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)?
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)?
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,
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,
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.
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.
Hi John,
If you are still there? I made a RCA shorting plug for the input on the amplifier. Wire from + to - .
The voltage on the DVM is from the B+ cap.
The probe from the scope was connected to the inside of the chassis on the binding post solder lugs. Ground from probe attached to chassis grounded binding post. Passive probe attached to positive binding post.
23 step GoldPoint attenuator all the way off.
Spikes are 120hz apart, every other spike ( all positive spikes on sine wave ) are 60hz.
How bad is it? I'm braced and sitting down.
If you are still there? I made a RCA shorting plug for the input on the amplifier. Wire from + to - .
The voltage on the DVM is from the B+ cap.
The probe from the scope was connected to the inside of the chassis on the binding post solder lugs. Ground from probe attached to chassis grounded binding post. Passive probe attached to positive binding post.
23 step GoldPoint attenuator all the way off.
Spikes are 120hz apart, every other spike ( all positive spikes on sine wave ) are 60hz.
How bad is it? I'm braced and sitting down.
Attachments
Paul,
To measure what's actually being fed to the speakers, move your scope ground probe to the -ve speaker terminal.
Now re-evaluate.
The spikes are probably rectifier switching. To prove this, disconnect the heater transformer from on side of the rectifier while the thing is running (careful!), then, in the second before the heaters actually go out, see if the spikes disappear.
There are ways to reduce these, but let's be sure of the cause first.
Paul, If you want to "wake me up", there's no problem with sending me an e-mail. I'll probably reply on the forum though, as others might be watching for hints.
Cheers,
To measure what's actually being fed to the speakers, move your scope ground probe to the -ve speaker terminal.
Now re-evaluate.
The spikes are probably rectifier switching. To prove this, disconnect the heater transformer from on side of the rectifier while the thing is running (careful!), then, in the second before the heaters actually go out, see if the spikes disappear.
There are ways to reduce these, but let's be sure of the cause first.
Paul, If you want to "wake me up", there's no problem with sending me an e-mail. I'll probably reply on the forum though, as others might be watching for hints.
Cheers,
Hi John,
Could you explain what -ve is? Would this be the negative speaker binding post? That is where I connected the probe ground.
The heater transformer wire to the GZ37 tube? Could I temporarily solder in a SPST toggle switch? This would be the safest way, I think.
John, I also noticed on the Audio Asylum that you are an Audio Professional, just curious as to what exactly it is you do? You obviously know your electrons.
Thanks again.
Could you explain what -ve is? Would this be the negative speaker binding post? That is where I connected the probe ground.
The heater transformer wire to the GZ37 tube? Could I temporarily solder in a SPST toggle switch? This would be the safest way, I think.
John, I also noticed on the Audio Asylum that you are an Audio Professional, just curious as to what exactly it is you do? You obviously know your electrons.
Thanks again.
Attachments
Paul,
Sorry about the negative binding post confusion. I was thinking that you were connected to your star earth point. (Which is a valid point for all other measurements).
Yes, I'm an electronic engineer.
Most of my recent work has been in Broadcast. But I've been around for longer than I'd like to mention
.
Cheers,
Yes, that sounds fine.
Sorry about the negative binding post confusion. I was thinking that you were connected to your star earth point. (Which is a valid point for all other measurements).
Yes, I'm an electronic engineer.
Most of my recent work has been in Broadcast. But I've been around for longer than I'd like to mention
.Cheers,
. I need to put the switch in series with one of the 300b filament wires?
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.