25 Hz question

[dtvmcdonald]

I bought an old Atwater Kent Model 44F receiver, supposedly very rare.
That seemed odd ... until I found out that "F" means 25 Hz.
I also bought another 44 of unknown suffix. That likely means 60 Hz.
The seller has not shipped yet, though (from Ontario).

These things need power supply recapping. Its a strange, by
modern standards, supply: transformer, type 80 full wave
rectifier, then three paper caps at roughly 1 uF around two
filter chokes, unknown values.

Its going to be interesting to see what the inductor and
capacitor values are! I just hope at least some capacitors are
still original value. They probably should be, since they
are safe from water under all that tar. I intend to restore
so it will still work at 25 Hz.

This should be interesting. I have a Variac ... if I can find it ...
that should be capable of working at 25 Hz since its rated at
30 amps at 60 Hz. Would a regular 7 amp Variac work? I
intend this to step UP the output at 25Hz from my Phase Linear
400 amplifier to power the radio.

Doug McDonald

[bob91343]

You can run the radio at 60 Hz for a while at least. The main difference is that the eddy current losses in the core will be greater, perhaps resulting in more power dissipation. That may be offset by the reduced flux density, with less hysteresis loss. Other parameters are probably not a problem.

For a Variac, well the voltage rating needs to be reduced in proportion to the drop in frequency, so a 120 V unit (at 60 Hz) is only good to 50 V at 25 Hz to keep the flux density the same. The current rating would be unchanged.

[Chad Hauris]

I always thought that equipment designed to work on 25 hz would be just loafing along on 60 hz...the transformers are bigger with more iron and there would be more filtering. Personally I don't see any point to using a power oscillator to generate 25 hz.

An Atwater-Kent radio shouldn't draw more than 1 to 1 1/2 amps off the power line.

I think if you use equipment designed for a higher frequency on a lower one, say 60 hz equipment on 50 hz, or especially 400 hz equipment on 60 hz, that the transformers are smaller and lighter and couldn't dissipate the heat well enough.

[bob91343]

Transformer size is not governed by heat dissipation. Low frequency transformers have thicker laminations, and the eddy current losses are greater at higher frequencies. The higher frequency transformers are made with thinner laminations.

Low frequency transformers must be larger to support the larger flux density required.