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Switched capacitors in output pi tank

Discussion in 'Amateur Radio Amplifiers' started by KX4DV, Nov 2, 2018.

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  1. KX4DV

    KX4DV Ham Member QRZ Page

    I know one of the Henry amps I had used a "camshaft" to engage a array of doorknob caps for the plate tuning side of a pi tank. I'm working on a linear build that I'm trying to find an affordable means to build the sizeable pi tank needed. I've got a 15kv 800pf variable vacuum cap. But I'm still needing to sort how to work out C1 and C2 for this setup. From what I've gathered, I need up to about 400pf of capacitance n C1. And up to 1800pf or so on C2. It wouldn't be hard to find a 500pf variable in 15kv range for C1. But finding the 1800pf variable for C2 seems spendy since I'm only finding new. Could I possibly use a fixed 1000pf vacuum cap that I switch into the network via a cam/conductor or vacuum relay?
  2. W1BR

    W1BR Ham Member QRZ Page

    You have a few choices you might consider.

    Design the Pi network for 200 ohms at the load, and use a 4:1 matching transformer... or go to Pi L tank. Either will lower the required max capacitance needed for the load cap, but it will raise the needed voltage rating. do you really need that much voltage rating for the load cap to use a fixed value vac cap on the lower bands?
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Frankly I wouldn't even use a vacuum variable for plate tuning, unless it was a much smaller range variable that paralleled larger value fixed capacitors switched in by plungers and a camshaft, or a very good switch. An 800 pF vacuum variable will take a lot of shaft rotations when bandswitching from, say, 10m to 80m. More exercise than I'd want.

    C2 voltage rating doesn't have to be that high. With a 50 Ohm load, at 1500W output power it only has to stand off 274Vrms (387Vpk). If the load Z is even as high as 200 Ohms, that's still only 548Vrms or 774Vpk at 1500W output power. A variable capacitor rated >1500V should work, and conventionally we usually switch in padding capacitors in parallel with that for the lower bands, so the variable cap doesn't need to be so huge.

    I like capacitors that have no sharp edges anywhere (resists corona), even if they cost a little more.
  4. KU3X

    KU3X Ham Member QRZ Page

    15 KV for C 1, what's your plate voltage? Do you really need a 15KV cap? Just like Henry did and so do MOST HF amps, you can pad down C1 with another fixed cap. Watch your current rating for this cap.
    My 4-1000 amp runs 4500 volt, no load and my (C1) Vac Vari is only 7.5 KV. My 3CX1000A7 amp runs 3400 volts, no load and my (C1) Vac Vari is only 5KV. In my 38 years of all of my home brew amps I have never flashed over on the C1 cap. I've used Bill Orr's rule of thumb, " C1 should be 1.5 time the plate voltage" for me.

    Both of my amps use only the Vac Vari of C1 and they both easily cover 10 to 160 meters.

    As far as C2, usually stardard Door Knob caps will work just fine. You don't need 15 KV there. Stardard 5kv door knobs will work just fine. 1000 pf @ 5kv will work fine. If you were closer I'd help you out, I have a bag of them.

    Barry, KU3X
    Last edited: Nov 5, 2018
    N8ZL likes this.
  5. K9AXN

    K9AXN Premium Subscriber QRZ Page

    Without knowing what tube/tubes you plan to use and the power supply capabilities anything you do will be a guess. Pi section design is fairly simple if you know the capabilities of the tubes and power supply.

    Regards Jim
    N2EY likes this.
  6. N8CBX

    N8CBX Ham Member QRZ Page

    The times "1.5" is not what he shows in his tables in Figure 33, page 14-17, 23rd edition?
    What he shows in the first table is "Usual Breakdown Ratings of Common Plate Spacings" for air gap capacitors. As an example, 3000 peak voltage breakdown, has an air-gap of 0.070".
    The second table gives recommendations of air-gaps (assuming using a DC-blocking cap between the plate & C1, or typical Ham radio amplifier using CW/SSB). That table indicates a 0.200" air-gap for plate DC voltage of 3000. Well, it you divide 0.200 / 0.070 = 2.8. That's not a 1.5 multiplier.
    Now go back to the first table and find the breakdown voltage of a 0.200" spacing and it's 7500 voltage. So I take it that Orr's saying for a 3000 plate voltage, one needs a minimum of 7500 breakdown voltage for the tuning capacitor?
    Jan N8CBX
  7. KX4DV

    KX4DV Ham Member QRZ Page

    Plate voltage will be 5-6Kv. I did win an auction for a vacuum cap last night on ebay. 10kv rating. Tubes are two 4-1000a's.
  8. N8CBX

    N8CBX Ham Member QRZ Page

    What is your calculated plate loading? The tank "Q" might be very high with that capacitance that you mentioned.
    Jan N8CBX
  9. WN1MB

    WN1MB Ham Member QRZ Page

    Why only two 4-1000A's? :rolleyes:
  10. KX4DV

    KX4DV Ham Member QRZ Page

    2300-3200. I know according to the data sheets the tubes show 4600 as a single at 5kv.

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