5000V output tube amplification design

Discussion in 'Amateur Radio Amplifiers' started by LAZYDOORMAN, Oct 13, 2021.

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  1. LAZYDOORMAN

    LAZYDOORMAN QRZ Member

    A439DB64-66B6-4CBC-9198-41FEADFEF992.jpeg I’m building a custom Voltage amplifier that will be amplifying “pure” sine waves from a signal generator. Input signal will range from .01V to 1V and will be a set frequency ranging 500-5000hz.


    I’m new to tube amplification. I was originally going to wing it and just use a 5f1 but thought I’d maybe put some more thought and not just outright waste money blindly. So I bought a book “Amplifying With Vacuum Tubes” by Carl Gauss. I’ve read and reread it about 4 times, and I’ve learned quite a lot, but I’m still short on knowledge in a few places.


    Currently I don’t have any parts other than a signal generator and an o scope on the way.


    The signal generated by the signal generator .01V-1V (it’s a cheap FeelElec FY8300) needs to have a final amplification factor that will bring the signal to 5000v or more. This signal out is a broadcast antenna Containing multiple coils in series. I’d like the final sine wave signal to have as least distortion as possible.


    Here is my custom design I’ve designed. It includes cathode biasing, 1M pots all over to help refine the signal. I’ve included a similar B+ filter as the 5f1 however I’m not sure of the C7,C6,R15,R14 values. I think I’d prefer to use a choke filter instead bc I’m under the impression it will aid in less distortion, but I don’t know how to determine values of L1, C55B.


    I believe with the 12AX7 and 6SN7 I should be able to achieve a 250V signal and then amplify it through a microwave transformer at a conservative 1:20 ratio to achieve 5000V. I’ll be using the 5Y3 rectifying tube. The OT is me thinking outside of the box to achieve my goal..... is the coupling at C5 as well as the 1M Pot load at R13 going to be sufficient to ignore the usual impedance requirements that are necessary to work around with most OT and still allow strong low distortion voltage gain though the OT.


    Lastly, I’m not afraid to start the design over from scratch I’m just a newbie and this is my newbie design in dire need of external input from those who know much more than I do.


    As a side note.... I’ll be building 3 of these units, so cost is a factor within reason, but I understand the need to spend what’s necessary.


    Thanks
     
  2. WL7PM

    WL7PM Ham Member QRZ Page

    What is the frequency range of operation ?
     
  3. LAZYDOORMAN

    LAZYDOORMAN QRZ Member

    Samples will be between 500-5000hz pure tone
     
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    What you are proposing is completely impractical as drawn.

    A "microwave transformer" is optimised for 50/60 Hz operation, and core losses
    together with winding capacitances will consume all available power (at the most a few tens of milliwatts) from the 6SN7 voltage amplifier before it reaches the secondary winding.

    It is literally "ages" since I last designed transformers, but from memory it takes
    a very special kind of iron together with sectionalised windings to make a working transformer with such a large step-up ratio at audio frequencies. Also, the insulation requirements have to be seriously considered.

    The closest you may come could be a variant of the old TV flyback HV transformer, but they only operated in narrow frequency ranges and with miniscule output currents.

    Something that also *might* work is an old oscilloscope or radar indicator CRT power transformer, rated for 400 Hz operation. It still requires a quite substantial driving power, several watts, for overcoming the core losses in order to produce a high secondary voltage.

    73/
    Karl-Arne
    SM0AOM
     
    LAZYDOORMAN, KA9JLM and WA5VGO like this.
  5. LAZYDOORMAN

    LAZYDOORMAN QRZ Member

    Interesting. So you’re saying that using a microwave transformer would not work out bc there isn’t enough current from the 6sn7 to overcome the core losses and have an undistorted signal exit the transformer and that it’s custom parameters will be an additional hurdle.

    Now what is the benefit of Sectionalized secondary windings? And would the primary winding need to be sectionalized as well? If I were to build my own 1:20 toroid OT that used a particular core that you may suggest material wise and then sectionalize the secondary with say 20 sections would that remove the shackles of a repurposed OT?
     
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    Wide-band transformer design is an art in itself, especially if large step-up ratios are required.
    Dividing the secondary winding in sections makes the influence of winding capacitances somewhat less, otherwise the combination of leakage inductance and winding capacitance will make up a
    low-pass filter or create severe distortion of the passband curve.

    The core material, permeability, saturation flux and hysteresis curve shape, needs to be selected very carefully, in order to reduce the losses, and to use the windings most efficiently. This can be more easily handled for small AF transformers, where impedance transformation ratios of several hundred can be made, but for larger transformers this becomes quite difficult.

    I am no expert of transformer design, having only done it a few times since Uni, but have instead learned to consult expertise in magnetics when requiring specialised transformers.

    In some 40s and 50s designs of CRT-based indicators there were high-voltage transformers
    which operated off 400 Hz aircraft power buses. Those used much better iron than the usual 50/60 Hz power transformers and could most likely be pressed into service at a few kHz. But they still require substantial driving power from a low-impedance source to overcome the transformer losses.

    73/
    Karl-Arne
    SM0AOM
     
    PY2RAF likes this.
  7. KM3F

    KM3F Ham Member QRZ Page

    Are you possibly thinking a 5000 volt audio signal will radiate from your >>coil loaded<< antenna?
    For what reason are you thinking of trying this?
    You would get better results with a 1000 watt audio amplifier with metal rods driven into the earth, spaced out.
    It takes power to do these things.
    I did not hear anything about Current involved.
    The Current in the antenna is what would generate a radiated signal. I don't see this happening.
    .
    Just for perspective; the old analogue TV sets developed close to 40kv for color CRT use as a by product of horizontal sweep functions.
    The front of the screen was at least 18 inched from the electron gun to light up the phosphor.
    And they did not radiate much of a signal at that level further than a few inches off the front of the CRT and that was not at audio frequencies or at high Current levels.
     
    WB2GCR likes this.
  8. AH7I

    AH7I Ham Member QRZ Page

  9. WB2GCR

    WB2GCR Ham Member QRZ Page

    Well, he joined on Oct 13th.
    Posted on the next day.
    Never returned (apparently).
    Hmmmm... wonder how many other places he posted the same question?
    I always find this sort of thing "interesting".
     
  10. WA9SVD

    WA9SVD Ham Member QRZ Page


    But standard " TV flyback" transformers are designed for high audio frequencies (a dozen kHz or more, and within a very narrow bandwidth) and would be very inefficient at the much lower audio frequencies indicated.
     

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