Using my O-scope on Tube amp

TubeHound · 13th January 2003, 07:45 PM

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)?

Joel · 13th January 2003, 07:54 PM

Well, if it's 60Hz it's filaments. If it's 120Hz, it's power supply.

dhaen · 13th January 2003, 08:02 PM

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,

TubeHound · 13th January 2003, 09:30 PM

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.

TubeHound · 13th January 2003, 09:33 PM

Here are the peak to peak volt readings with attenuator turned all the way down.

TubeHound · 13th January 2003, 09:35 PM

Here is pk-pk volts attenuator full volume.

dhaen · 13th January 2003, 09:58 PM

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,

TubeHound · 13th January 2003, 10:07 PM

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?

dhaen · 15th January 2003, 03:53 PM

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,

TubeHound · 16th January 2003, 12:08 AM

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.

13th January 2003, 07:45 PM	#1
TubeHound diyAudio Member Join Date: Dec 2006	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)?

13th January 2003, 08:02 PM	#3
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	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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13th January 2003, 07:54 PM	#2
Joel diyAudio Member Join Date: Oct 2002 Location: Brooklyn, NY	Well, if it's 60Hz it's filaments. If it's 120Hz, it's power supply.

13th January 2003, 09:58 PM	#7
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	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,

15th January 2003, 03:53 PM	#9
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	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,

16th January 2003, 12:08 AM	#10
TubeHound diyAudio Member Join Date: Dec 2006	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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