tuning indicator on tube amplifier

Discussion in 'Amateur Radio Amplifiers' started by SV2SBE, Oct 13, 2019.

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  1. SV2SBE

    SV2SBE Ham Member QRZ Page

    one friend of mine, had an old tube GU-84b linear amplifier. He is asking if he can add an LCD tuning indicator, like the ACOM 1000 amplifiers etc, the one thatwhen the arrow is in the middle you have tuned ok the amplifier. Im good on arduino and electronics, and want to help him in the design of this part.
    Please can you tell me what is this meter calculate? Output current?, anode current? or something else?

  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Usually it's a phase detector; when the output tank is tuned to match the plate load impedance of the tube(s), that balances a bridge detector circuit and yields "zero."

    I have not checked what Acom does, but that's what Collins always did.

    It works!
    SV2SBE and AH7I like this.
  3. SV2SBE

    SV2SBE Ham Member QRZ Page

    Any more help in this please? some schematics or something to start?

  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Sure, see below. Note the detector diodes used are extremely low leakage devices (25nA @ 25V, and <5uA @ 125V @ 150C).

    30l-1 bridge writeup.PNG
    30l-1 bridge circuit.PNG
    KD2ACO likes this.
  5. W4NNF

    W4NNF XML Subscriber QRZ Page

    There are several ways of doing this, including several published schematics for tuning indicators. However, it seems like a lot of trouble when a wattmeter will tell you when you're properly tuned...

    Here's an example of one... https://www.w8ji.com/TOF1.htm
  6. K9AXN

    K9AXN Premium Subscriber QRZ Page

    George, the following url is a fairly detailed explanation of the (30L1) design. It is a collaboration of Gene Senti and Warren Bruene used in the 30L1. It's purpose was to enable the amplifier to be tuned up at low power (300 watts) in such a way as to result in perfect loading at max power; protecting the 811A's from protracted max out tune ups. It was also used to indicate when tubes were becoming soft.


    Regards Jim
  7. SM0AOM

    SM0AOM Ham Member QRZ Page

    Since the GU-84 is a tetrode, it most probably uses a grounded-cathode circuit.

    A "tuning indicator" for such an amplifier would ideally consist of two circuits, one which indicates when the plate circuit is tuned to resonance, which also means that the phase shift between grid and plate is 180 degrees.

    If the grid circuit is tunable, it must first be brought to resonance using another phase detector.

    The other indicates the load-line so the plate impedance becomes correct, and which most easily realised by comparing the grid and plate voltages.

    For hand-tuned amplifiers, the grid and plate-circuit phase detectors may often be dispensed with, and the circuit just tuned for maximum output.

    Guidance for designing such circuits may be found in the first edition of the textbook
    "Single-Sideband Principles and Circuits".
    US patent US3355667A also describes the principles.

  8. W5LZ

    W5LZ Ham Member QRZ Page

    [: I use a watt meter.

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