Probing around valve amps with an Oscilloscope

[diydidi]

Hi.
Its been a while since I have worked on valve gear. I was wondering if someone could clear out something for me. I have a Tektronix 453 Scope, as well as a Rigol DS1102E scope.
Can I probe those 250V-550V DC Valve- plate pins if I set my probes to x10 and the scope to AC coupling????
If not, how would I go about doing so?

[soundguruman]

usually OK

[g1]

Your probes themselves will have a voltage rating (as do multimeter probes). For safety reasons you don't want to exceed the voltage ratings of your probes.

[Enzo]

Yes, a X10 probe or a 1/10 set for X10 will take 250 volts and only show the scope 25v.

[soundguruman]

I got 2 tektronix scopes that never complain about plate voltage...
then I got this Goldstar scope, that spews FETs when I measure high voltage. hahahahahah!
I will take tektronix over panasonic, but where else can you buy a new scope for $225?

[Dave H]

Yes, a X10 probe or a 1/10 set for X10 will take 250 volts and only show the scope 25v.

A x10 probe is a 9M resistor. If you put the scope on ac coupling it will divide the ac by 10 but not the dc. eg. if you connect it to the 500v B+ to measure the ripple say it's going to put 500v across the scope's input capacitor. It's probably OK as there's the 9M resistor in series but I'd check the input spec. of the scope to be sure.

[JoeM]

A x10 probe is a 9M resistor. If you put the scope on ac coupling it will divide the ac by 10 but not the dc. eg. ....

??? How does the series resistor only work on AC?

[nickb]

For safety's sake I use a x100 high voltage probe rated to 1000V. About $50 from B&K. It's cheap insurance. Don't forget that on the output plates you can get twice the supply voltage.

[Dave H]

??? How does the series resistor only work on AC?

When you select AC coupling on the scope it switches in a DC blocking capacitor meaning there is no DC in the series resistor and therefore no voltage drop across it. The capacitor sees the full DC voltage on the probe tip but the AC passes through the capacitor so the 9M resistor in the probe and 1M scope input impedance make 10:1 divider to AC.

When you select DC coupling on the scope the capacitor is switched out of circuit so the probe 9M and scope 1M make a 10:1 potential divider to both AC and DC.

[g1]

A 10:1 probe works the same for AC or DC. If you scope 500VDC on the x10 setting it will show 50VDC on your scope. If you are on AC coupling of course you won't see any DC.
Try it. Even with a 9V battery or something.

[Chuck H]

I use a /10 probe (didn't use an "X" because it's a division, not a multiple). But I guess, for whatever reason, it's called a 10X probe!?! Anyhoo... My scope hase a 400V input capacity. So I do fine. I've never understood why so many insist on 100X (or 100/) probes. But I only know my own scopes specs. I suppose if a scope has an input voltage limit of 100V you would need to divide a B+ of 500V by greater than ten to make it safe for your scope. So... It's as much about the scope specs as the probe division. Like anything else, it has to be figured for. So I can't say "Always use a 100X probe." or "You don't need a 100X probe." What is the input voltage spec for the scope???

[JoeM]

I use a /10 probe (didn't use an "X" because it's a division, not a multiple). But I guess, for whatever reason, it's called a 10X probe!?!??...

Think of it how the scope vertical amp is set with the probe at 10X. If set for 5V/Div, with the prope on 10X, it's actually 50V/Div.

[Chuck H]

Thanks. I do see how it could go either way. But it seems to me that it would be clearer to call it a 10th probe, or a /10 probe. Just sayin'. 10X, to my mind, implies "10 times". And that the opposite of what the probe does. I suppose it's all good as long as the implemented nomenclature is understood. But with translation of languages, as well as typically understood grammar, it seems that the standard designation for scope probes is less than ideal. We need a scope probe designation revolution!

[nickb]

Probes use "x" because you multiply the scope reading to get the input voltage. The reason for my use of x100 (occasional) comes from the rating of the probe, not the scope. You can see that in some cases it is possible to exceed them for the probes I use. Best to check the datasheet for your own probes.

[Dave H]

A 10:1 probe works the same for AC or DC. If you scope 500VDC on the x10 setting it will show 50VDC on your scope. If you are on AC coupling of course you won't see any DC.
Try it. Even with a 9V battery or something.

If it's AC coupled and you put 500V DC on the input the coupling capacitor charges up to 500V through the 9M and 1M resistors as in the sketch below. I think the capacitor in my scope is only rated at 400V. If I want to check the ripple on the B+ I set the scope to DC and use a 10n high voltage capacitor clipped in the probe.