Cro Probes: Using Cro for 400+ V Tube Circuits

Hi Eman,

I'm trying to learn all I can about electronics. Could you please tell me what cro (CRO?) stands for?

Best Regards,

Mike

CRO is acronym for Cathode Ray Oscilloscope. Was used for several decades as an indispensable testing device by Electronic Technicians. Most now use DSO or Digital Storage Oscilloscope. CRO's were recurrent sweep (that is: the beam would sweep from left to right at a predetermined frequency) As the beam would sweep, it would be deflected up and down by an electrostatic field. This would provide a graphical representation of the waveform of an AC voltage in real time. These older CRO's are still extremely valuable in diagnosing complex waveforms (Video, Chroma, Sync, etc..) and are great for measuring noise and hum in audio circuits.

Hey Techart,

Thanks for the info! I was wracking my brain trying to figure what the acronym stood for.

I actually have one of these, given to me by my stepfather some years ago. I've just now decided to pull it out and see if it still works.

Again, thanks for taking the time to give me an answer!

Best regards,

Mike

Mikerho,

Hey those things are not only great tools, they can be a whole lot of fun too. If you get that thing working and have any more questions, I'll be glad to answer. I've used one regularly for 47 years so I must have picked up some knowledge, I think, or heck I forgot what I was talkin' about.

Have Fun

I have been using one for over 40 years, and this is the very first time I have ever heard it called a "CRO".DAMN THE ACRONYMS!! FULL SPEED AHEAD!!

Bill

cro means cathode ray oscilloscope and if you want more search the wikipedia

Combining only a 90Meg resistor with a 10:1 10Meg input divider will corrupt your signals response because dividers are internally frequency compensated with capacitors.

Such a capacitor with 1/10 value of the divider`s capacitor (including cable) will be required to frequency compensate the cro`s response.
If only lower frequencies (up to some hundred kHz) or slopes of >= 1µs have to be observed very cheap dividers including frequency compensation are readyly available on the market.

True, but using 9 x 10 meg resistors wound onto an insulating former and covered, can easily have the necessary parasitic capacitance of 0.3 pF to allow a reasonable compensation.

Most x10 probes have input capacitance of 3 pF 10 Meg so the compensation is only 0.3 pF approx.

Also the OP does say he only wants it for audio use, so a few pF either way isn't a major problem.

John.

Before you attempt to read the waveforms on your amps, I would suggest that you check carefully with a voltmeter to be sure the common connection is floating from earth. If the amp is Totally transformer powered providing complete isolation there should be no problem. Also there should be no problem if the CRO is old enough that it does not have a Ground wire in the AC Mains Line. Some CRO's are grounded (earthed) through the ground line and the Probe Common Shield is therefore grounded. If the amp is not isolated from the mains, attaching the sheild clip from the probe to the common line in the amp can result in destruction of your CRO, your amp, and your dignity. Or perhaps just a blown fuse. Even if the schematics show that both units are OK, a leaky capacitor can cause problems. Don't take any chances, use your voltmeter to verify first. Good luck.

Very good advice.

I'd recommend an HV probe to protect your o-scope...even at 440 V because surges and weird powerful harmonic distortion can overwhelm when an amp is faulty.

If you are willing to fabricate something, you can make a tunable voltage divider with a heavy duty potentiometer...you would have to measure the resistance of the pot while cold and use that in your calculation. Be sure to get one with a linear taper, though.

It's safer to to put the entire signal through a network of fixed resistors so you can really control how large your sample ends up...like if you put your 70VPP into a 100K in series with a 10K, you'd see only 1/10th (7VPP) over the 10K resistor.

Also very good advice.

Hi

I was able to download an old scanned manual for the model i bought , it states

"Direct reading controls enable voltages below 10mv to over 500v from DC to over 10Mhz

against time scales..." yet it also states "max input Voltage 400v or 250v AC or 400vAC

and DC p-p combined. i can get some 500v rated probes from a 'good' electronic parts

supplier .......

but if i want to work with say el34's with 550v on the plates .... this will be no good.

any suggestions of where to source 500v+ probes (in Australia?) and would they be unnecessary

thanks

pyrotechno

Here is a url to a site that probably has what you need (a high voltage probe kit):

http://www.testpath.com/Categories.aspx?Category_ID=536201&c1=ppc&source=ggl&ad=tekHVprobes&gclid=CJSfz4SP-ZACFQlxOAodJjrb0g

Unless you want to trash your scope, I highly recommend using a high voltage probe...just be aware of your frequency response needs if you purchase one.

That's a good one FKIA...that particular device fits well within the Nyquist requirements for working with audio circuits. The 200 MHz range is 10x higher than the top of human aural response (20 kHz)...and the neat thing about it is that it could pick high voltage transient events like noise spikes.

10,000 times... you are going from KHz to MHz. The rest is quite correct though.

Bill

Oh Duh, guess I was distracted! Yes, 10,000 x, and noise spikes (especially from ESD events) can have such fast rise times, that it's nice to have that kind of frequency response to see them. It would also pick up very high harmonics from unexpected sources that could do what I call "sub-aliasing". I had a friend who could never understand the uneven volume response in certain tonal ranges...until we looked outside and saw the huge radio station antenna next to his property.