Need help from amp builders

Discussion in 'Amateur Radio Amplifiers' started by KN4CTD, Nov 16, 2018.

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  1. KN4CTD

    KN4CTD Ham Member QRZ Page

    I am in the early stages of testing a homebrew dual gi46b amplifier I’ve built/retrofitted into an old chassis. This is my first, so be easy on me, I’m still learning. I’m using a w4zt bias board. I applied hv and keyed amp using no drive, and the idle current soars....takes off. Doesn’t draw current without being keyed which indicates to me it’s not a tube short....opinions please? These are wired in a standard parallel grounded grid configuration. I was measuring plate current using a fluke meter tied in series with the bias fuse. Any and all help will be appreciated
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    I'd need a lot more information, like the schematic.

    You can't measure plate current with an external meter having "leads" unless what you're really measuring is the voltage drop across a very low-resistance metering resistor; otherwise, unless the meter is very well decoupled from the circuit, the meter and its leads can cause an oscillation.

    More details.
     
  3. W1QJ

    W1QJ Ham Member QRZ Page

    Need to know a lot more information. What did you set the operating bias voltage to and where and how are you applying and switching it?
     
  4. KN4CTD

    KN4CTD Ham Member QRZ Page

    I don’t guess we really ever “set” the bias voltage on the board to a specific voltage. We were keying the amp with a foot pedal and no rf drive applied. When in standby, the ht B- is tied to the cathode through a 100k ohm cutoff resistor through the 3pdt input/output/keying relay. When the amp is keyed, the ht B- is going to the black wire on the w4zt bias board (via a 1 amp fuse) and the red wire from the board is tied to the cathode.
     
  5. KN4CTD

    KN4CTD Ham Member QRZ Page

    What would be the correct way to set the bias voltage? Is there a way to bench test the bias board for proper operation without installing it into the amplifier? We may not have been measuring the idle current correctly but when keyed and no rf applied, there was a huge load on the power supply....verifiable by sag in hv.
     
  6. W1QJ

    W1QJ Ham Member QRZ Page

    Those Russian tubes require lots of operating-bias to keep the idle current low. I’m thinking 30 volts. The zt board should have a pot that allows you to set the bias. Start high first to see where you are. Are you using an RF choke in series with the cathode in the bias line?
     
  7. KN4CTD

    KN4CTD Ham Member QRZ Page

    I wound a single wire choke around a small length of rod that I got from somewhere. If memory serves correct , I believe it measured 30-40uh....I can check my notes. I’m unsure as to what the core material I wound it on though....ferrite or powdered iron. It was wound using magnet wire.
     
  8. KN4CTD

    KN4CTD Ham Member QRZ Page

    The b- connections to the cathode go through this choke
     
  9. AC0OB

    AC0OB Subscriber QRZ Page

    Most of us are visually oriented when it comes to circuitry and it helps us help you when we have a schematic.

    As WIK said, currents should be monitored via currents flowing through low resistances.


    Pheel
     
  10. KN4CTD

    KN4CTD Ham Member QRZ Page

    I was wrong about the cathode choke......it measured about 150uh
     

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