Design Example - Transformer for Pi-Filter Modulator

Frequency range 100 to 5,000 cycles.

Audio output 400 watts.

Power amplifier E_B/I_B = 10,000 ohms.

Voltage ratio primary/secondary (1,180 + 1,180)/2,000.

f₁ = 60 cycles.

Core: 4-in. stack of silicon-steel lamination B, Fig. 44.

Turns primary/secondary (800 + 800)/1,380 No. 26 wire.

Ac = 7.2 sq in. net; 8.0 sq in. gross.

l_c = 12.75 in.

l_g = 0.012 in.

Possible tube current unbalance = 0.032 amp.

X_L at f₁ = 6.28 · 60 · 36.5 = 13,800 ohms. Z_s = 115 per cent R at 100 cycles from Fig. 146. Winding arrangement as in Fig. 158, to reduce layer voltage and capacitance.

Winding resistances: Total primary 90 ohms. Secondary 80 ohms.

Secondary leakage inductance = 53 millihenrys.

Capacitances (referred to secondary):

P₁ - S₁(at A)         = 292 μμf

P₁ - S₂(at B)         = 0

P₂ - S₁(at C)          = 112

P₂ - S₂(at D)         = 58

Secondary layer to layer    = 140

Primary layer to layer       = 170

Stray and tube       = 50

Power amplifier r-f bypass = 500

----------------------------------

Total = 1,322 μμf

Fig. 158. Section of transformer coil wound for low layer voltage.

At high audio frequencies f_r = 19,000, D = 0.6, and Z/R₂ from Fig. 119 (p. 159) is 87 per cent at 5,000 cycles.