Re: Output transformer - when needs must
Chris,
I assume you mean the output valve is a 220 HPT?
According to the Cossor data sheet the optimum load is indeed 17K Ohms, which is not unusual for a battery pentode, where the anode current is kept quite low (for obvious reasons) and therefore a matches to a higher impedance load than a thirsty mains pentode. Indeed some battery tetrodes/pentodes where specified for optimum loads of up to 24K.
As the load impedance is pretty much a compromise anyway and specified to be at a point on the 2nd/3rd harmonic distortion curves where both are 'reasonably low' the actual value is not critical. It is unlikely (unless you have 'golden ears') you will be able to tell the difference between a 17K or 15K load, the size of the transformer and its construction are more likely to be noticeable though in terms of frequency response!
p.s. The RCA AR88 receiver is designed for a speaker impedance of 2.5 Ohms IIRC, so 3.0 Ohms is not always the answer, in any case it is only nominal (measured at 1KHz?) since speaker impedance varies greatly with frequency, especially at resonance!
Adrian
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