Scope Probes and Cap

[GregS]

I just bought a used oscilloscope from a local surplus parts place. It is a 15 MHz B+K Precision 1477 which I got for $100. I got 2 new probes thrown into the deal for free - one a switcheable 10x/1x, the other a 1x. There is no indication of what voltage they are rated for though and the shop wasn't able to tell me either. The shop owner seemed doubtful about putting the high voltage of a tube amp into the scope. My understanding is that if using the 10x probe the scope will only see 1/10 the voltage so it shouldn't be a problem provided the probe is rated for the high voltage.

He suggested using a capacitor in series with the probe to block the high DC voltage (just in case) as it's only the AC that is of interest anyway. It makes sense to me but is there something I'm missing? Some reason not to use the capacitor? If not what would be a good value to use to ensure no relevant frequencies are blocked?

Thanks,

Greg

[Robert M. Martinelli]

Hi Greg,
the cap you're willing to use will have to be series-connected, so, together with the o-scope's input impedance, it will form an high-pass filter. The o-scope will have a very input impedance, ( ideally it should be infinite, in the real world it will be several megaohms ), but we can use a "worst-case" input impedance of 1 Mohm and a lower frequency of interest of, say, 1 Hz:

By reversing the known F=1/(2pi*Z*C) formula ( solving for C ) we get C=1/(2pi*F*Z), which, substituting our worst-case values, F=1Hz and Z=1Mohm yields C=15.9 nF.

So a 15 nF capacitor will be most likely ok.

A word of caution, the cap will have to be able to withstand the highest voltages which can be found inside an amp, so a 1KV cap ( or even a 1.5KV one for additional peace of mind ) is not out of question; also, be aware that, after each measurement, the cap will be fully charged, so, even if its capacitance is not as high as an electrolytic, it is advisable to discharge it after each and every measurement.

Hope this helps

Best regards

Bob

[stokes]

Most scopes are capable of taking the high voltage of a tube amp with the 10x probe,I would bet the B&K you have is no different.Even using the 1X probe you wont damage anything,you just wont be able to read the full trace.If you cant find a manual or schematic for it,open it up and see what the input cap is rated for,I would bet it at least 600v's.Do a search for B&K 1477 and I am sure you will find the manual for it.

[GregS]

Thanks for the replies. From what I read on here the scope shouldn't have a problem with the voltage - especially if using the 10x probe. However, the shop owner put some doubt in my mind about that, saying what happens if the probe is not rated for that voltage - which brings up the other issue:

It's not known what voltage the probes are rated for. Are most probes rated for the voltages is a tube amp? I would think a 10x probe would be intended for high voltages, otherwise what would be the point of it. No?

Greg

[defaced]

Manual and schematic:
MZE-Electroarts Entertainment - MZEntertainment.com: Dr. ZEE WORKSHOP B&K Precision Dual-Trace Oscilloscope model 1477.

I was just looking at buying one. Haven't decided if I'm going to or not yet.

[Enzo]

I wouldn;t worry about the scope itself at all. A X10 probe will step that 500v down to 50v at teh scope input. No sweat. My only conccern would be the voltage rating of the probe itself. You don;t want the cable arcing while it is draped across your wrist.


That said, I have found over the decades that the basic cheap $30 probes you can get everywhere, seem to work just fine, and I have never had any difficulties working with the high voltage in guitar amps.

[GregS]

Wow! Thanks a lot for the link to the manual and schematic. That's great! I did a quick search for it yesterday and came up dry.

So it sounds like the probes I have will be ok and I don't have to worry about using a capacitor.

Thanks to all for the info,

Greg