Choke input filter power supply "Critical value of Inductance" questions

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John K5PRO:
The micas get stressed from both current and voltage. Find a Hp 4815A vector Z meter, WITH A GOOD PROBE. I got mine years ago, a friend just found one on ebay last month for <$200. All it had was one bad fast recovery diode in the sampler board, not the probe. You can see that the old tank micas of values like 0.01 uF are very inductive and have ESR above a few MHz. This eats their lunch when running current through them. Forget trying them at 10 meters. Depending on the Z, you can either stress them from RF voltage, or from I^2xR heating. I didn't know about them being often dead in the drawer ('DID') as well. That sheds a new light on what some sell as 'NOS'. I have a lot of them, both the stand up type pictured and the flatter ones that look like my VW's key fob.

I use old Jennings, CRL and High Energy 5 kV ceramic doorknobs for some amplifier jobs, the little ones like HT-58 series and 850 series. They aren't stable in tuned circuits, unless you use < 100 pF values, due to the type of ceramic used. If they are just DC blocking capacitors then they are fine. If you have too much reactance (too low a value of C for the lowest frequency) then they can overheat. In bigger industrial/scientific amps, I use 7.5 kV rated ones that are couple inches long, for things like screen bypasses (100 kW tubes).

KA2DZT:
Most all the old micas I have are recycled ones that I don't know the history of.  They've been sitting around in draws for decades.  I have some of those big tank style ones also.  A lot I pulled from scrapped GR equipment, they're in different draws (the maybe they're useable draw).  A lot of those postage stamp silver micas I also wouldn't trust.  I have hundreds of more modern silver micas (the little brown ones) that I do use.

As for the OP's two micas, I couldn't tell what circuit they're in.  I thought they were possibly some loading caps, but, I guess it may be the input grid circuit.

Fred

KJ4OLL:
John,
The HP 4815A looks really nice!
I sure would like to add one to the pile'o HP test equipment.
But after some searching, these seem to suffer from the same syndrome as the LCR and power meters - the probes went one way, the meter went the other way!

Lots of HP 4815A's for sale in lots of places, not one probe to be found anywhere.

I paid more $$$ for the fixture for the HP 4262A and the power sensor for the HP 436A than for the meters themselves, due to this issue.  

Frank

KJ4OLL:
John,
Thanks for the tip about how an HP Vector Impedance meter can help with the amp circuit design and component testing.

Completely addicted to old HP test equipment anyway, and thus did not need any other justification to acquire
one of these wonderful instruments (Thanks to Paul, K0UYA for selling me his HP 4815A)



Would appreciate some guidance on practical usage tips.
I read the manual and even found an HP app note for the 4815A.

Of course, this part was completely obvious:



A more serious example is the cathode input circuit, where there are currently two Micas installed:



These test fine on the LCR meter, and so the next test is to see where they are resonant and/or become inductive at the frequencies the amp will initially be used at, 75 and 160M.

When I sweep these caps w/ the 4815A, from 1.8Mhz to 3.85Mhz, this is the result:

www.youtube.com/watch?v=uUWh_8qOmN4

Not an engineer here, have NO IDEA what the "Phase Angle" means about the suitability of these capacitors.

Google tells me that "Phase Angle is the phase difference between the voltage applied to the impedance and the current driven through it"

Google also found this explanation, which gives me a dim and opaque level of comprehension of the principles involved, but not how to determine if the meter readings indicate capacitor suitability for this application:

"The phases angle (measured as cos phi) is the degree that current lags behind voltage in the sine wave (voltage lead).
So, if you consider that a sine wave passes through 360 degrees and the voltage (v) starts at point zero rising through 90 following to 180 (and similar on the negative side of the sine wave) the phase angel would be the point at which the current (A) starts to follow the voltage lagging behind it.
Lets say by 10 degrees then the phase angle would be cos 10 = 0.97.
This "lagging phenomena" effect is due to the the reactive load on the circuit and has an effect on the power available,
For instance in an ac circuit power = VxA cos Phi so for a load of 10 amp from a 10 volt supply you would have a true power output of 97 watts (as opposed to 100 watts in a dc equivalent circuit).
Most supply load are inductive reactant which results in this lag, however capacitive reactant will have a lead effect - this is why capacitors are used as power factor correction to bring the angle back to as near as unity (angle of 0) as economically feasible* "


Going to receive some doorknob caps in a couple weeks @ the Orlando Ham fest, will test them in the same manner and see if there is any difference.
73
Frank
K4ILL

KA2DZT:
Current leads the voltage in a capacitive circuit.  Current lags the voltage in an inductive circuit.  When both reactances are equal and opposite the circuit is at resonance.

Fred

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