[AMPS] rheostat/autotransformers
Ian White, G3SEK
G3SEK@ifwtech.demon.co.uk
Fri, 5 Nov 1999 19:25:33 +0000
John Lyles wrote:
>
[lots of interesting stuff, as usual...]
>variable autotransfomers
...
>three are paralleled for current
Didn't know you could do that. Do you have to take any special
precautions to make sure they are sharing the load equally?
>I have used rheostats in some of the commerical FM broadcast rigs, as
>they were inexpensive (competitive pressure) and they do work well if
>they are run within ratings. I used them on the filament primary for
>4CX3500,7500 tetrodes. With a small value of 100 Watt rheostat, I
>could get the variation needed to cut back the filaments up to 5% in
>voltage, to practice life extention techniques.
Coming back down to amateur size, I'm currently using a fixed 100-ohm
resistor in the 230V primary of a 15V transformer, to power the 12.6V
heater of a GS35b. That is slightly too high a resistance, and in the
process of selecting a suitable resistor of about 1k to connect in
parallel, I noticed that the 2nd and 3rd places of decimals on the
heater voltage continued to vary for some time. Presumably the heater
was acting as its own resistance thermometer, while the whole
heater/cathode mass was coming up to temperature.
Anyhow, it took all of 3 minutes, exactly as the data sheet said. It
might be worth using that technique to determine how long it really
takes the cathodes of some other indirectly-heated tubes to come up to
temperature.
BTW, "Variac" is used as a generic name for variable autotransformers
over here, just like "Hoover" is used for vacuum cleaners... to the
disgust of all other manufacturers.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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