[AMRadio] Resistive load on interstage audio transformers in tube

D. Chester k4kyv at charter.net
Wed Mar 12 12:47:15 EST 2008

I have always loaded down the secondary windings of audio transformers to 
the manufacturer's specifications.  For example, I would put a 50k resistor 
across the secondary winding of a 500/600 ohm to 50k input transformer.  But 
I have consistently noticed schematics of broadcast and other high quality 
audio equipment, that most circuits use a much higher load resistance, 
typically 200k or more across a 50k secondary winding.  Often, there is no 
loading resistor at all. This is  for class-A amplifiers, so grid current 
loading is not a factor.

I have quite a comprehensive collection of radio engineering books, plus 
engineering data in some of the transformer catalogues, but none carry much 
discussion of this topic.  The RCA Radiotron designer's handbook says 
loading a transformer down gives smoother and wider frequency response, but 
increases both the tube and transformer distortion, implying that a 
resistive load should be avoided if possible.

Has anyone seen any data on this subject that goes into any more detail? 
Those who have designed and built high quality audio amplifiers, how much 
loading did you use on your interstage transformers, if any?

Don k4kyv 

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