Interstage Transformers: Mere SET Amp Design Afterthought?

Even though it offers audible and engineering improvements in designing and building single-ended triode amplifiers, do most SET amp designers still view interstage transformers as a mere engineering design afterthought? 

By: Ringo Bones 

Most zero feedback single-ended triode amplifier designers seem to have yet to understand that directly heated triode power amp tubes that date from the 1930s – including the famed 300B tube – are not that easy to drive and therefore necessitates the need of a well-designed interstage transformer to exploit their full sonic potential. But why are well-designed interstage transformers still a rarity in the world of zero feedback single-ended triode amplifiers?

Books and data on how to design interstage transformers seem to be forgotten as soon as the world discovered solid-state semiconductor amplifier design. Folks who know or have datasheets on the correct winding configuration and transformer core material to be used are very reluctant to share their secrets. Getting a symmetrical square wave out of an interstage transformer that you’ve desined and wound yourself is still one of the blackest arts in the realm of vacuum tube audio. Unscrupulous designers often steal from those who successfully designed one then claim it as their very own design.  But still, SET amplifiers incorporating a skillfully designed and built interstage transformers, are one of the most amazing wonders of the hi-fi world – sound wise. 

Given that the most powerful directly-heated triode tubes specifically designed for audio use that are designed during the 1920s and the 1930s only offer 10-watts or so, audio designers often press directly-heated triode transmitter tubes in designing single-ended triode power amps that offer greater power output. For example, an 845 directly-heated triode transmitter tube is capable of 50 watts and yet even driving it with mere capacitor coupling from the plate of the 300B tube to the grid of the 845 transmitter tube will only give you 28-watts or so.

 The interstage transformer acts as a parallel choke on the 300B making it as a driver tube. At about 28-watts, the impedance of the grid of the 845 transmitter tube (like all other directly-heated triode transmitter tube designed around the 1930s) drops dramatically, necessitating the need for more current from the previous amplifying stage to drive the 845 tube further. That’s where the interstage transformer takes over, providing additional current to drive directly-heated triode tubes to their maximum designed power output.  

And not all interstage transformers are designed – and even created – equal. A case in point is the beautiful WAVAC 4304B SET amplifier that uses the famed Nobu Shishito designed inverted interstage transformer, is probably the flat-out the best amplifier I’ve heard – tube or solid-state. Nobu Shishito’s inverted interstage transformer designs are very noteworthy for their delicacy and blinding transient speed – more than good enough to trick you as if you are listening to an actual live musical performance!