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Fabio Berutti
 
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Many considerations:

- the higher the load, the lower the available power (you get V swing but
not A swing, and being the power VxA...)
- the higher the primary/secondary ratio, the more difficult is to wind a
good OPT. A 2K5 OPT is much easier to make (and therefore often better
sounding) than a 10K.
- a rule of thumb says that triodes work best into a load 2-3 times their
plate resistance (ie, a 2A3 with a plate R of 800R is usually used with a
2K5 OPT)

If You want to be more precise, You need the anode curves of the tube You
chose and a ruler. Choose a working point, then draw many loadlines and see
what happens. Usually a higher load is needed the more You "push" the tube
towards its limits (high V at working point).
But if You want to be sure, just find the original D/S of the tube You
prefer and stick on to recommended values for ALL parameters. After all,
these leaflets were written by professionals: they must have had some sound
reason to tell that ie. a 6V6 needs 8K a-a in PP, isn't it? Would You play
the same "it-seems-to-me" way with the braking pads of Your car?

By the way: the 300B needs a helluva driving voltage (some 250V p-p). I'd
start from something simpler.

Ciao

Fabio


"at" ha scritto nel messaggio
...
Hello,

What things should be considered to choose the best output transformer for

a
given tube and situation?

If low output impedance is important for good speaker damping and control,
why won't all amps just use output transformers with the biggest possible
primary winding impedance? That would give the biggest output transformer
impedance ratio and lowest output impedance of the amp.

Since it doesn't seem to be this way, what other considerations are there?
Tube type, Plate voltage, etc?

If I wanted to make a class A push-pull power triode amp with a 300B

output
tube, run at 400V Vp, what would be the best output transformer primary
impedance to use, secondary impedance is going to be 4ohms.

Thanks!

-at