A quick look at the data sheet for the 4-400A  and a little research on existing designs indicates I will need on the order of 3,000 volts on the plate of the tube. Interestingly enough I found that neither the commercial gear or HB amplifiers use choke input power supply filters these days. I grew up in the era of dual section choke input filters.  Apparently, for good linearity, it is desirable to have a constant voltage source for the DC plate circuit.  This makes sense when you remember that an inductor tries to maintain a constant current and will sacrifice voltage to accomplish this. Wow! I never thought about that before. The instantaneous DC current from the power supply will vary considerably with a SSB input signal. That can cause the DC voltage output of a choke filter to  swing wildly (for a short period of time) as it attempts to maintain a constant current drain out of the filter section. Not exactly what the tube likes for best linearity, IMD, etc.

So much the better! I didn't have an appropriate choke anyway. Now on to the plate transformer. Potentially a lot of money to get what I needed. However, a little research on the WEB brought my attention to a discussion of microwave oven transformers.

Well it seems there are a lot of microwave ovens rated at 1000 to 1500 watts. Yes they use magnetron tubes. But watts are watts! And if I have a tube that has a maximum plate dissipation (heat) of 400 watts and I can count on at least 50% efficiency on the amplifier it seems that a transformer that would support 800 watts would do the trick. And a 1000 watt hunk of steel and copper would be just great! And guess what! These magnetrons (bless their little rotating magnetic fields)  need on the order of 3000 volts on their plates! I have found that a lot of people have no idea that a common kitchen item they use every day is associated with this kind of voltage!

Yep! There are a few complications. One side of the secondary is tied to the transformer frame and there is the matter of the MAGNETIC SHUNTS to deal with. I also wanted to remove the filament winding.

All microwave oven plate transformers seem to have magnetic shunts (pieces of steel laminate stock) that create a low magnetic path around the primary winding. I assume this is to help regulate the secondary voltage (with variation in the 110 primary side voltage). I found my info regarding this on the WEB. What a resource!

At any rate,  the bad part of this (from my perspective anyway) is that the shunts result in a high idling (no load) primary current. Several amps will flow in the primary without any load on the secondary. To avoid this simply use a drift pin and carefully pound out the shunts. I know, this sounds bad but that is how it is done! I cut and pulled out the filament winding first to give me some working room. It's just a few loops around the core and I didn't need it anyway.

I have included a few pictures ( see previous page) of the transformer as I was testing it. I insulated the end of the secondary that was attached to the frame with a porcelain insulator and used the capacitor and diode from the original microwave just to test the transformer in this configuration. The magnetic shunts and filament winding are still in the transformer.