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Old 13th January 2003, 07:45 PM   #1
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Default Using my O-scope on Tube amp

Hi Guys,

I have a Tektronics oscilloscope, model 2246 Mod A. It has been recently calibrated and has multiple probes. It is 100mhz 4 channel rated to 400v, but have heard that it is conservedly rated.

What kind of tests can I make with this scope and some recommendations on the " how to's" would be appreciated.

I would like to start eliminating hum from my JE Labs 300b monoblocks. What should I look at first? The a/c filaments to the first signal tube (76)?
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Old 13th January 2003, 07:54 PM   #2
Joel is offline Joel  United States
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Well, if it's 60Hz it's filaments. If it's 120Hz, it's power supply.
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Old 13th January 2003, 08:02 PM   #3
dhaen is offline dhaen  Europe
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Default Tek scope

Paul,

I think this one has a button that toggles between t and 1/t.
Use 1/t, and you'll get a frequency readout.
Measure between 2 peaks or 2 troughs using the cursors.

Don't forget to connect the probe's earth to chassis.

Cheers,
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Old 13th January 2003, 09:30 PM   #4
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Hello,

Thanks for the replys.

Here are the pics of the scope readings. I just hooked the probe to the binding posts, I assume I'm looking at what the speakers are seeing.
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File Type: jpg hz1.jpg (36.3 KB, 703 views)
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Old 13th January 2003, 09:33 PM   #5
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Here are the peak to peak volt readings with attenuator turned all the way down.
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Old 13th January 2003, 09:35 PM   #6
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Here is pk-pk volts attenuator full volume.
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Old 13th January 2003, 09:58 PM   #7
dhaen is offline dhaen  Europe
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Paul,

It's the full volume hum we need to sort out first, though there are other problems.

5 volts peak to peak equates to 5*0.707/2=1.7v RMS which is far too much.

Ah,

I have no more time, sorry....

Cheers,
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Old 13th January 2003, 10:07 PM   #8
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Hi John,

Quote:
Ah,
I have no more time, sorry....
This scares me.


I do notice that there are spikes at 60hz and that the voltage for full volume is 11.?? volts. The scope was setup to take a/c readings and not DC. A flatline would be perfect at full volume? in a perfect world, right?
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Old 15th January 2003, 03:53 PM   #9
dhaen is offline dhaen  Europe
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Paul,

Can you tell me the test conditions?

Is there an input connected to the amp when you get this?

To test it properly, we normally do one of the following:

1) No input connected, but the input shorted.
2) No input connected, but a resistor connected across the input equivalent to the output impedance of the device normally connected.
If the source is SS, we normally assume the worst case of 1K ohm.
If the source is tube, really, it could be anything from 50 ohms to 10K ohms.
But you must do something with the input. An open circuit input is no test at all.

I'll wait for your reply berfore commenting on the spikes.

Cheers,
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Old 16th January 2003, 12:08 AM   #10
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Hi John,

I only have a cd player for input, (Tjoeb 99, 6922 tubes) and I had the rca connector plugged in and player turned on, no cd playing.

I could easily make a rca short plug and take another test.

I am eventually going to hook the amps to a Foreplay preamp that I'm in the slow process of building. The Foreplay uses two 12au7's.

Thanks again John.
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