On Aug 9, 3:04*pm, "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.

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?)

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

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

Unfortunately, proper audio transformers are getting scarce these days.

Thanks and 73.

Tony I0JX

Was looking at a smallish ex-microwave oven transformer other day and
wondering same. using one at much lower current as single ended OP
transformer?

Typical small scrapped m.wave transformer. Thousand watts at 115 volts
means primary handles around 8 to 9 amps.
Secondary at say 2000 volts handles 0.5 amps (500 ma.) Suppose we were
to use it at say less than 100 ma. plate current; to avoid saturation?
Note:

If the output of the transformer is around 2000 volts RMS (before DC
voltage doubling etc.) and has a 115 volt primary it has a voltage
ratio of around 17 to one. So a (single ended) tube impedance match
from an 8 ohm speaker of about 2400 is possible?

That could be suitable for a triode connected (non P-P) output tube?

Also wondered if other power transformers could be used at low DC
currents to avoid saturation etc. and perhaps cathode connected?

For example a centre tapped 700 volt power transformer (350-0-350)
would have a voltage ratio to its 6.3 volt winding of 111, hmmm!
That's a very high impedance ratio!

And it's 6.3 plus 5.0 volt winding (11.3) it would have a voltage
ratio of about 62, also very high!

But to its 115 volt winding the voltage ratio is 700/115 = 6.1 for an
impedance transform of 37.

Thence 37 times 8 = 296; adequate to match the cathode follower
impedance of P-P 'grounded' plate OP tubes?
With DC currents in the cathode circuits tending to cancel each other?

Just playing with thoughts at this stage!

Note: Seen use of those ex. m.wave transformers for battery chargers,
their primaries are pretty rugged and a 12 volt secondary can be not
too many turns of something capable of quite a few amps!