Bang.
fuses changed and all well till the next jam night. When switched on the volume was very low and then nothing. Power light was on but no sound.
got it home yesterday and found the first two valves as pictured.
i put two spares back in and volume was very low and within about a minute those two valves were hotter than the sun.
i've tested the resistors on the valve bases and the ones I can see that look associated on the pcb and they all seem fine.
repair is probably beyond my limited knowledge but any ideas what might be causing the valve meltdown?
cheers!

https://i.imgur.com/1yBllyh.jpg

ICBM

If beer went into it, it could quite likely have conductive residue all over the inside even if you dried it out. It sounds like something is either shorting out the cathode bias, or leaking HT through to the grids.

If you're not up for taking the whole thing apart, thoroughly cleaning and drying it - front and back of any PCBs that could have got wet as well as the valve sockets - and probably powering it up with a Variac, it's best left to someone who's done this before.

This is why I don't like amps with top-mounted control panels and/or 'beer traps'...

ecc83

Yes, this can be a protracted process. We had whole, sq ft TV PCBs with cola in them. The only recourse was a long wash in water followed by hanging in front of the air blow heater in the industrial unit we were in for a week!. The telly mnfctrs would not supply PCBs and so we did not want to write off £300 of colour TV.

Mind you, I did rescue a Mersey Super 15 PCB (Dommy clone) which had suffered in a damp garage. I emptied a whole, large can of WD-40 over the board then left it in a dry loft for a fortnight. Been fine even since.

Dave.

ICBM

ecc83 said:

Yes, this can be a protracted process. We had whole, sq ft TV PCBs with cola in them. The only recourse was a long wash in water followed by hanging in front of the air blow heater in the industrial unit we were in for a week!. The telly mnfctrs would not supply PCBs and so we did not want to write off £300 of colour TV.

Mind you, I did rescue a Mersey Super 15 PCB (Dommy clone) which had suffered in a damp garage. I emptied a whole, large can of WD-40 over the board then left it in a dry loft for a fortnight. Been fine even since.

At least damp is clean. I rescued a Peavey bass combo which had been left out in the rain for two days - there was half an inch of water sloshing around in the bottom of the cabinet - by doing nothing more than leaving the chassis on top of a radiator for a week and then powering it up .

The problem with drinks is the organic matter that stays behind when the water has evaporated...

bandmaster188

Cheers chaps! The main pcb looks as clean as a whistle. The beer residue looks pretty much confined to the output valve area. One of the valve bases looks quite gunged up with beer and possibly looking like it's made a connection between 2  of the pins. I've given it a good clean with solvent and will test it when it's dried.
is possible that beer could of got into the transformer. It's one of those round types wrapped in plastic?

ICBM

bandmaster188 said:

Cheers chaps! The main pcb looks as clean as a whistle. The beer residue looks pretty much confined to the output valve area. One of the valve bases looks quite gunged up with beer and possibly looking like it's made a connection between 2  of the pins. I've given it a good clean with solvent and will test it when it's dried.
is possible that beer could of got into the transformer. It's one of those round types wrapped in plastic?

No, it will be fine.

Wash the valve sockets thoroughly with clean water - if you live in a hard-water area, it's genuinely worth using cheap bottled drinking water for this - since the contamination is water-soluble! Solvent cleaners don't always work as well, surprisingly.

bandmaster188

Cheers icbm! 
Have done that. I'll report back when dry.

ICBM

Just be careful with any new valves - one possibility is that the HT got fed through to the cathode network and the cap is shorted - and is still so when the socket is clean. That means no bias voltage will be developed and the valves will run wide-open, which would certainly explain the melting. Keep a very close eye on the valves when you power it up and tun off immediately at the first sign of trouble.

In theory it shouldn't happen - the EL84 is a well-designed valve with the high-voltage pins on the opposite side of the base from the low-voltage ones and gaps next to them as well, so shorting directly between pins is largely prevented - unlike the notorious pin 3 to pin 2 or pin 4 (or 6 if used as a support for the screen resistor) to pin 5 failures in octal bases. But put enough beer in and all bets are off…

Also, do the valve sockets have a metal ring/tube in the middle, and if so is it connected to anything? From memory it isn't in these Laneys, but some amps do use it as an anchor for the cathode connection - including some AC30s. Arcing between the HT pins and the edge of the ring on the valve side of the socket is a common problem. I remove the tube from the sockets, which is a bit of a fiddle but prevents the problem.

jpfamps

Worst drink I've dealt with in an amp is orange juice.

That was not great.

bandmaster188

Everything looking clean and dry now but when switched on the amp emits a low growly hum and those first 2 output tubes closest to the transformer are red hot within seconds. (you can smell the heat)
The valve bases do have that central pin icbm but nothing to suggest any arcing.
tech time me thinks!

ICBM

If it's just those two, it sounds like HT getting to the control grids somehow - it will affect both valves since they're in parallel. Not the cathode cap since that would do all four.

Tech time definitely.

bandmaster188

Thanks for the input chaps!

DJH83004

bandmaster188 said:

Thanks for the input chaps!

Would be interested to see how this one pans out when you get it to a tech, as IC quite rightly says the EL84 pin layout is particularly good at avoiding shorts, generally with non used pins as separation. I have a VC30 on the bench at the moment and can't see anything obvious track wise, but then again I can remember if the layout is the same as the LC30 without digging out old photos. Anyway good luck. 

bandmaster188

Well, I've gone back to this a few times. Stripped the pcb out to check for anything causing a short. Nope. Checked some more components. Put it back together and it's working.
the nasty hum has gone, and the valves have stopped melting.
 The only thing I found that was possibly a bit suspect was a globule of solder on one of the cables to a valve pin (v7) that was just touching the cable on the next pin. Wether this had just melted through the insulation and was causing a short or not I don't know. Couldn't see without a magnifying glass and I forgot to check before I put it all back together. 
But all good for now!

ICBM

It's possible that if there was a very near short which had been there since the time it was built but which was not quite making contact, that the beer could have completed the circuit. It does sound like you've accidentally fixed it, anyway!