Antonio Vernucci wrote:
I'd like to hear someone having tried to use a power transformer as output
transformer for the single-ended final stage of a tube communications receiver
(power 5W, maximum, frequency not higher than 4,500 Hz).

As a matter of fact a plain power transformer with a 220V primary winding and 6V
secondary winding would well match a 4-ohm speaker to a 5400-ohm plate load.

If it were a push-pull stage, it would be easy to say yes. You can do it
with some transformers, not with others. The frequency response won't be
wonderful, but it will be fine for communications work.

But with a single-ended stage, forget it. You need a transformer that is
designed for impressed DC. You might be able to take a filament transformer
and cut a gap in the core, but the result will not be predictable and it
will take a lot of trial and error to get it right.

Basic questions:

1) does the DC current flowing through the transformer create problems? (do
proper audio tranformers have a gap in the magnetic path to avoid saturation?)

Yes, massive ones.

2) would a typical transformer work up to 4500 Hz, while maintaining the
expected impedance ratio?

Many will. Some modern transformers won't... the Talema toroids have very
poor high frequency response, deliberately to reduce power line trash from
getting into your power supply. Most cheaper E-I core types will work.

3) would modern tranformers wound on a toroidal core offer better performances?

Depends on what the core is made of.

Unfortunately, proper audio tranformers are getting scarce these days.

Hammond is still around!
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."