Scope Probe light show! :mad:

[eschertron]

Forgive me if this has been reported before; I couldn't find any thread - or any other internet discussion - that mentions this phenomenon. Here goes:

I had a scope probe on the plate of my Champ's 6V6. I'd probed the power tube plate on this and other amps before with no problem. I was monitoring the signal from my signal generator, sine wave, about 600vpp on the plate. Resistor load. I pulled the signal generator's 1/4" plug from the amp input, to move to input 2, just to see the difference in output level.

When I pulled the plug from the input, I started hearing a 'sizzle' sound, and the scope probe began to glow (internal arcing?) behind the cal knob. Before I could react, the 1/2A slo blo in the amp blew. I suspect that the internal grounding contacts in the input jack are not doing what they aught to, and there is some kind of UHF oscillation going on when I pull the plug. Comments?

[Tom Phillips]

…about 600vpp on the plate…

600Vpp on the plate of a Champ?

…I pulled the signal generator's 1/4" plug from the amp input, to move to input 2…

Most all of us do things that are not best practice and it usually works out fine. That is, until it doesn’t. Any time you make an abrupt change to a signal that is passing through a magnetic circuit (The Output Transformer) high transient voltages can be generated. If there is a weak point and arcing starts then the acing can continue to destruction of something. That’s what I think happened to you and I think the scenario started in the OT rather than at the input jack.

I have had similar things occur. Once you experience it personally you never forget. Funny thing is that you can't convey the seriousness to others and there will always be people saying "I do that all the time and it's no problem."

[eschertron]

So if the signal had been, say, a guitar being strummed then chances are the signal would be absent (me not strumming the guitar) when swapping inputs, and there would have been no light show. Makes sense. I did play the guitar through it *after*, and there was no funny business. The amp did hum more with the guitar out than in (at least with the guitar vol turned down), so I generated the input jack hypothesis. Maybe dirty jack, but not related...

[eschertron]

600Vpp on the plate of a Champ?

'78 SF champ, at my house it runs about 380vdc on plates and screens, so yeah, I get 600vpp before gross distortion.

[Enzo]

600v p-p, so 300v either sode of rest. 380v on plates plus 300v makes 680v. Is your scope probe rated for 680v?

The thing with ratings is they are not brick walls. If it is rated at 300v and you use it at 600v, it will probably work a lot of the time, but that doesn't make it OK. I think your probe was just waiting to strike an arc and die in the process.

[eschertron]

600v p-p, so 300v either sode of rest. 380v on plates plus 300v makes 680v. Is your scope probe rated for 680v?

The thing with ratings is they are not brick walls. If it is rated at 300v and you use it at 600v, it will probably work a lot of the time, but that doesn't make it OK. I think your probe was just waiting to strike an arc and die in the process.

The scope probes are simply marked with a sticker that says "coline", no model number or any other ID. From the sketchy info that I've found, it's possible they were rated for 600v, and possible they weren't. Yes, I know 600v is still less that 680!

It appears to be molded construction. So, assuming they are history, what should I be looking at for a replacement? Thanks for your input

[Enzo]

I won't stand here and tell you that I don't do the same thing with probes. I do. I just have gotten away with it.

To be honest with you, I have been using the basic cheap probes you can get all over for $30-35. Wait, I better go price them, I have not bought any in quite a while.

But the basic ones have always given me good service. They are rated for 600v, which hasn't failed for me in 40 years.

I have bought these:
3900 Series, 100 MHz passive probe Mfg by Probe Master

and these:
Tenma 100MHz Oscilloscope Probe Kit | 76-102 (76102) | Tenma


Those were 100MHz probes. I work on audio, and I also work on digital stuff. If you are just doing audio, then even the 60MHz ones will be fine.

Over the years I have seen probes that were about 3 feet long and others about 5 feet long. I definitely like the longer ones. If I have some smallish flat thing on the bench, like a rack mount processor of some sort, then a short cable probe would reach it OK, the scope is right in front of me on a riser. But chasing a signal around a big mixer, and a short cord can run out of reach.


And who knows, you could have had a tiny spider crawl up your probes adjust hole and set off an arc.