Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => AmpTools/Tech Tips => Topic started by: six el six on November 20, 2011, 02:53:24 pm

Title: Which Oscilloscope probe to use on high voltage?
Post by: six el six on November 20, 2011, 02:53:24 pm

I'm going to experiment w/my o-scope tonight (tektronix 2235).

Do I use my 10x probe?

Apparently I want to stay under 4oov so I'm wondering does the 10x probe allow me to see the waveform on plates and other high voltage areas of amps?

Title: Re: Which Oscilloscope probe to use on high voltage?
Post by: six el six on November 20, 2011, 03:29:01 pm

I think I've answered my question.

If I'm trying to see the ac waveform of say, a power tube plate, then a 10x probe will effectively give me 1/10 of say, 450 v dc plate voltage in which case I'll be well within the limits of <400v peak.

Correct?

Title: Re: Which Oscilloscope probe to use on high voltage?
Post by: jjasilli on November 20, 2011, 06:17:14 pm

Yes I think you are right.  See:  http://www.tek.com/Measurement/App_Notes/tds360-p5100/eng/40W_11343_0.pdf (http://www.tek.com/Measurement/App_Notes/tds360-p5100/eng/40W_11343_0.pdf)

But Murphy's Law is always lurking.  If the probe is accidentally set to X1, you may damage the scope.  Another solution is to read the plate's AC signal through a large plastic cap, say .22uF or larger with a proper voltage rating.  That will block the high DC voltage to protect the scope, and should allow the full AC signal to pass to the probe.

Title: Re: Which Oscilloscope probe to use on high voltage?
Post by: PRR on November 20, 2011, 09:19:44 pm

No. The compensation cap in a 10:1 probe is rated 500V. If you apply more than 550V to the probe, this cap ARCS and ruins the probe.

So you may be able to monitor all the low-level stages, and the power stage plates at NO SIGNAL, but with any significant signal a typical 400V amp will be kicking the power tube plates over 550V, ZAP.

I used to have a collection of zapped probes.

If you must monitor power tube plates, build a dedicated divider. 10Meg:100K will reduce 700V kicks to 7V. Frequency response extends to 50KHz which is ample for our purposes. A 1X 1Meg probe across the 100K will cause 10% calibration error, but we are mostly interested in "shape", not absolute measures. 10X 10Meg probe across 100K gives only 1% error. You can calibrate by comparing idle voltage with DMM.

Title: Re: Which Oscilloscope probe to use on high voltage?
Post by: DummyLoad on January 21, 2012, 03:21:07 am

i have something similar to this:

http://www.ebay.com/itm/1-250MHZ-x100-Oscilloscope-clip-probe-Tektronix-HP-2KV-/170469789648?_trksid=p4340.m263&_trkparms=algo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D15%26pmod%3D250873031146%26ps%3D63%26clkid%3D5762650975350465854 (http://www.ebay.com/itm/1-250MHZ-x100-Oscilloscope-clip-probe-Tektronix-HP-2KV-/170469789648?_trksid=p4340.m263&_trkparms=algo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D15%26pmod%3D250873031146%26ps%3D63%26clkid%3D5762650975350465854)

--DL

Title: Re: Which Oscilloscope probe to use on high voltage?
Post by: six el six on January 21, 2012, 07:28:10 pm

Quote from: DummyLoad on January 21, 2012, 03:21:07 am

i have something similar to this:

http://www.ebay.com/itm/1-250MHZ-x100-Oscilloscope-clip-probe-Tektronix-HP-2KV-/170469789648?_trksid=p4340.m263&_trkparms=algo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D15%26pmod%3D250873031146%26ps%3D63%26clkid%3D5762650975350465854 (http://www.ebay.com/itm/1-250MHZ-x100-Oscilloscope-clip-probe-Tektronix-HP-2KV-/170469789648?_trksid=p4340.m263&_trkparms=algo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%26otn%3D15%26pmod%3D250873031146%26ps%3D63%26clkid%3D5762650975350465854)

--DL

I decided on a high voltage probe too.