Using the oscilloscope

[thomasdj]

Hi all, I've just bought a used scope (leader lbo-522 20mhz) for monitoring and troubleshoot my builds..

The scope has 2 channels that shares ground and I want to see the 6.3VAC filament and the output signal of my tube amp simultaneously..

..so how can i do this..without getting shocked or short something in my tube amp!

Best regards

Thomas

[olddawg]

Put one probe, AC coupled across the filament supply and one probe AC coupled across the speaker out (under load). You probably want to have it set for line trigger as well. You probably want a 10:1 probe for looking at the speaker out. Set your amplitude and time base appropriately.

[thomasdj]

Put one probe, AC coupled across the filament supply and one probe AC coupled across the speaker out (under load). You probably want to have it set for line trigger as well. You probably want a 10:1 probe for looking at the speaker out. Set your amplitude and time base appropriately.

Thanks for your reply!

If i do that..wouldn't i connect the heater to the OT secondary..through the common ground in the scope? Btw the heater windings are center tappee..

[supawah]

it's ac coupled inside the scope through a capacitor, so I don't think so. it's never happened to me.

[diydidi]

Why do you want to check the filament waveform at the same time as the output?
As Thomasdj said, you will be shorting one of the heater terminals to ground via the other scope lead.
Don't do this!! Measure with respect to ground, ALWAYS!! Rather just connect one probe to the output, and with the other measure on only one of the filament wires (with respect to ground) .Both filament wires are balanced anyway.

[thomasdj]

Why do you want to check the filament waveform at the same time as the output?
As Thomasdj said, you will be shorting one of the heater terminals to ground via the other scope lead.
Don't do this!! Measure with respect to ground, ALWAYS!! Rather just connect one probe to the output, and with the other measure on only one of the filament wires (with respect to ground) .Both filament wires are balanced anyway.

well, there is a guy on youtube..search....the hunt for buzz & hum..he connect the scope as olddawg suggested..i think..however..my heaters are elevated 80v so i am afraid og frying something here..however, i want to do this to trace any hum from heaters.

[olddawg]

well, there is a guy on youtube..search....the hunt for buzz & hum..he connect the scope as olddawg suggested..i think..however..my heaters are elevated 80v so i am afraid og frying something here..however, i want to do this to trace any hum from heaters.

Well ... If the scope probe shield lead is not isolated, I guess you could short something out. I've never had that problem. But truthfully I usually check heater voltage with a meter. You might want to use the center tap as ground for that channel scope probe ground. I also use an isolation transformer on my bench to power the amps and such I'm repairing.

[thomasdj]

Well ... If the scope probe shield lead is not isolated, I guess you could short something out. I've never had that problem. But truthfully I usually check heater voltage with a meter. You might want to use the center tap as ground for that channel scope probe ground. I also use an isolation transformer on my bench to power the amps and such I'm repairing.

well, I think I'll just scope with one channel only..I've got a nasty 50hz hum..I'll start a new thread.

[g1]

I never had a ground clip on more than one probe. Ground clip to chassis. 2 channels grounds are automatically connected internally as far as I know.

[Weez]

Hey thomasdj,

Here's a pretty good video if you're just starting to use a scope:

EEVblog #279 - How NOT To Blow Up Your Oscilloscope! - YouTube


I get a kick outta that guy.