Quote:
Originally Posted by kalee20
Interesting article!
For interstage transformers, winding self-capacitance is a significant limiting factor for high frequency response (together with leakage inductance). Even with just a 3:1 step-up ratio, 1pF extra secondary capacitance hurts as much as 9pF of primary capacitance.
I therefore - and I have only wound a couple of intervalve transformers - wind the secondary FIRST.
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Yes the capacitance in each winding is effectively transformed into the other by the square of the turns ratio and the main issue is the secondary capacitance. So it is especially bad with a 1:5 ratio type.
Despite the expense and weight, they do provide "voltage magnification" which really helps in many radios with only a few valves. They also provide a BPF function in the audio range eliminating the RF carrier and DC isolation between stages. I have seen the transformer replaced (repairs) in some sets by R-C coupling and its much more problematic and less effective. However interstage transformers are adroit at picking up magnetic radiation (almost as good as a hearing aid's telecoil) and will sense a switchmode psu nearby!
One other version of them, which is less problematic from the design perspective, is the type made for class B operation. These are usually step down rather than step up as they have to provide grid driver power.