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 tranformers are getting scarce these days.

Thanks and 73.

Tony I0JX

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."

"Antonio Vernucci" wrote in message

Tony I0JX


Tony, this has been discussed numerous times on the
AntiqueRadios.com forums. Yes, it will work, and apparently
it works quite well by all reports. The problem is core
saturation, as you suspected. You will probably get a bit
less audio (before clipping) than you anticipated for a given
core size. If you have a few filament transformers, it might
be well worth your time to experiment and try it for yourself.
Since most filament transformers are designed for secondary
currents in amps, it is logical that they will handle a watt of
audio without much issue.

Increasing the gap between E and I core laminations will
to reduce saturation will dramatically reduce the low
frequency response.

73

Peter K1ZJH

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 transformers 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 transformers wound on a toroidal core offer better
performances?

Unfortunately, proper audio transformers are getting scarce these days.

Thanks and 73.

Tony I0JX


A 6.3V filament transformer will work but will be far from ideal. Far
better to take one from an old valve broadcast receiver.

Steve H

On Sat, 9 Aug 2008, 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).

I haven't done it, but it was a pretty common trick to see in the
magazines in the old days.

At least one article pointed out that if you started with a 400Hz
transformer, which were common in surplus from aircraft, you'd likely
get better frequency response where you needed it.

Michael VE2BVW

Thanks everyone for the replies.

However no one touched the issue whether the proper output transformers have E
and I laminations arranged so as to create a gap (i.e. like non-swinging filter
chokes).

I would not like having to disassemble an audio transformer just for the sake of
determining whether there is a gap in the magnetic path or not.

Buying audio transformers is not cheap today:

- buying in the US implies a minimum shipping charge of about 30$ (international
USPS rates have gone crazy. Forget UPS or Federal Express)

- buying on e-bay not easy either! You'll find yourself within hordes of
esoteric hi-fi aficionados prepared to pay almost any price for plain things. If
you wish to buy a Silver Rock audio transformer (
http://www.mantrasound.it/AC_components/AC2.htm ) be prepared to pay at least
4,200 Euro (about $6,300 with the US$ now getting stronger).

73

Tony I0JX

http://www.antiqueradios.com/forums/...a99607a05f2b09

http://www.antiqueradios.com/forums/...61700 d5d187f

http://www.antiqueradios.com/forums/...a99607a05f2b09

Some reading regarding this subject.

Pete

"Tio Pedro" ha scritto nel messaggio
...
http://www.antiqueradios.com/forums/...a99607a05f2b09

http://www.antiqueradios.com/forums/...61700 d5d187f

http://www.antiqueradios.com/forums/...a99607a05f2b09

Some reading regarding this subject.

Pete

Thanks, reading those discussions and other references, it was possible to
determine that single-ended audio transformers must have a gap in the magnetic
path, this fact making power transformers generally unsuitable for that
application.

73

Tony I0JX

The same problem with the DC flow in the
transformer exists when a power transformer
is used with a halfwave rectifier.
Typically the transformer rating is degraded by
60% when be using with halfwave rectication
due to the DC current flowing in the secondary.

Many folks have used filament transformers
as audio outputs replacements with good results.
The transformer rating is decreased by the
standing DC current, but it will
still handle a good amount of audio before
saturation on peaks occurs.

Peter

You can avoid direct current from flowing in the filament transformer used
as an output transformer by using a choke with sufficient inductance to act
as a load, then capacitive coupling the filament transformer to transform
impedances only. That way, the filament transformer is only used with ac.

This method is often used with modulation transformers, to prevent dc from
saturating the core of the modulation transformer.

73, Colin K7FM