3-500Z amp drive me crazy! Guru needed

Discussion in 'Amateur Radio Amplifiers' started by VA2NA, Apr 26, 2019.

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  1. VA2NA

    VA2NA Ham Member QRZ Page

    Hello,

    Here the story:

    I got an not complete dual 3-500Z amplifier conversion ( from commercial amp ) and i try to get it finished.
    There was a lot of mistake in the RF section and i got almost all fixed up.
    Now i can tune 50-54 Mhz using 2500ohm in the plate without problem (static reading using vna),

    Where thing get worse is when i try to bias the 2 tubes, i have a simple circuit using 33K in the return of the cathode and a 10V zener 50W. First time i have try to key the amp just to see how much resting current i have..in almost 2 sec both tube cam red , both meter (grip and plate) get full scale and the resistor in my parasitic get RED!!! holy S*** i created an oscillator!

    So i was thinking my problem was the parasitic...i do another pair folowing the SB-1000 conversion and got the same issue .. 1-2 secondes of closing the 33K ohms and here we go.

    For your information i am using 2 Eimac socket with cheminey and all the grid connection are grounded using wide silverplated strap.

    On the cathode side, fillament choke is winded around a toroid and have decoupling capacitor on each leag.

    Remember...i have not yet apply any RF at all...

    So new i am listen for clue

    Picture will follow.

    Thank

    Guillaume VA2NA
     
  2. VA2NA

    VA2NA Ham Member QRZ Page

    Pictures
     

    Attached Files:

  3. W1QJ

    W1QJ Ham Member QRZ Page

    Don't feel bad. This happens to everyone when they try to make a 3-500z 6 meter amp. Many give up.
     
  4. VA2NA

    VA2NA Ham Member QRZ Page

    As a note..i disconnected the link between the pi network and the coupling capacitor....same.. problem dont come from 6 meter circuit. Only tube, plate choke and parasitic was connected.

    Guillaume
     
  5. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Can you draw out the circuit and post it?
     
  6. AA5CT

    AA5CT Ham Member QRZ Page

    Suggestion: Use your VNA and measure S12 and S21 with power off, through tubes in circuit, just to get idea what isolation factor is.

    Question: Is your input on the grid, or is the amplifier cathode driven?

    Successful amplifier conversion in the past have actually resonated the cathode filament choke with the required capacitance value.
     
    Last edited: Apr 26, 2019
  7. VA2NA

    VA2NA Ham Member QRZ Page

    Yes i will do over the weekend!
     
  8. VA2NA

    VA2NA Ham Member QRZ Page

    My amplifier is cathode drived,

    Good suggestion for isolation measurement, I feed rf trough a T pass circuit and then to the cathode,

    I never measure tube isolation before,

    what is the best way to do it? keep the tune input circuit in feed 1 port of the VNA there, and the other port to the plate circuit?

    My output circuit, ( PI) is not responsible in the problem, as i disconnect it and still get the problem..

    Guillaume
     
  9. KD2ACO

    KD2ACO Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Also, if you can, take a picture under the tubes. The layout may give some clues about what is happening under that cover.
     
    Last edited: Apr 26, 2019
  10. VA2NA

    VA2NA Ham Member QRZ Page

    Pictures is in the 2nd post.

    I have do a input-output vna sweep and find a nice VHF dip.

    It is look like the small hole in the plate and the input circuit are leaking there and kill isolation.
    Look at the Marker 2
     

    Attached Files:

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