|Those who advocate their use will tell us that "in interstage transformer 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 (from Hans Lohninger's page)." They'll say that "the advantage of using this type of transformer in certain applications is that it gives a different type of clipping behavior, the output tube grids can be driven somewhat into the positive range (because of the low DC resistance in the grid circuit), if a low impedance bias supply is used." More evangelical even is AudioNote UK: "It is becoming increasingly obvious that applying transformers at every stage of an amplifier yields a great benefit in sound quality, provided of course the transformer in question is of an appropriate quality and design (for AN designer Andy Grove's full presentation on the topic, click here)."
Lynn Olson adds: "The most important requirement for the interstage transformers is a precise phase match between the secondaries, with no more than a few degrees of deviation from 20Hz to 50kHz. Another advantage of no RC coupling between driver and output tube is instant recovery from overload. RC coupling usually requires hundreds of milliseconds to recover the correct bias point for the output tubes. This is great for guitar amps, not so good for hifi applications where immediate recovery is much more desirable". Hashimoto points at further challenges: "Input/interstage transformers require low-noise characteristics rather than power efficiency." As does another transformer vendor: "They probably represent the greatest challenge to a transformer designer due to the DC current in the primary and the very high impedance grid circuit across the secondary." Bud Purvine of O'Netics is realistic: "Sonically, a good non-inductive cap like MultiCaps are the equivalent of good interstage transformers except in one area. The caps all have an unrealistic increase in perceived physical size of an instrument as it increases in amplitude. The transformer raises amplitude without this artifact. Both will provide excellent musical performance depending upon how much money you want to spend, with the transformer usually being more expensive."
Technically, interstage transformers reduce losses in passive plate-loading circuits where resistive loading suffers up to 10 times higher DC resistance. Yet inductive loading imposes size and cost constraints. It is easily more than 100 times as expensive as resistive loading while a quality inductor could weigh 1000 times more than a resistor. SAC Thailand uses the example of a typical 50Henry 30mA choke with 300-ohm DC resistance as compared to the 5K of an equivalent load resistor. "If a 30mA current has to be drawn through a 5K resistor, it will generate a 150 voltage drop across that resistor for around 4.5 watts of heat dissipation. The inductor will only drop a 9-volt equivalent for 0.27 watt heat dissipation. This demonstrates the efficiency gain of inductive plate loading. An inductor also stores energy from the power supply to deliver transient energy to the tube load. A resistor can't store energy so voltage fluctuations will be higher. A coupling capacitor can be eliminated if a second winding to the plate-load inductor core is introduced which then becomes an interstage transformer. Its turns ratio determines the extraction voltage which couples magnetically for superior low-frequency transmission." For some SAC performance graphs for their Silk IT, click here; for Harvey Rosenberg comments on the subject, here; for perhaps the premium forum where to post questions on the subject, visit Kevin Carter's K&K Audio.