Oscilloscope help -setup issue

[mac0611]

Hi guys! I have an HP 1741a scope, and want to check the ripple on the B+ rail of an amp I'm building. The manual says the scope cannot have more than 250V at input. I have a 10001 high voltage 10X DC probe rated for 650V. Can I use this probe safely, or do I have to build a resistive divider network to keep the voltage in range? Thanks!  

[sawdust]

This link may help or not, it has a bunch of info about using an o'scope on guitar amps.

http://www.tone-lizard.com/


Dave

[mac0611]

thanks sawdust. I've read that entire sight a bunch of times, and there isn't anything specific about setup for high voltage applications. Hopefully someone can answer this, as I won't risk blowing my scope.

[PRR]

> want to check the ripple on the B+ rail

Build this.

Multiply 'scope reading by 10. (If it looks like 1.3Vpp at the 'scope, it is 13Vpp at the rail.) (Actually the multiplier, for 1Meg 'scope, would be 12; but how accurate do you need to be? If you are 10% fussy, change 100K to 120K.)

> I have a 10001 high voltage 10X DC probe rated for 650V. Can I use this

Probably. Maybe. Of course only up to 650V. Can't find a data-sheet for it, don't like to risk fancy probes on dumb jobs, and others may have the same problem and not have a 10001 on hand. The stupid 0.01u+1Meg+120K kludge lacks treble response (falls above ~~10KHz) and exact calibration, but will read any ripple or most audio-trash you may find on B+ rails. And if you smoke it, you are out about a dollar.

The cap must be rated for well past your expected supply voltage. 600V or 1,000V is wise.

The Ground connection -MUST- stay connected!!!! Connect that to circuit common first.

[Fresh_Start]

The Ground connection -MUST- stay connected!!!! Connect that to circuit common first.

Please forgive my ignorance here - do you mean that the ground of the voltage divider is connected to chassis ground of the amp and then the probe ground is connected to the same point?

Just want to be sure that I understand this.

Thanks,

Chip

[mac0611]

thanks PRR!  I'm going to build it today. I want to be able to use the scope as a tool for checking the B+ rail during diagnosis. Just another tool in the box. I have a tweed pro clone with noise issues and a low hum in the backround that I woit.uld like to check while playing

[Aurora]

The 250V limit of the scope is the limit of the input at the scope connector. If you use a 10x probe, the 650V limit is of the probe, - the scope will see 65V and is OK. If you need to measure higher voltages, get a 100 -probe, and your scope will be good up to 2500V. The snag is of course that the scope resolution increases with the probe's divisor, i.e if the lowest range of the scope is 1 mV , it will be 10 or 100 mV resp with a 10x or 100x probe. If the probe is not an original accesory to the scope brand, you must remember to multiply "buy head".... "original" probes for quality brand scopes usually autoswith the range marker......
( I've been using HP scopes for well over 30 years, but off my top I cannot recall  the 1741.......)

EDIT: Remember - Use the scopes own cal. point and adjust the probe trimmer for correct waveform response........
and - that little circuit above is of no use at 50 hz..
at 50 hz the reactance of a .01 uF is over 300k, giving you over 20 % error..
( OK.. somewhat less on 60 Hz...)

[PRR]

> be sure that I understand this.

"Keep one foot on the ladder at ALL times."

> The 250V limit of the scope is...

Welcome Aurora.

Yes, there are several limits. The input sensitivity switch can spark-over; if you set it wrong, 250V at first grid/gate may blow something up. The 10:1 probe "seems like" that should multiply to 2,500V at the probe, but the parts inside the probe are small, typically 500V rating. Which might be 550V max at probe tip. Dunno what the 10001 does to claim 650V but I can believe it.

HowEVER, _my_ 10X probe is melted and bodged from years of sticking it in tube amps. I always found something which not only exceeded nominal rating, but drew arcs or exploded the R and C inside.

Remember that most small parts have low voltage ratings. Half-Watt resistors often have 350V ratings. We know that they can stand more; the rating is about long term stability. For a quick-check divider, I'm not worried you may put 500V on that resistor.

> that little circuit above is of no use at 50 hz..
at 50 hz the reactance of a .01 uF is over 300k, giving you over 20 % error..

Hmmmmm.... you are correct, it IS inaccurate.

I say it is accurate-enough for the stated purpose of "I.. want to check the ripple on the B+ rail". So you read "2V" and it is really 2.4V. But when we check ripple, mostly we want to know "very low? or high enough to hurt?" If it is a problem, we'd typically want to fix/change something to reduce ripple to 1/3rd or 1/10th, not a little 20% change. If we hear the 2V or 2.4V ripple, we want to make it 0.5V and see if it goes away.

i.e.: often a +/-20% reading is good enough to know if something wants fixing or not.

This isn't Government Calibration.

Also: "ripple" should be 100/120Hz and higher. The error is 12% or 1dB at 120Hz, less further up. 1dB differences in audio are hardly worth noting. I grant that, if he has a supply rail issue so bad he resorts to a 'scope, he may have 50/60hz trouble too. But hum is as likely to be ground-borne.

> "original" probes for quality brand scopes usually autoswitch the range marker...

I've been using non-HP scopes for well over 30 years and just last year got one which would "auto-switch". It was not a common thing in the 1960s. In shops which need good 'scopes to turn a buck, auto makes a ton of sense; but we mere audio techs don't need fancy frills and often use lesser 'scopes. Two of mine are pre-WWII. My Heath and Leader are fine low-price 'scopes and certainly not auto-switch. I got the Velleman Pocket Scope which is too crude for frills. And I finally got a Tek but not the original probes.

Has anybody spotted the reason I re-posted with the 1Meg resistor on the "hot" side of the cap? This relates to the reason I picked a "too small, inaccurate" 0.01u cap.

[sluckey]

and want to check the ripple on the B+ rail

If you just gotta 'see' it, put a 0.1µF cap in series with your probe. If you want to measure it, use a good DVM set to AC VOLTS mode.

[Aurora]

I'm not trying to patronize anyone, nor will I engage in any longwinded debate about measurements.
One main point, is that I feel that a measurement error of 20%+ is way to high...
Another point is that when you start poking around in HV circuits, the "awareness factor" is of major importance, both from a safety POV and in terms of knowing what you really are doing and measuring, including the voltage limits of you measurement devices.
Here in Norway, our power grid is fully balanced ( no neutral), and many a "wanna be" TV tech, have had the big surprise when hooking the scope ground to chassis in older TV sets, where the chassis was connected to one live phase, as in our setup.

On a fairly regular basis, I have to do symmetry adjustments on kW level class D shortwave transmitters, - for which I use 100x probes  - with a 4kV max. input.

Most "pro's" agree that oscilloscopes are not precision instruments, - very often as you say " just to have a look".... but you still must be able to rely on what you see.
As for the DIY scope probes, there are lots of circuits on the net, which easily gives you "pro" accuracy for just a few dollars.. radio amateurs build these regularly, - and new probes are available for 40$ and upwards - 100x are probably somewhat more expensive.
- And I've been in the electronics business for well over 30yrs, - using a myriad of different scopes,  ranging from old antiques to the abolute top notch brands ( and prices) - they all fill their purpose, provided you can rely on what you see!

Government calibration ...??  That's a totally different world!
I work as Sen. Engr. for an ISO certified company servicing the science community - we spend 40.000 $+ a year on instrument calibration only.............everything from small DMMs to microwave spectrum analyzers. Instruments without proper cal. tags are forbidden on our premises..........................

[mac0611]

Hi guys!

 sorry i didn't reply. If i don't at times, it is because I am very busy with work and school. Tubes amps is my hobby, and the misses frowns when I devote too much time to my passion.

 PRR- here are the specs for the probe:

    HP model 10001B

    max voltage: 600V

   operating range: DC to 25MHZ

   input impedence: 10 megaohms shunted by approxemately 20pF

   division ratio (when connected to an instrument having an input impedence of 1M shunted by not more than 45pF): 10:1

   I saw the scope method of checking the DC waveform, and using it to determine ground or cap problems as well as oscillations on the B+ line. Looks like a great diagnostic tool when building from scratch or diagnosing pesky problems. I've heard conflicting stories on the safety of my scope when using probes on high voltage lines. I'm just looking for the "word" from an experienced tech on this issue. Many thanks to all that have responded.