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Homebrew 811 amp bias cut-off

Discussion in 'Homebrew and Kit Projects' started by M0OYR, Sep 11, 2021.

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  1. M0OYR

    M0OYR Ham Member QRZ Page


    Apologies if this is in the wrong forum - but the amplifiers section seems to concentrate on commercial designs.

    I'm in the planning stage of a 811 x 4 amplifier. The transformer I have does not have a centre-tap on the filament supply - which I have seen used in multiple circuits. In the absence of a centre-tap, what is the best circuit to use for applying cut-off bias when not transmitting? Its a basic grounded grid in AB2.


  2. W4HAY

    W4HAY XML Subscriber QRZ Page

    I have no idea if this will work with higher power amps running in AB2, but in some old radios using an output tube without a cathode (such as a 47), two equal-value resistors in series were placed in parallel with the filament and the bias was applied at the junction of the two resistors.

    47 base diagram and characteristics:

    (Would you believe I recalled that tube number from memory?)
    Last edited: Sep 12, 2021
  3. K0IZ

    K0IZ Ham Member QRZ Page

    There was a single 811 amp project in QST back 1960's? that used a filament transformer without center tap. Had one 22 ohm 1W from each side of filament winding to ground. So possibly could do similar, with resistors going to tie point. And single cutoff resistor 22K, 47K whatever, to ground. Relay to short resistor.
  4. K0IZ

    K0IZ Ham Member QRZ Page

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