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Gs-35b legal limit load cap recommendations

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Oct 22, 2019.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    Does anyone who has built a multiband Hf have any recommendations as to which load cap to use. I have a nice load cap from a kenwood tl-922 that I picked up somewhere. It has one section good for 1100pf and another good for around 700pf which would (by my calculations) only leave me having to add a bit of extra load capacitance on 160. I realize the tl-922 was never originally designed as a full legal limit amp. My concerns are the plate spacing of the cap.....not very wide at all. The more I read about this, the more I realize there’s much more involved when doing the calculations for plate spacing. Should I even attempt to mount/use this tl-922 load cap or should I source another cap with wider spacing? The output tank is gonna be a standard CLC pi network using tapped inductor/inductors via a multitech industries model 86 band switch. I welcome any suggestions/recommendations. Thanks
     
  2. W1QJ

    W1QJ Ham Member QRZ Page

    That depends. That cap spacing is about 300v so assuming you keep swr low near 52 ohms that cap should be good to 1750 watts. However to be on the safe side if you go to a 1000 volt rated cap your capability goes way up but that makes a much longer cap for same capacitance. There is always trade offs.
     
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  3. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    In a pi-net, the LOAD cap voltage is all based on what the load really is; not much to do with the tube and not even that much to do with the power.:p

    The voltage across the cap is only 316Vpk at 1kW output into a 50 Ohm load; it's only 387Vpk at 1.5kW output into a 50 Ohm load.

    Unfortunately, it may be looking into a 200 Ohm load sometimes, and then those peak voltages will double to 632Vpk and 774Vpk respectively. And sometimes it might be worse than that.
     
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  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    So, even though I use a matchbox “tuner” adjusted to very low swr, an amplifier may be seeing a 200ohm load? Or are you saying that because of the amplifier tank when tuning/loading the amplifier?
     
  5. KN4CTD

    KN4CTD Ham Member QRZ Page

    Lou, do you know the range of capacitance the Al-1500 load caps use? They are reasonably priced. I don’t wanna waste my time drilling holes/mounting a capacitor that won’t be up to the task and then have to redo it etc. I found one from another ham that has about .02” spacing and my handbook suggests that should be good for 1200volts. It has a high range of capacitance also but isn’t priced much cheaper than the Ameritron. I’m just trying to avoid having to switch in any more padding than necessary. I’m using a software program that is based upon formulas from a very old handbook to obtain the likely values for the tank components.
     
  6. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    No, the load is whatever it is.

    The "200 Ohms" was to provide an idea of how the load impedance is more important than almost any other parameter when sizing the load capacitor.

    If you have a good external tuner that can provide a 50 Ohm load to the amp, then that's what it's going to see.
     
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  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    As far as I know Ameritron uses the same LOAD cap in the AL-80B through the AL-1500 so the "big box" amps don't use one that's any different from the AL-80B.

    That one seems to be rated 800 pF, 1200V.
     
  8. KN4CTD

    KN4CTD Ham Member QRZ Page

    Ok, thanks. That’s what I thought. I guess my tuner is ok. It tunes my 80 meter dipole just about dead flat 1.1-1 most every band.
     
  9. KN4CTD

    KN4CTD Ham Member QRZ Page

    I did look up the schematic after I asked that question and saw the rating for it. The software I’m using to design with obviously takes into account plate current and voltage along with Q. I’m seeing I’ll need about 2000 +/- pf on the load to cover 160. Does this seem about right? The bandswitch I’ll be using has contacts for switching in extra capacitance but I’d like to keep that to a minimum if at all possible
     
  10. W1QJ

    W1QJ Ham Member QRZ Page

    Right, if you always use an antenna tuner after the amp and adjust the tuner for 1 to 1 swr the amp will always see around 50 ohms and the voltage at the load cap will be lowest. The Al-1500, 82, 1200, SB-1000, AL-80,A,B,C , SB-220,SB-200 (newer style) Dentron's Amp Supply's and many others all use the same load cap. 850pf at 1000-1200 volts. It is the most popular load cap of all. Ten Tec actually made their own variable caps in house. Their plates were larger and so that the cap could be shorter in length. It really is a treat to able to see almost every amplifier ever made for ham radio and be able to compare quality, circuitry, and design.
     
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