Input Transformer

Author: Hans Lohninger

To terminate a 500-ohm line and apply input to push-pull class A grids as in Fig. 129.

Fig. 129. Input transformer driving push-pull grids.

Primary voltage is 2.0 volts. Frequency range 100 to 5,000 cycles. Step-up ratio 1:20. No direct current flows in either primary or secondary winding. Nickel-iron laminations are used. Refer to Fig. 57 for dimension symbols and winding arrangement.

A_c = 0.5 sq in.

l_c = 4.5 in.

Permeability (initial value) = 5,000.

Coil mean turn 4.5 in.

Window 0.578 in.

Primary 400 turns No. 30 single enamel wire.

Secondary 8,000 turns No. 40 single enamel wire (total).

Primary layers 7; layer paper 0.0015 in.

Secondary layers 44; layer paper 0.0007 in.

Vertical space factor 0.9.

b = 0.75 in.

c = 0.008 in.

Capacitances:

With the primary winding located at audio ground on the secondary, there is virtually zero winding-to-winding capacitance. Secondary-to-core capacitance is based on a coil form 1/16 in. thick.

Total secondary load resistance = 500 · (20)² = 200,000 ohms.

Based on inductance with 0.001-in. gap, X_N = 339,000 ohms and X_N/R₁ = 1.7.

Response is 0.3 db down, in Fig. 108.

Resonance frequency of 0.62 henry and 127 μμf is 18,000 cycles and X_c = 70,000 ohms. B = X_C/R₁ = 0.35. f/f_r = 5,000/18,000 = 0.28. Response is 0.6 db down at 5,000 cycles (from Fig. 109).