Oops, flames...

[vibratoking]

I was working on a 183 clone at the time of the incident so I'll post it here. I was actually using a scope to set the PI trim. I was probing around looking at different waveforms with signal generator input. I decided it was a good idea to probe the plate of the PI at the socket. I accidently and VERY briefly shorted the plate to the heater with the tip of the scope probe. I probably shorted them for something like 100ms, just guessing. Wow, the light show came on fairly long after, maybe a second or two, the short occured. The two 120 Ohm resistors for the false center tap on the heaters caught fire and burned up in a somewhat impressive display of fire and smoke. The soot, there was a lot of it, emitted during the event made a big mess - sticking to everything it came in contact with. I shut her down, checked the wiring, checked the impedance of the heaters on all tubes and measured the output of the heater winding with no tubes installed. Cleaned her up and replaced the resistors. Nothing else damaged except for the two resistors. I guess I will have to be more careful next time. I was guessing that a tube or transformer might have been damaged? Anyone else ever done this and suffered more significant damage? I am thinking high wattage resistors in this spot might be a bad thing.

[David Root]

I did something that had a similar result a few years ago and burned up two 100 ohm 1W metal films.

I keep one of them where I can see it occasionally when I'm working just to remind me.

[DonMoose]

Once you get an arc going, it goes for a while - sometimes inside the tube where you can't see.

[talbany]

Once you get an arc going, it goes for a while - sometimes inside the tube where you can't see.

Here Here Moose!!.. Get a good arc going on a plate wire or cracked tube socket around pin 3 with a little back EMF you got yourself a little arc welding clinic..

Tony

[ampdork]

Had a client who had some electronic background.... well he just had to know the plate voltage so he could set his bias "just so"...
Replaced burned resistors and (knock on wood) 2-3 years later no reports of issues...

[heisthl]

They do make flameproof resistors (lots less gunk when burning) .

[Zippy]

I accidently and VERY briefly shorted the plate to the heater with the tip of the scope probe.

Reminds me of when my Fiat 124 VERY BRIEFLY was running at about 5000 rpm with a broken cam drive belt... When I pulled the intake manifold, I found chunks of piston in the throat of the Del Orto carbs. So sad.

Maybe this is a good time to remind people that eye protection is a good precaution when probing inside a live chassis.

A VERY BRIEF arc with burning shrapnel is not a good thing.

<flash, pop> Oops, I'd better put on my safety glasses. Damn, I can't see a thing. Where are they? Too late. Nevermind...