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Old 14th January 2011, 04:16 PM   #1
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Default Recommended O'Scope Reading?

Is there a practical guide for using an O'Scope to take measurements on a tube amp?

I recently read a posting about using a Scope with a dummy output load that confused and concerned me. Perhaps I need to study the subject a bit more.
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Old 14th January 2011, 04:56 PM   #2
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Use a dummy load on the output (e.g. an 8ohm wirewound resistor) but remember that this may get hot. Don't test with speakers attached. The input should normally be shorted to ground if no input signal is used (e.g. when checking for hum and noise). Visible clipping on the trace roughly corresponds to audible clipping.
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Old 14th January 2011, 05:08 PM   #3
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Thanks,

The post I read had to do with the danger of grounding the scope.

www.diyaudio.com/.../40051-scope-connection-audio-dummy-load.html
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Old 14th January 2011, 05:11 PM   #4
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Quote:
Originally Posted by DF96 View Post
Use a dummy load on the output (e.g. an 8ohm wirewound resistor)
Just to emphasize -- always use a dummy load on a tube amp.

You might want to invest in a decent set of probes -- you probably don't need the expensive Tektronix/HP/Agilent variety at the beginning -- but they do make life a lot easier.

Some of the Heathkit distortion tester manuals (from the tube era) have illustrations of what the correct and incorrect oscillograms are going to look like. I think that this is true as well for the oscilliscope manuals from Heath, Knight and Eico of the same vintage.
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Old 14th January 2011, 05:22 PM   #5
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Troncones: Do a search in the tubes forum for oscilloscope, scope, and o'scope, searching titles only. There is a fair bit of info out there, but it's a bit scattered.

Do you have a function generator also?

some basic tests are:

check for noise on the output (across 8R load) with input shorted. Just put the scope probe across the load R and measure noise.

check square wave response for ringing (oscillations), rounded leading edges (reduced HF resp), or sloped tops (reduced LF resp). Usually done at 10Khz from function generator with square wave output.

Check freq response by inputting sine wave and varying freq from 20hz-20Kz +/-. You are looking for the freqs were the amplitude of the sine wave starts to drop. When the amplitude is half of the max value, that's your -3db point. Some amps are flat way beyond 20khz.....

Check power out by inputting a sine wave and measuring voltage across the load R with the scope and an AC voltmeter. Increase input sine wave voltage until clipping on the scope is just visible, then record AC RMS volts. Power =Vsquared/R. Usually done at 1khz.

Check power supply ripple voltage by connecting scope probe to B+, set the scope for AC coupling, and zoom in for the ripple voltage. NOTE: Make sure that you are using a probe rated for the voltage, and preferrably 10:1 voltage attenuation. I use a 600V 10:1 Pomona probe that I picked up on ebay of few years ago.

Be sure to calibrate the probe on the scope with the built-in sq wave generator. You want to adjust the probe to get nice clean sharp square waves out of the scope test jack.


Understanding Scope Trace Square Wave

amp freq response with o-scope

Last edited by boywonder; 14th January 2011 at 05:26 PM.
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Old 14th January 2011, 07:26 PM   #6
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Tektronix' guide 'XYZs of oscilloscopes' is an excellent resource all round:

XYZs of Oscilloscopes Introduction > Application Note : Tektronix
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Old 14th January 2011, 08:05 PM   #7
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Default Scope Grounds

Quote:
Originally Posted by Troncones View Post
Thanks,

The post I read had to do with the danger of grounding the scope.

www.diyaudio.com/.../40051-scope-connection-audio-dummy-load.html
The two major dangers from scope grounds are shorting AC line to ground if connecting to "hot chassis" equipment without using an isolation transformer to power the equipment under test (usually blows grounds in scope) and shorting audio to ground/blowing outputs in floating ground output type amps (I look from ground to either speaker terminal with the scope, then add the two p/p voltages to get total.)
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Old 14th January 2011, 08:37 PM   #8
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Pete Millet's website has a whole section nearly devoted to oscilloscopes, and being vintage texts, they are mostly referring to tube technology.

Technical books online

Scroll down 3/4 of the way to the bottom, in the test and measurement section. Don't forget to stop and view the other attractions while you're in the area.

..Todd
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Old 17th January 2011, 08:33 PM   #9
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Quote:
Originally Posted by boywonder View Post

Check freq response by inputting sine wave and varying freq from 20hz-20Kz +/-. You are looking for the freqs were the amplitude of the sine wave starts to drop. When the amplitude is half of the max value, that's your -3db point.
Edit/Correction: I just learned something reading the Chrish/Tubelab 6L6GC thread.......the -3 db point is when the output voltage is .707 x max value, not half. Newbie mistake....
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