Interstage Transformer

In interstage coupling the impedance level is high, to maintain both high load impedance and high grid excitation in the following stage. The limit on the secondary side is the highest resistance which affords grid circuit stability. There is no impedance limit on the primary side except that imposed by transformer design. Usually a 1:1 ratio is about optimum. A step-down ratio gives less voltage on the following grid. A step-up ratio reflects the secondary load into the plate circuit as a lower impedance. This reduces the voltage gain, especially with pentodes which have high plate resistance compared to load resistance. Under this condition, equation 58 becomes

ep/eg = μZL / rp = gmZL

[76]

or the voltage gain is proportional to load impedance. Interstage transformers commonly have many turns and high OCL.

Suppose that a transformer is required to connect a 6SK7 tube to a 6L6 operating class A with 10 volts rms on the grid over a frequency range of 300 to 3,000 cycles. 6L6 grid resistance is 90,000 ohms, which is to be reflected into the 6SK7 plate circuit as a 90,000-ohm load. Hence a 1:1 turns ratio is used. 6SK7 plate current is 10 ma. The same core as in Example (a) is used, except that here it is made of silicon steel, and the stacking is reduced so that A_c is 0.32 sq in. Primary and secondary windings are single sections; with the primary start lead connected to 6SK7 plate and secondary finish lead connected to 6L6 grid. This leaves adjacent turns in both these windings at zero audio potential, and effective primary-secondary capacitance is zero.

Primary turns = secondary turns = 6,600 turns No. 40 enamel wire.

Primary and secondary layers = 37.

Primary mean turn = 3.3 in.

Secondary mean turn = 4.2 in., l_g = 0.005 in.

f_r = 10 kc. X = 90,000, D = 1, and response is 1 db down at 3,000 cycles (from Fig. 127). X_N/R₂ = 6.28 · 300 · (55/90,000) = 1.04. Figure 118 shows a Z/R₂ of 0.72; therefore the response is 3 db down at 300 cycles.