Peter Avgeris take on the legendary Telefunken RS241 and his dedicated output transformers:
“It’s past midnight and the 16kohms output transformer is fresh, just taken out of the winding machine. It is specially designed for one of my favourite Directly Heated Triodes and one of the best audio tubes ever made, the legendary Telefunken RS241.”
“The load linearity in the frequency domain is exceptional as always. The frequency response with 600 ohms primary loading is very good as you will see in the graph, but this OPT is designed to operate with a tube of Rp=4500 ohms and a mu of 17.
With 4500 ohms loading you will notice a shift in the frequency response in the ‘plus’ territory on the oscilloscope.
The unique study here is related to the Miller effect. I doubt if any transformers manufacturer has ever calculated the implications of the Miller effect in OPT design for HF response compensation. Due to the relatively high amplification ratio of RS241, the interelectrodes capacitance will tend to shift the frequency response to the ‘minus’ side at frequencies exceeding 15kHz, so in the end the overall frequency response in actual conditions will be extraordinary, keeping in mind that here we are dealing with a tube of Rp=4500 ohms and mu=17! On the pics you will see the response with 600R and 4k5 ohms of loading conditions, with secondary set at 16 ohms. It is 10kHz square wave response. Not bad at all!”
