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RE: Nice OPT

In my experience, the reference for good bass with 300Bs is the Magnequest FS-030, which has about 40 henries inductance with generous flux headroom and small copper losses. That means it weighs about 11 pounds (5kg); I would be cautious about anything smaller. (The FS-030 is not available new, of course, and anyway there are transformers with better treble, at least in my experience and memory.)

The tradeoffs are:

1) Copper losses can be high when inductance and magnetic headroom are high - 1dB is 12% power loss. (Be sure to include both primary and secondary losses, data sheets are sometimes ambiguous about that ...)

2) Most large transformers, including the FS-030, use interleaved windings to reduce leakage inductance, at the price of increased capacitance and some ultrasonic resonances which may or may not be audible (at least in the absence of feedback; YMMV). I learned this from Mike himself, and my experience is consistent with it.

Good luck!

The '50s want high voltage, 400-450 volts plate to filament, and are limited to 55mA cathode current. The data sheets call for around 4K ohms load. But 3.5K and 5K are much more readily available and probably close enough. Either is also a good choice with a 300B as well, running a bit more current.

The reason they can both work at the same impedance is that the 50 has more that twice the plate resistance of a 300B. For the same reason, the '50 is less efficient and produces a lower(*) damping factor. To compare then at the same damping factor would call for a ~2K primary with the 300B.

(*) corrected 11/5/2024


Edits: 01/05/24
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