90* Audio Phasing

Düubttff Or Suffer

T'jnfld To R.Fv Injection Frequency

Düubttff Or Suffer

T'jnfld To R.Fv Injection Frequency

Duol Eolonced Modulator Grids

Fig. 3a. A typical 90° rf phasing circuit

Duol Eolonced Modulator Grids

Fig. 3a. A typical 90° rf phasing circuit

Tune Doubler

■150V. Regulated

Fig. 3b. 90° rf phasing circuit preferred by author due to voice wave complexity, The diameter will vary, but the pattern will remain essentially circular.

i he Schmidt phaser makes use of the driving transformer inductance and the coupling capacitance, C1? to accomplish the 90° phase shift, i heoretically this can occur at one frequency only. It is believed that Rx and P1 effectively lower the Q of the resonant circuit, extending the 90° phase shift throughout the speech frequency band.

But the adjustment of utmost importance is the phasing of the rf injection to the balanced modulator stage. Fig. 3-a and 3-b are suggested circuits for driving the 6BU8 control grids with 90° phase rf injection.

Fig. 3-b is preferred by this writer. This is an R-C bridge used by Heath and others. C was chosen at 27 mmfd. Its capacitive reactance at 900 kc is close to 6700 ohms, Resistors, R, were 6800-ohm composition type. Two small ceramic trimmers, C1 and C2? are in parallel with the phasing capacitors. Their purpose is to compensate for inequalities in resistance, stray capacity due to lead dress, etc. Coupling capacitors to the 6BU8 control grids are 27 mmfd. Grid leak values are 1 megohm.

To VFO Outpul

Fig. 4, Mixer with conversion gain

It is difficult to use a 'scope to check the rf phasing here due to stray lead capacity between the exciter and the 'scope. But the rf injection phasing must be very close to 90° or the signal will not be clean.

The injection frequency to the balanced modulator, 900 kc? was chosen because of (1) the availability of 450 kc crystals at 10c each, and (2) the desire to use one dual triode as a crystal oscillator and an un-neutralized isolation stage. A 12AX7 fills the bill by working as a crystal oscillator and doubler. It happens that 900 kc injection also produces a higher image frequency after mixing than 450 kc injection, giving an added bonus of less possibility of spurious emissions being transmitted.

Your VFO can operate either 900 kc above or 900 kc below the desired operating frequency, Sidebands are switched by Sw^, Fig. 2-b,

On the subject of mixing, it was desired to select a mixer tube which would render some conversion gain. (See Fig. 4.) A 6CL6 was chosen for this purpose, its control grid receives the balanced modulator output, while the screen grid is driven from the VFO output (about 10 to 15 volts if) superimposed on the normal screen grid dc potential. rl he mixer output then drives a 2E26 output C3ass-A amplifier directly without the use of any buifer stage.

In operating the exciter the microphone preamplifier and phaser driver 12AX7, the oscillator-doubler 12AX7, the Schmidt phaser 12AU7, and the balanced modulator 6BU8's all operate continuously from a 150-volt regulated supply. The 6CL6 mixer and the 2E26 output stage are supplied with 365 volts unregulated, continuously supplied also.

The two stages receiving the 365 volts are blocked-grid keyed. Blocking bias is obtained from a voltage divider to ground from either hot side of the power transformer, A point of the voltage divider feeds a single silicon diode

IM f

6BUB Control Grids IM ^

Blocking Negative Bios (See Teal)

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