using step-down transformer as step-up

[AndyS]

I'm sorry. I didn't mean to upset anyone. This is a real question to
me since I need a high-voltage step-up transformer for an RF amplifier
and was planning to use a power distribution transformer backwards,
but it's expensive so I wanted to make sure it will do what I want
before I buy one. Again, sorry I'm being a pain. I won't ask any
more dumb questions. Robert

Andy comments:

When I was in high school, I did exactly what you are suggesting to
generate the high voltage to power four 811A s in parallel, running
around 500 watts input. I used 866 mercury vapor rectifiers....

The transformer was a 5 kva pole pig that I bought from the power
company, taken out of service, for around $10 ( in 1960 )...

I drained the transformer oil out.....since I wasn't running it
at
anywhere max ratings, it didn't need the cooling.

I "think" the secondary was two 220 windings and the primary was
somewhere around a couple thousand volts..... I most likely ran
the primary from 110 and ended up with around 1200 volts for the
811A s .... I don't remember the voltages accurately anymore, but
it was very straighforward..... The idling current is not very high,
no
more than an amp or so maximum, so don't worry about blowing
out fuses since your 220 is probably at least a 20 amp circuit.
Those
pole pigs are well made, and very efficient, since the power company
doesn't want to waste power on magnetizing current any more than
you do...

Andy in Eureka, Texas W4OAH since 1960

 

[Baron]

Thanks for the tip. I'm looking for something more powerful, but this
is a great idea for other projects. I see microwaves put outside on
the curb all the time, and never thought of cannibalizing them for
their transformers!

I've used salvaged MW oven transformers in a few projects. They are
easy to rewind too.

I suppose the magnetron might be useful to some
folks as well, but perhaps it would be burnt out in a discarded
microwave.

I've come to the conclusion that most magnetrons are perfectly usable !
It seems that the controllers or power supply rectifiers commonly fail.
The actual magnetron seems to wear to about 70% of its full output from
new, fairly rapidly (8 to 12 months) and then remains constant for a
very long time, read years.

(Although, you'd be surprised how much perfectly
functioning stuff gets thrown out by consumers and industry alike. I
remember when I was a grad student, the amount of good stuff I
scavenged from the university's garbage bin was astounding! And later
when I worked at a big company too. Until they got wise and locked
the garbage. To make sure the perfectly good equipment actually DID
get turned into landfill or whatever. Sad, sad, sad....) Robert

Yes ! That is something that I too hate ! Rather than allow
enthusiasts to take advantage of old kit, to destroy it !
I understand the reasons why its done, but its nice to salvage gear that
you may otherwise not be able to afford !