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Old 9th February 2015, 10:42 PM   #1
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Red face Now I have a scope and want to test amplifiers... but how?

I am not finding any reliable information for how to test high voltage amplifiers with an oscilloscope. Is it safe to step the voltage down with my own voltage divider/dummy load?
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Old 9th February 2015, 11:30 PM   #2
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Based on your own knowledge and experience, how safe would you feel?
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Old 9th February 2015, 11:39 PM   #3
Einric is offline Einric  United States
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100:1 Step down probes would be useful.
For every 100V getting in to the probes you get 1V out into the O-Scope.
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Old 9th February 2015, 11:41 PM   #4
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Yeah, just buy a 10x, 100x, or 1000x probe.
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Old 10th February 2015, 12:05 AM   #5
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Quote:
Originally Posted by counter culture View Post
Based on your own knowledge and experience, how safe would you feel?
I feel comfortable with high voltages. I have good experience in commercial electrical.

I am a newb when it comes to a scope though and the last thing I want to do is destroy one or whatever I am testing.

You still have to put some kind of dummy load on the amp though right? Won't it destroy the 100x probe if you don't? But then you have the dummy load and the 100x probe's resistor in parallel and that screws everything up right?

What about the probe introducing some kind of distortion with the divider inside of it? Will it not change the impedance into the scope or interact with the output impedance of the amp?

There are a lot of variables that I am not confident enough to just jump into. Does anyone have a schematic or drawing to point to that will be the best way to connect things?
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Old 10th February 2015, 12:18 AM   #6
rayma is offline rayma  United States
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Quote:
Originally Posted by shredhead View Post
You still have to put some kind of dummy load on the amp though right? Won't it destroy the 100x probe if you don't?
But then you have the dummy load and the 100x probe's resistor in parallel and that screws everything up right?
I would get two x10 probes. They must be compatible with your particular scope inputs. Be very careful about grounds.
You can only measure voltages using these probes with respect to ground, and the probe's ground clip MUST be connected
to the circuit ground at all times. The probe will present a very high impedance to the circuit being measured, and usually
won't affect the results. You do need a pair of 8 Ohm non-inductive dummy load resistors of at least 50W or more, and you
can buy such resistors at electronic distributors. Two of these would be 4 Ohms 100W in parallel as well.

Last edited by rayma; 10th February 2015 at 12:41 AM.
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Old 10th February 2015, 04:10 AM   #7
benb is offline benb  United States
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Quote:
Originally Posted by shredhead View Post
I feel comfortable with high voltages. I have good experience in commercial electrical.

I am a newb when it comes to a scope though and the last thing I want to do is destroy one or whatever I am testing.
The biggest worry here is slipping the probe and shorting something out with it. If you have a steady hand and/or use the little clip thing for the probe tip, you should have no problem.

Quote:
You still have to put some kind of dummy load on the amp though right? Won't it destroy the 100x probe if you don't? But then you have the dummy load and the 100x probe's resistor in parallel and that screws everything up right?
No, the probe and scope input are such high impedance that they have no effect on the amplifier's output, nor just about any other point in any reasonable circuit.

But the amplifier should have a load for measurement of its parameters. A solid-state amp won't be hurt if no load is connected, BUT a tube amp (with its transformer output) CAN be hurt with no load. A large signal can cause high AC voltage on the primary site of the transformer, and arcing in the tube or inside the transformer.

Quote:
What about the probe introducing some kind of distortion with the divider inside of it? Will it not change the impedance into the scope or interact with the output impedance of the amp?
Look up 10x oscilloscope probe schematics. There's a little trimmer capacitor across the "10x" resistor in the probe that acts as a 10-1 reduction with the capacitance of the scope's input capacitance. There's instructions of how to adjust this while looking at a square wave input.

Quote:
There are a lot of variables that I am not confident enough to just jump into. Does anyone have a schematic or drawing to point to that will be the best way to connect things?
Oh, here we are, these should explain a bunch:
https://www.google.com/search?q=10x+...obe+schematics

Your scope has vertical gain steps, probably to 10V/division, 20V/div or 50V/div, so that with the last, with a 10x scope and vertical gain set at 50V/div, if the trace goes up two divisions, you've connected the probe to 100V. Be sure to check the max voltage on the probe. I'm not sure what they usually are, but I'm thinking they might be only 250V. Double-check that when you buy probes.
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Old 10th February 2015, 08:14 AM   #8
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Quote:
Originally Posted by shredhead View Post
Now I have a scope and want to test amplifiers...
How about mentioning what you have there?
According it true potentials you will receive better quality of advices about probes.
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Old 10th February 2015, 03:46 PM   #9
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No use in measuring output of amplifier without a relevant load. Many faulty amplifiers will show a perfect output when unloaded, only to find that when loaded, they can't deliver any current at all. make yourself a dummy load 4ohm, 8ohm etc. Google is your friend here.
Then use both channels of your scope to do differential measurements. This is a must when measuring amplifiers in Bridge mode!!
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Old 11th February 2015, 02:43 AM   #10
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Quote:
Originally Posted by benb View Post
a tube amp (with its transformer output) CAN be hurt with no load. A large signal can cause high AC voltage on the primary site of the transformer, and arcing in the tube or inside the transformer.
Interesting, I always heard it was bad but it makes sense finally. I'm reading the service manual to my old school Tektronics analog scope (being shipped now) and have 2 10x probes on the way as well. Thanks for the tips everyone. I already have some 4 and 8 ohm 20W non-inductive dummies laying around to get started with.

Will you look at my picture here? I don't have an understanding of resistor networks yet. Will you explain to me why this won't put the 4ohms and the 1M ohms in parallel?

Instead of using the probe to attenuate the input, would it be possible to make your own divider out of power resistors which will act as the dummy load too?
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