It is not clear whether you are saying that your (unidentified) scope already has AC mode, or whether you think you need a new probe to get AC coupling?
Ripple is typically a very small signal. Using a 100x probe will probably make the ripple nearly invisible.
You don't need a 100x probe unless you are trying to measure a very large signal (350V). But you are not asking about measuring the 350V. That should be measured with a DMM. You are asking about measuring the ripple which is a very small AC component resting on top of the 350V. You need a 1x or 10x probe with AC coupling.
you can use a Tektronix P6006 Probe from flea market , 10:1 , <600V
It is build for glowing scopes but useful with any 1M input BNC
Martin
I think you are well aware that in AC coupling mode the DC component is not attenuated at all. So, if I use a cheap 100:1 passive probe for probing a 350+ V rectified signal in AC coupling mode I have good chances of damaging the scope. Hence, my question.
Quite to the contrary, AC coupling completely "attenuates" the DC component 100%. That is its whole purpose. It completely blocks any DC component and allows only the AC signal through. That is exactly what you want when measuring the small AC ripple signal on top of the large DC signal.
You have not revealed what scope you are asking about. Do you have some scope that has no input protection? If the capacitor charges up to the 350V, that means it is doing its job with 350V DC at the tube amp end, and 0V DC at the oscilloscope end. That is exactly how AC coupling works.
Are you saying that the AC coupling capacitor in your mystery scope is not rated for 350V? If so, then simply add a high voltage capacitor between the amp and your 1x scope probe.
If you use DC coupling with a 100x probe, you will never see the ripple.
Here is my understanding of the issue.
A 10:1 probe is a complicated 1/10 voltage divider.
You want to apply a 350V AC RMS voltage and look at the ripple.
That means you are apply 350*2*sqrt(2) AC peek-to-peek at the probe tip, or 990V -- Let's call it 1KV.
Then, 1/10 of 1KV is 100V -- well bellow the scopes spec of 400V.
p.s., make sure that there is no way the ground clip on the probe can be disconnected while probing high voltages.
As pointed out, the AC mode of the scope places the full DC voltage across the scope's AC coupling capacitor, regardless of the probe ratio or rating.
If you are unsure about the max. rating of this capacitor (which is expected to include the specified max. voltage of the scope alone plus some headroom), leave the scope in DC coupling and place a suitable capacitor between the B+ rail and the probe tip.
So if you scope is rated for 400V max. input voltage, this would be safe to use on 350V DC in AC coupling mode with any probe. Usually one wants to use a 1:1 probe here for best display resolution of the ripple voltage.
I've had spectacular ka-bangs come out of scopes when I disconnected the probe and switched back from AC to DC coupling. This hard shorts the scope's input blocking cap as it discharges through the switch.
The 1MEG input resistor is in the scope, and AFTER the blocking cap when you are on AC-coupling. So the blocking cap charges right up to the full HVDC potential on B+ even though on x10. You can measure ripple at 350VDC but you have to be careful. X1 is very risky, I would not use it without a jig with a blocking cap and voltage divider and bleeder resistor.
Connecting the scope to the HVDC present is hard on the scope's input protection (clamp) as you are fast charging the input cap to the HVDC present. Disconnecting (and touching GND or anything else in the circuit) the blocking capacitor discharges through the scope's protection clamp which is also hard on the scope and can damage it.
That would work, but one last trap.
Keep in mind changing VOLTS/DIV switch does switch in and out divider resistors, so I would start and end (measurements) at high settings like 10V/DIV.
Don't do this (turn amp power on/off or connect probe) at sensitive settings like 50mV/DIV where the scope's front end is vulnerable.
Looking at B+ you will see mains ripple, rectifier switching noise and the DC voltage will bob up and down due to mains voltage moving around, as well as load from music playing.
Here is my understanding of the issue.
A 10:1 probe is a complicated 1/10 voltage divider.
You want to apply a 350V AC RMS voltage and look at the ripple.
That means you are applying 350*2*sqrt(2) AC peek-to-peek at the probe tip, or 990V -- Let's call it 1KV.
Then, 1/10 of 1KV is 100V -- well bellow the scopes spec of 400V.
p.s., make sure that there is no way the ground clip on the probe can be disconnected while probing high voltages.
When you're using a 1x probe, it's closer than I'd cut it but it's within spec. When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope. Just make sure your probe is rated well enough.
Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?
When you're using a 1x probe, it's closer than I'd cut it but it's within spec. When you're using a 10x probe, sure, cause that's only 32V on the actual frontend of the scope. Just make sure your probe is rated well enough.
Please notice that I'am referring to AC coupling mode, so the blocking cap is charged at full 320V even when using attenuating probes. So, since I'm not sure of the blocking cap's rating, I can only infer it from the scope input rating (400V pk). BUT: is 400V pk enough for 320V DC?