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100:1 probe for measuring ripple in a tube amp power supply
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Andreax1985:
All in all, I think that the safest solution is to use DC coupling mode with an external AC coupling capacitor (even if it sickens me not to being able to safely use the AC coupling mode in my scope, if they feel that I cannot use full input in AC mode they should have specified it in the datasheet). Now, for computing the minimum value of this capacitor I'm using the formula f = 1/(2*pi*R*C) where f is the desired -3db frequency of the high pass filter (let's say 2Hz), and R is the total resistance (let's say 10Mohm when using 10x probes). This yields C = 10nF. If I wanted to cut frequencies below 15Hz I'd choose C = 1nF. Is this correct?
trobbins:
To minimise amplitude differences between mains and 2nd harmonic, perhaps aim for one decade below mains frequency.
rf-loop:

--- Quote from: Andreax1985 on January 30, 2019, 09:23:06 am ---All in all, I think that the safest solution is to use DC coupling mode with an external AC coupling capacitor (even if it sickens me not to being able to safely use the AC coupling mode in my scope, if they feel that I cannot use full input in AC mode they should have specified it in the datasheet). Now, for computing the minimum value of this capacitor I'm using the formula f = 1/(2*pi*R*C) where f is the desired -3db frequency of the high pass filter (let's say 2Hz), and R is the total resistance (let's say 10Mohm when using 10x probes). This yields C = 10nF. If I wish to cut frequencies below 15Hz I'd choose C = 1nF. Is this correct?

--- End quote ---

Siglent internal AC coupling is designed so that it meets front panel information. 400Vpk. You can connect 400V DC to oscilloscope input using internal coupling DC or AC.

How ever it is, it is still good practice to use external coupling capacitor in Your case what you have told also for reduce scope internal parts stress.

Previously I have also told about external coupling capacitor and freq response.
Yes your calculus is ok. (Roughly because it is complex network and not only ideal R and C. In practice still well enough accurate.)



Also this can use
You can give known/wanted values and it calculate unknown.
http://www.learningaboutelectronics.com/Articles/High-pass-filter-calculator.php

As told previously Siglent this model internal AC coupling have quite low corner frequency. Around 1.2Hz what is low. Perhaps too low for many applications. (if example testing ripple this "dc" under test may have too much drift/fluctuation so with sensitive V/div settings it is difficult to observe due to large vertical movements. So, with external AC coupling it is easy to select suitable corner frequency.

Btw, when use input coupling DC with also external DC coupling,  it is good to note that you can select vertical mode for fixed offset voltage or for fixed vertical position and this last one is handy in many cases when looking some small signal riding over DC. Now when adjust V/div range it keep trace position. This feature can use even for lot of more high resolution DC level (indirect) measurements using offset because offset DAC is lot of more than 8bit. 10x probe, 5mV/div and offset max 20V and offset resolution 100uV.
(and same for horizontal, there can select fixed delay time mode or fixed position mode (example for keep trigger position other than center of screen. Example fixed position in nearly left side of screen and it stay fixed independent of t/div adjusting)
trobbins:
Also note that valve amp turn on can cause a much higher voltage than measured at idle,  as diodes conduct before typical amp valves.   A scope interface should aim to cope with at least the peak of the power transformer hv secondary ac voltage, plus margin.
2N3055:
I would not use anything that is not safe up to 1000V on valve amplifiers..
If you include peak voltages, lifted grounds, grid voltage variations, wrongly positioned voltage selectors, and you need some reserve..
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