The resonating oil cap value is easily measured with any digital lcr meter.
The real problem is measuring the choke.? I went through this with dahl / hammond a couple of years ago. A standard digital LCR meter won't read the inductance correctly, but on the low side.
My B+ K 875A / and also my 875B? will read from .1 uh? up to 200H.?
When clipped onto my dahl? 4 H? choke ( 127 lb hypersil, rated at 3 amps CCS)... it reads like 3.26 H or a bit more.
The only way to measure the inductance on those chokes accurately per dahl / Hammond was to use rated? DC current flowing through the choke, while conducting the test.? They sent pix of the 2 units that are used for that job.? One is 20 amp rated, the other is 100 amp rated.? ?I will have to dig out the emails, but it used a bridge circuit and other items...along with the 20 amp dc supply, to arrive at the actual inductance.?
The problem with the resonant choke setup is.... you have TWO currents flowing through the choke.? The normal rated? DC current to the amp...and the 2nd current is the circulating current that flows through the resonating cap and choke.?
What was tested with resonated chokes? was 1H, 2H, 4H, 8H.? The smaller the choke, the bigger the resonating cap has to be.....and the more the circulating current.? On a 4H choke, the resonating current was as much as the actual DC output current.? Ok, the light bulb went on.? For resonant choke use, the choke has to be rated for a helluva lot more than? your dc plate current.?
You can use the LC calculator online to see what value cap required to resonate any choke to any freq.? ?Typ it's aprx .44 uf to resonate a 4H choke at 120 hz.? On the Collins? 30S1, the B+ supply is resonant at 120 hz.? ?On the resosnat choke setup for the screen supply, they resonated it at 165 hz.?
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John Lyles, K5PRO..back when he designed FM broadcast tube amps for BE ( broadcast engineering)..... this is on there small 5 kw output FM? PA..... John used a pair of .9 uf? @ 5 kv oil caps wired in series to make up the resonating cap.
The resulting .45 uf cap assembly was then sent to the choke manufacturer....and they designed the choke such that it resonated exactly at 120 hz....when rated normal plate current drawn.? This is on the 5 kw,? ?240 vac /? 60hz / single? phase B+ supply used in that PA.? ? So it's just a resonant choke...followed by a pair of paralleled oil caps, typ standard setup.?
Interestingly enough,? he said there is a spec for FM PA's...called an ...'AM noise spec'...and to pass that spec, extra filtering in the B+ supply was required.... to kill ALL the harmonics of 120 hz ripple freq.? ?The resonant choke only kills the 120 hz, and the main pair of oil caps are not big enough in value to kill all the harmonics of 120 hz (240-360-480 etc).? ? To pull that off, a 2nd choke was used( not resonated), followed by some more oil caps.? The 2nd choke + oil caps is a simple LC filter....which has 12 db attenuation? per octave.? Depending on the value of the 2nd choke and caps... will determine the staring point or corner freq, where attenuation 1st starts.?
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I got a hold of the 4 H dahl choke from a ham across town.? It uses the same pair of hypersil cores? as the dahl 10 kva plate xfmr.? several of em were used at the now defunct satellite uplink? station? 50 miles north of me.?
I'm using it in an experimental? ?C-L-C? config..... with a spst vac relay across the choke terminals? ( and also an adjustable spark gap across the choke).? Choke is in series with the B+, and is insulated from grnd with 4 x huge glastics.? ?So far, so good, with 120 hz ripple down to just 20 mv? ( -121 db) on a 7 kv loaded B+ supply...loaded to 2.5 amps CCS.
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Software only shows P-P ripple at 120 hz. BUT the C-L-C continues attenuating above 120 hz.....at the rate of 12 db per octave. IF I have done the calcs correctly, it's 1st modelled as plane C filter, (just C1) the 120m hz ripple is down to -54db. Then the LC is added to the mix. Then it's down to -119 db....(an additional 65 db) It works out the corner freq is just 2.9 hz......then attenuates at the rate of 12 db per octave above that.
If u only have X amount of money to spend, spend it wisely.? Money is better spent on a bigger hypersil plate xfmr...with a plane jane C filter...like a string of lytics... vs a plate xfmr + resonant choke.?
The problem with the FWB + resonant choke setup is.... the sec HV has to be a lot bigger.? Since the B+ is only 90% of the ACV...... vs? 41% more on a FWB? + high C filter.?
HaL Mandel