Pi Network, Plate tuning cap, etc

Discussion in 'Amateur Radio Amplifiers' started by K9FV, May 14, 2011.

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  1. K9FV

    K9FV Ham Member QRZ Page

    Hello all, here I am asking a silly question for you knowledgeable folks again.

    For a 6 meter amp, KW power (maybe more?), 3200 VDC no load plate voltage, using a vacuum capacitor for plate tune, and an air variable for antenna load.

    How much distance is required from vacuum cap to chassis deck or side of aluminum enclosure? Since vacuum caps tend to mount from each end, the middle glass part is not a real concern?

    Can the ant load air variable frame sit on chassis deck? OR should it be lifted up to clear deck... by how much?

    Can both be mounted to front panel for chassis ground? OR should they be set back from front panel a tad with support bracket to chassis deck?

    I've looked at photos of many different amps, but no-one has really commented on this distance.... is it important as long as it's over 1/4"?? 1/2"

    Thank ya'll again for the help,

    73 de Ken H> K9FV
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I prefer vacuum caps that have a threaded front bushing so they can mount directly to a panel, although this certainly isn't required. You can have panels and chassis components close to the glass or ceramic envelope, the tiny capacitance added by that doesn't do anything and you only have to watch the distance from the "hot" end (high RF voltage) to ground to prevent arcing.

    I usually mount the LOAD cap directly to the same panel, although that isn't a requirement, either. The voltage there is much lower so arcing is unlikely if you're connected to a 50 Ohm load.

    In my 1500W 6m amps I mounted both caps directly to the grounded front panel, and made sure I had a very good bond from the panel to the main chassis.
     
  3. K9FV

    K9FV Ham Member QRZ Page

    Very good Steve - that was what I would thinking, that the arc distance was the big concern. Mounting to front panel is an easy mount, and that's my plan - unless Tom, yourself, or some of the other brains on here who have been providing guidance suggested different.

    Thank you,

    73 de Ken H>
     
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    If you look carefully at my 6m amp construction articles you'll see both caps mounted to the front panel.

    But remember usually the "rest of the amp" is grounded to the main chassis, and the inductance in the path between those things and the panel mounted components does add some. I make sure the panel is very well bonded to the chassis at several locations along that interface, that's why you see all the screws along that area.
     
  5. K9FV

    K9FV Ham Member QRZ Page

    Thanks Steve, there are 6 screws bonding the front panel to the chassis on my cabinet - and this is with clean aluminum. The layout I'm planning is coming together and I'm just about ready to cut the hole for the tube mount - BUT I do think I'll wait until I get the final cap - I've ordered a vacuum capacitor for plate tuning and think it would be better if I wait until the cap is actually here so I can better tell position for tube.

    Thank you and all the others again for all the help and input for this 6 meter amp project.

    73 de Ken H>
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I agree it's best to start with all the parts on hand.

    If you read my 4-1000A 6m amp article I think it points out that amp was built literally in one weekend. Saturday from 6AM to about 11PM I built the power supply; Sunday from about 7AM to 10PM I built the RF deck, and Sunday night at 11PM it was on the air and making contacts.

    But I did have all the parts on hand, first.:)

    That amp was built the weekend before the June VHF QSO Party in 1984, and its maiden voyage was that contest. It never failed, then or since.
     
  7. K9FV

    K9FV Ham Member QRZ Page

    Steve, do you have a switch to prevent HV from being applied during filament warmup? OR is it ok just not to key amp to cause plate current to flow?

    Thanks for all the help - I've spent more time getting materials together, AND most of all, learning/understanding what's going on rather than just following a cookbook design.

    73 de Ken H>
     
  8. W8JI

    W8JI Ham Member QRZ Page

    Ken,

    You ALWAYS want the path from anode to grid to be as short and wide as possible, and anode to chassis path to be as short and wide as possible. Always. That's a golden rule, just like using an energy limiting resistor between the tube and filter cap with oil caps and a warm up timer and grid trip circuit is when using indirectly heated tubes.

    The anode path should have the lowest possible impedance.

    This does several important things:

    1.) It reduces harmonics.

    2.) It makes the amp much more stable.

    3.) It increases efficiency by reducing tank loaded Q.

    The ONLY way you should mount to the front panels is if the front panel is solidly bolted back to the main chassis at the edge nearest the caps, and the tube socket is up near the panel.

    If you look at this SW broadcast 3CX15,000A7 amp you will see the vacuum tube cap is right against the tube. The vacuum cap is mounted to a FULL sub-front panel. That sub front panel has screws every 2 inches clamping the sub front lip to the main chassis, and about 10 inches of space back to the socket. This makes the return path to the cathode and grid very wide, very good, and as short as possible for the physical size of components. Nearly all of the stability and gain flatness you can affect with layout will depend on following that rule. The loading cap can be further away and only needs to have a good short ground path back to the tuning cap, and to the output coaxial line shield. The loading cap will not affect efficiency, gain flatness, or stability. It will seriously affect harmonics though, so you have to treat it just like it is bypassing the coax from center to shield (or part of a lowpass feeding the next section of an LCL filter).

    This is a little bit bigger amp (three phase supply with a 350 pound transformer) and covers multiple SWBC bands, but it is a good example of careful layout because the harmonic specs are very tough (like there are at VHF). This amp, because of the layout, didn't even require additional external harmonic traps. Since you are building at VHF, you probably want the same attention to layout (if you want to meet FCC specs).

    [​IMG]
     
  9. K9FV

    K9FV Ham Member QRZ Page

    Thank you Tom, that is the type of info I've been looking for. My cabinet currently has screws every 5", I'll add an extra screw between each of those to bond a bit more.

    Due to the size of the anode cooler it will be hard to get the tube much closer than 7" from front panel - that will put it right at the rear of the vacuum capacitor.

    I'm taking a summary of all these posts and putting them together in a single document to review and learn from.

    a BIG THANK YOU to all who have contributed.

    73 de Ken H>
     
  10. W8JI

    W8JI Ham Member QRZ Page

    To give you a better idea how I do HF cabinets, look at this page:

    http://www.w8ji.com/shortwave broadcast amplifier.htm

    maybe you can get some ideas from it.

    I'm looking for VHF pictures that are non-cavity that are not proprietary. Everything I found so far is cavity. But the general layouts for non cavity low VHF would follow the same rules as HF amps.
     
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