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Tube Input Z ?

Discussion in 'Amateur Radio Amplifiers' started by K6BSU, Mar 13, 2018.

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  1. K6BSU

    K6BSU Ham Member QRZ Page

    I have finished building an amp using three 811A tubes in GG. For the input tuned circuit, I designed Pi networks to match 50 Ohms to the tube input Z. I used a value of 107 Ohms for the tube input. Plate voltage is 15oo V at full load. The driver transmitter (Kenwood) shows higher than ideal SWR driving the amp. The Kenwood doesn't "fold back", driving the amp, so it will still work.

    I'm wondering if I have the correct value for input Z to the tubes. The Pi networks are not adjustable, and the inductors are toroids.
  2. W1VT

    W1VT Ham Member QRZ Page

    In practice, the low permeability cores typically used at HF do allow for adjustment with the proper selection of wire and core sizing to allow the turns to be expanded or compressed on the core. Though the first article I recall where this was done was a set of VHF converters.

    Zak W1VT
  3. KD8DEY

    KD8DEY Ham Member QRZ Page

    there was somebody that made a very nice input board with relay controlled band switching, BUT I can't remember who it was offhand..
    If one tube is 320 Ohms. then 107 should be about right, BUT 1500v on an 811 sounds a little high...shouldn't it be more like 1250v?...Might want to consider 572B's...

    I know that the ICAS says 1500, but I always heard that it's not a good idea to push things to the max,...and your dealing with Chinese tubes, not good ole Taylor/RCA...
    Last edited: Mar 13, 2018
  4. WA5VGO

    WA5VGO Ham Member QRZ Page

    Consider the calculated values a starting point. I have never built an input circuit that came out perfect with the calculated component values. There are just too many undefinable variables. You will have to work with the values to optimize the network.
  5. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    Me, too.

    That's why most amps have tunable (adjustable) input tuning networks.

    When homebrewing GG amps I've used a small mockup board with all three elements (C1, L1, C2) adjustable and installed right near the tubes, like under the sockets; activate the amp (full power into a dummy load on the output) and adjust all three for a nearly perfect match on one band, then remove that board and measure all three components to see what the settings were. I write those down and use those values for the "real" network for that band, later, then go on to mocking up a network for the next band.

    If I do that, I can then use fixed components in the real bandswitched network to be used. But calculations alone never got me there.:(
  6. K1ZJH

    K1ZJH Ham Member QRZ Page

    I'd suspect a quad of 811 tubes would be 1/4 of 320 ohms, or closer to 80 0hms Z. Ideally a pi network is designed to be close, then has two variable elements to allow for fine trimming. Fortunately, in most instances it isn't that critical. You can use an antenna analyzer, and the tubes in circuit (COLD!!! no pwr!) with a carbon resistor between the cathode and ground to tweak your input circuit.

    Regarding relay controlled boards, Carl KM1H has been selling NOS from an commerical amp design. I have used a few and his prices are cheap. Second option is a TU-6B from WD7S, , but they are a bit pricey, although I did use one in my SB-220 when I added 160 meters, QSK and a bunch of other changes.

  7. VE7DQ

    VE7DQ Ham Member QRZ Page

    Hey, Pete!

    I tried the link you posted and I've been trying to get to WD7Ss site for some time now from two different computers, and the URL doesn't seem to be active any more. I wonder what's going on.
  8. G0JUR

    G0JUR Ham Member QRZ Page

    Just clicked the link and took me straight toWD7S site
  9. K6BSU

    K6BSU Ham Member QRZ Page

    Those input circuit boards are nice, but the capacitors are not provided. So you still have to calculate values and then test/fine tune.
  10. N8CBX

    N8CBX Ham Member QRZ Page

    No, he provides all the values in the PDF that you download. Or look for Bill Orr's 23rd edition, he has them too.
    Jan N8CBX

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