HB 813 amplifier question

Discussion in 'Homebrew and Kit Projects' started by K0IZ, Apr 30, 2021.

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  1. KL7SG

    KL7SG Platinum Subscriber Platinum Subscriber QRZ Page

    I looked at your page on QRZ.
    Fantastic job on the Covid project!
    Plus, you ski and like horses.

  2. K0IZ

    K0IZ Ham Member QRZ Page

    Thanks, Mike, for kind comments.
    Peter, I first became acquainted with Antek transformers from KG7TR. I'm using one of their 300VA for the "new" vintage radio power supply. Mike uses the 8A800 and uses the doubler circuit due to concerns that you voice. I would rather use bridge, but ... Antek spec is 3500 volts but might be primary to secondary and not also between secondaries. Not sure how to proceed.
    Now to get back to watching the Derby ....
    N2EY likes this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Yes, very nice valve rig !

    I was born into the Transistor age . . . but always found it SO much easier to build equipment using Valves then Transistors . . . quite apart from all the Valve rigs I built working first time, it was so easy to just hang most of the components off the valve bases, just the occasional tag strip.

    I built something very similar to your rig in the 70s . . . but made it cover 80m & 20m, as it was so simple to do so with the same VFO. (surprised you didn't)

    Roger G3YRO
  4. W9BRD

    W9BRD Ham Member QRZ Page

    The grid (finer wire) must be protected from overcurrent relative to the screen (heavier wire) if the tube(s) will be driven into control-grid current. The usual grid self-bias resistor(s), bypassed, can serve. Don't just ground control grid(s) and screen(s) without including the ability to monitor the current in each. A tube's max screen- and grid-current limitations apply regardless of the circuit configuration.

    I see that W8JI prefers directly grounding grids for arc-over protection; very well, but note he also insists that grid current be monitored--good.

    An important aspect to keep in mind is that a tube is a voltage-controlled resistor. With common-cathode operation we have -- neglecting other concerns -- maximum flexibility in scaling control-grid voltage v drive. Our control-voltage-v-drive options narrow with cathode drive because cathode impedance is generally low (however varying); this is why, for instance, we don't see grounded-grid amps operated in class C: Achieving the driving voltage necessary to overcome cutoff bias would require prodigious driving power into a cathode impedance of a few hundred ohms.

    813s date from the common-cathode era and were designed to be used as screen-grid tubes. As such they need their specified high positive screen voltage to draw their rated plate current at spec-sheet-max plate voltage. Hence we tend to see such older tubes, when used in grounded-grid, operated at higher-than-specified plate voltage to give the limited driving-voltage swing provided by cathode drive more supply headroom to work with.
    Last edited: May 23, 2021
    N2EY and SWL37632 like this.
  5. N2EY

    N2EY XML Subscriber QRZ Page

    Before you make any more decisions, check out QST for November, 1961. There's an article by K9LKA about "Single Band Grounded Grid Linears" which uses 2 x 813s in GG. This article also appeared in some ARRL Handbooks and some other publications.

    73 de Jim, N2EY
  6. KM1H

    KM1H Ham Member QRZ Page

    With good tubes that should be sufficient drive for 400-500W, they wake up with more HV. Due to their age Id recommend doing a good regettering job at ~1000V which wont support an arc.

    You can also experiment with lifting the screen and applying some low voltage. Ive done it with other tubes years ago but dont remember the details.

    BTW 300-350VDC is the normal operating screen voltage for the 813; the 750 is a maximum voltage as in standby.

    There is nothing wrong with swamped grid operation, it worked very well for the NCL-2000 of which I have 3 available to use on HF. No neutralization needed, stable even on 10M and reliable, all my tubes are ~67-75 date codes and run an easy 1200W on any band.

  7. W9WQA

    W9WQA Ham Member QRZ Page

    in gg i once clamped the 813 screens down with a power tube.
    then with a 6al5,rectified some input signal in a negative way.

    sent some of that to the clamp grid to let screens go up at the signal rate.

    got a little more output without really knowing what i was doing.
    having fun...
  8. W9WQA

    W9WQA Ham Member QRZ Page

    you could tinker with the clamp plate resistor and grid drive...
  9. SWL37632

    SWL37632 QRZ Member

    On your 20M homebrew, curious to know what you used to generate the SSB....is there an easy to use IC that does it?
  10. N2DTS

    N2DTS Ham Member QRZ Page

    Fantastic looking rig, is it ssb or/and AM?
    I see a carrier control....
    Did you make the cabinet?
    I would be interested in the diagram/details as well, as almost all my equipment is home brew.
    I once built a 4X 813 gg amp and ran it at up to 3500 volts.
    I tested it on AM and could get about 300 watts carrier out of it but on ssb I could see almost 3kw!
    Way too much heat and not much point over a plate modulated transmitter where a pair could run 600 watts carrier easy so I scrapped it.

    Not as nice as your stuff.

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