Plate-to-heater EXPLOSION!

[Buddha's Guitar Tech]

Orange OR-120 amp.

Came to me with a blown fuse. Replaced it, switched it on, worked fine...
Hmmm, i think. Let's take two valves out at a time, see if they're ALL working.

Ok, these two are alright. Let's put these other two back in.
FLASH BANG Zzzt! Oh that lovely smell of burning bakelite.
Char marks on the valve base and socket between pins 2 and 3 (EL34)

Never seen this happen in the flesh. The worrying thing is the blast pattern around the valve base. It looks like it happened at the pins themselves...unless this is normal?
(yes, the valve was in the right way)

Replaced the fuse, put the first two valves in (avoided the charred socket) and it plays ok.

i'm guessing a new pair of EL34s will do the trick. Maybe even replace the socket i dunno yet.

Anyone any thoughts?

[Phil_S]

There is no way to avoid replacing the socket. The residue is conductive and can't reliably be removed. You should know this if you're a tech.

[dynaman]

My buddy's ancient SuperLead did the same thing on the same pins. He'd scratch the stuff away and replace the tube. Only took three tubes in one year before he finally gave in and replaced the socket....

[xtian]

That's exciting! What caused it to arc in the first place? Bad tube?

[Buddha's Guitar Tech]

There is no way to avoid replacing the socket. The residue is conductive and can't reliably be removed. You should know this if you're a tech.

i do now! (life is a learning curve)
i actually knew that about the inside of fuses, not specifically sockets.

Thanks

[Buddha's Guitar Tech]

That's exciting! What caused it to arc in the first place? Bad tube?

Not sure at this point. 2-month old set of JJs.
Last i heard, the player played, it was fine, plugged it into a different cabinet, it squealed and died. Got it here, plugged it in, no problem to begin with, but as i probed.... see above

New sockets all around i think. May as well, the amp is as old as the hills. Thinking on it, it should probably be gutted and repopulated.

[Phil_S]

New sockets all around i think. May as well, the amp is as old as the hills. Thinking on it, it should probably be gutted and repopulated.

Bad move. Don't do it. Replace one bad socket only.

If the owner wanted a new amp, he'd have said so. I wouldn't want to be you when you have to explain 1) excessive cost and 2) why the mojo is gone. Particularly #2.

[dynaman]

Yeah, repopulating with new, high quality parts will only lead to...disappointment. That same SuperLead I spoke of was a mess and had plenty of out-of-tolerance parts. It had a bit of hum and the warmest and sweetest growl I ever heard. I'm convinced the electrolytics were ready to die.

But alas, I tried to clone it twice and neither of them were accurate.

[Buddha's Guitar Tech]

If the owner wanted a new amp, he'd have said so. I wouldn't want to be you when you have to explain 1) excessive cost and 2) why the mojo is gone. Particularly #2.

i'd never go 'whole hog' without their permission, i ain't that green.
Trouble is, this is the second time i've seen this amp. When is it the third? The fourth? A little preventive medicine can go a long way.

i probably won't do the whole thing, i just like reconditioning these old things. i prefer that to working with a landscape that works against you.

[Buddha's Guitar Tech]

Yeah, repopulating with new, high quality parts will only lead to...disappointment. That same SuperLead I spoke of was a mess and had plenty of out-of-tolerance parts. It had a bit of hum and the warmest and sweetest growl I ever heard. I'm convinced the electrolytics were ready to die.

But alas, I tried to clone it twice and neither of them were accurate.

Damn shame, oh well.
MY original plan was, really, to keep the coupling caps and controls. Really only replacing what are mostly CF resistors, diodes and electrolytics. Problem is getting them back in - not the most tech-friendly board in there at the moment. It's a noisy old girl and i think the earthing/grounding could be made better (ie quieter) as well.

i am cursed - i've made four of my own amps and now i compare so much of what i see with the 'perfect' amps i've made.

But i'll most likely change the sockets and be done with it....

[Buddha's Guitar Tech]

i read something interesting about residue left over from past arcing and how once it builds up, it makes it easier for the potential from the plate to make its way to ground via the (next door) heater connection. (as there is not much resistance to ground via the filament windings of the transformer, especially in the case where you have a centre tap.
Makes a lot of sense with what happened with this amp.

i guess there was some old arcing residue i couldn't see around pin 3 which eventually led to my small disaster.
(Phil, you may be rolling your eyes )

With a meter on the filaments of all valves, i still get 0.7 ohms. i might try fitting the "blown" valve into another socket.

Anyone any thoughts?

[dynaman]

i read something interesting about residue left over from past arcing and how once it builds up, it makes it easier for the potential from the plate to make its way to ground via the (next door) heater connection. (as there is not much resistance to ground via the filament windings of the transformer, especially in the case where you have a centre tap.
Makes a lot of sense with what happened with this amp.

i guess there was some old arcing residue i couldn't see around pin 3 which eventually led to my small disaster.
(Phil, you may be rolling your eyes )

With a meter on the filaments of all valves, i still get 0.7 ohms. i might try fitting the "blown" valve into another socket.

Anyone any thoughts?

Not sure if I follow you correctly, but if you hook your meter up to the socket pins that correspond with the tubes' filaments, then you should have a low resistance reading like you stated. Your basically just measuring the resistance of the power transformer's filament winding. If your meter showed a high resistance or open circuit, then you'd have a problem.

[Buddha's Guitar Tech]

No sorry, i measured the resistance across the valve pins; out of the amp.

i know the filament winding in the PT works, as after the 'explosion' i had the amp up and running again with only one pair of EL34s.
(without using the burned socket)

[Structo]

About a week ago I was measuring the plate voltage on my D'lite.
I touched pin 3 on the 6L6 and it slipped off and touched pin 2, the heater.
Big arc, and a weird little fire ball right there, then the fuse blew.
I was surprised that it took so long for the fuse to go.
I was frantically trying to find the AC cord to pull but then the fuse went.
There was a little carbon between the two pins which I scraped clean with a dental pick.

Replaced the fuse and it was fine, I was afraid I had blown something, like the OT.....

Lesson? Probably best to use a clip lead and measure the voltage on pin 3.
Using the tip of the probe is asking for trouble.

[Buddha's Guitar Tech]

Haha, i've done similar crap. i once tried the shunt method to measure plate current and hit pin 5 by mistake. Oops. Now i like to measure voltage drops across the OT primary legs.


Back to topic; thinking, it makes sense that the valve's filament should be ok after such an incident....

"Fellow electrons! We need to cancel out the evil Positive Voltage! Shall we go THROUGH this filament, whose resistance is now quite high, due to it being lit up and all... or... F**k it, let's go via the winding!"

Ta da! (i'm here all week folks)

An amp-tech friend of mine once told me a really good reason for the 100-ohm resistors on filament windings without a CT. APART from getting balanced voltages on the filaments themselves, these resistors will blow out in case of a plate-heater short, likely protecting any other big-ticket items from ruin.
So it has been said....