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100:1 probe for measuring ripple in a tube amp power supply

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Fungus:

--- Quote from: Peter S on December 04, 2022, 03:14:22 pm ---At the risk of beating a dead horse,  why not just build a simple resistor voltage divider,  two  1 Meg resistors with the scope at the centre tap.   Would this not reduce everything to safe limits,  AC and DC?

--- End quote ---

The impedance of the average oscilloscope input is also about 1Meg so when you connect up your probe you'll be loading the circuit down.

A 100:1 probe will have a very high impedance (99MOhms) so that won't happen.

You can do it with resistors but you have to take all the impedances into account.

Make sure your probe is rated for high voltage. There are 100:1 probes out there which are sold for their impedance properties, not their high voltage abilities.

TimFox:
Again, I hate to re-animate a dead horse, but I have successfully used a good polypropylene capacitor and normal metal-film resistor to form a high-pass network to pass frequencies above, say, 10 Hz to measure ripple on a Vbb supply at a few hundred volts.

For AC at 50 to 120 Hz, this is essentially 1:1, and the resistor can be, say 100 kohm to get only a 10% fudge factor (frequency response only) with a scope impedance of 1 megohm.
For the high-pass capacitor, 220 nF or so will suffice.
The AC loading of 100 kohms should be negligible for a Vbb rectifier circuit.

As always, proper high-voltage safety precautions must be taken.

tautech:

--- Quote from: TimFox on December 04, 2022, 06:48:39 pm ---Again, I hate to re-animate a dead horse, but I have successfully used a good polypropylene capacitor and normal metal-film resistor to form a high-pass network to pass frequencies above, say, 10 Hz to measure ripple on a Vbb supply at a few hundred volts.

For AC at 50 to 120 Hz, this is essentially 1:1, and the resistor can be, say 100 kohm to get only a 10% fudge factor (frequency response only) with a scope impedance of 1 megohm.
For the high-pass capacitor, 220 nF or so will suffice.
The AC loading of 100 kohms should be negligible for a Vbb rectifier circuit.

As always, proper high-voltage safety precautions must be taken.

--- End quote ---
100% however for the novice I consider this a very risky practice as there is zero indication on the scope display or within measurements to remind the user they are working with elevated voltages.
Therefore best advice is to invest in 100x probes that will pass DC for the scope to clearly display the elevated voltages worked with.
As a secondary reminder but primary to the user is selection of a brightly colored probe as it's not just about getting the job done but protecting yourself and the instrument.
Recommend these if you can find them locally:
http://www.pintek.com.tw/productDetail/land-ctop-2/index/pscsn/17073/psn/19272

TimFox:

--- Quote from: tautech on December 04, 2022, 07:21:11 pm ---
--- Quote from: TimFox on December 04, 2022, 06:48:39 pm ---Again, I hate to re-animate a dead horse, but I have successfully used a good polypropylene capacitor and normal metal-film resistor to form a high-pass network to pass frequencies above, say, 10 Hz to measure ripple on a Vbb supply at a few hundred volts.

For AC at 50 to 120 Hz, this is essentially 1:1, and the resistor can be, say 100 kohm to get only a 10% fudge factor (frequency response only) with a scope impedance of 1 megohm.
For the high-pass capacitor, 220 nF or so will suffice.
The AC loading of 100 kohms should be negligible for a Vbb rectifier circuit.

As always, proper high-voltage safety precautions must be taken.

--- End quote ---
100% however for the novice I consider this a very risky practice as there is zero indication on the scope display or within measurements to remind the user they are working with elevated voltages.
Therefore best advice is to invest in 100x probes that will pass DC for the scope to clearly display the elevated voltages worked with.
As a secondary reminder but primary to the user is selection of a brightly colored probe as it's not just about getting the job done but protecting yourself and the instrument.
Recommend these if you can find them locally:
http://www.pintek.com.tw/productDetail/land-ctop-2/index/pscsn/17073/psn/19272

--- End quote ---

In my projects building vacuum-tube preamps, I needed to check for ripple on the Vbb supply, but a 100:1 probe into a 5 mV/div scope input would not be sufficiently sensitive.
I did not build these hp filters as "probes", but as a circuit that could be connected to the power supply and CRO before turning on the supply.
Safety first!

trobbins:
A well made or restored valve amp should have a bleed resistor across the B+ supply. Add a fixed divider resistor to that bleed, and confirm it provides a 10:1 or 100:1 ratio, and then just probe the safe end of the bleed. Make the probing process as safe as you can. I use that form of divider to extend to a maintence socket that can easily connect to an external meter for checking rails and bias levels.

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