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Re: The secret to using iron IT's (like Lundahl 1660)

"I like the sound of my Lundahl 1660 IT with a 6H30 driver going through the IT and into a 20K ohm load (47K on secondary with 1:2.25 step-up)."

Whoa, Kurt, am I missing something here? The net turns ratio from the entire primary to the entire secondary is 1:2.25? And the load resistor on the secondary is 47K?

Wouldn't the load seen by the primary (and the 6H30 plate) be about 9.3K instead? (Impedance ratio = square of turns ratio; thus 47,000/2.25(sqr)=9284 ohms, neglecting DC resistance of primary and secondary.)

Of course, the transformer multiplies the Miller capacitance of the output DHT's as well, so adding a resistive load to the secondary means the 6H30 plate is seeing a real resistance as well as the reactive component reflected from the output DHT's. I wonder where the inflection point between real and complex load begins ... at a wild guess, I'd say somewhere above 2 kHz. Maybe the 6H30 likes a flat load over part of the spectrum compared to a purely reactive load.

In my setup, I had the opposite experience (added resistance on the secondary sounded worse) but the Amity is a pretty different critter in terms of IT turns ratio (1:1), PP driver, and PP 300B's requiring 120V rms at low distortion. A step-up IT would be a whole different world than a 1:1 IT impedance-wise.

I'd like to extend a great big "Thank You!" for sharing your experiences with your circuit; so few people have built IT-coupled PP amps that every posting we see in the Tube DIY Asylum helps.

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