Hi, Vacuumlanders, here's a somewhat strange topic...
I may have found a use for salvaged microwave power transformers. I
exhausted all the usual notions, e.g. spot welder, fishing sinker,
landfill (this is close, these transformers are not very useful!) So,
how about using the HV secondary as a choke in a tube B+ supply?

Problem (but easily overcome): One side of the microwave power
transformer HV secondary, the inner end of the winding, is attached to
the core, i.e. must be at chassis voltage (don't even think of floating
the mount!) Even if disconnected, the winding has poor insulation at
that end, so we don't want to expose it to the full B+ at the capacitor
end of a CLC filter, nor the 120 Hz FW rectified Vpeak at the tube
cathode end of a choke-input filter.
However, in a FW rectifier with CT transformer, we can move the choke
from the rectifier cathode and put it between the secondary CT and
ground where it will do exactly the same job. If you want a CLC
filter, just connect the reservoir capacitor between the rectifier
cathode and the CT (but cap. negative will be live so don't use a can
type.) Now we can leave the HV secondary winding connected to the core
with its HV end connected to the B+ transformer CT. The 115 VAC primary
is left open.

Before we go on, here are some characteristics of a typical 1 KVA uwave
power transformer... quite ugly, I fear:
Core centre limb: 1 3/4 x 1 1/2 inches cross section (that's the good
news, but it quickly gets worse!)
The 115 VAC primary is only about 144 turns of quite thick wire - this
is only 1.25 turns per volt. Resistance is 3 ohms or so. At no load
the magnetizing current alone is close to 3 amps!
The 2000 VAC secondary is about 2500 turns of thin wire, pile wound in
layers, no interlayer insulation except the enamel covering (but it
works at 2 KV so who's arguing.) Resistance is about 115 ohms.

BTW, for other experiments, I had already pushed out the laminated iron
blocks used to increase the leakage reactance for magnetron service,
leaving a straight transformer with a 17:1, or so, ratio. I see no
merit in leaving these magnetic shunts in for choke service.

Question: what's the inductance of the HV secondary at zero DC and how
would it change under, say, 100 mA DC? I looked for an equation for
inductance as a function of turns, core size, etc, but could only find
solenoids and toroids, no EI lamination equations. Any ideas?

So, to go on with the experiment...
I tried the HV secondary as a B+ filter choke (as above) in my "Scratch
10" amplifier, a 10 watt P-P amplifier using two 6V6's with LC (choke
input) B+ filter configuration. Doing this the power trans. secondary
winding CT gets the Vpeak voltage stress to core but, as a B+ winding
it is well insulated - there was no problem. I worked, no hum, just a
somewhat lower B+ suggesting the inductance was larger than I had
before at the rectifier cathode.

So that's about it. Do we have a source of "free", albeit quite bulky,
tube PS chokes from the uwave landfill?

One last question... How could we best use the uwave PT primary? It
could just be wired in series with the HV winding for about 12% more
inductance (or 12% less, I suppose). I tried putting a FW rectifier on
it to get a "free" bias supply but the result was only about 15 VDC
peak. Pity.

All comments welcomed...

Cheers,
Roger

Disclaimer: Don't do any of this unless you competent to build high
voltage tube power supplies. They contain lethally high voltages with
high current capacity. Also, don't play with microwave power supplies
unless you know what you are doing. The transformers are around 2,000
VAC RMS and the rectifier/voltage doubler can still deliver a lethal
voltage from the capacitor after the unit is switched off.