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Run Plate Transformer at Half Voltage?

Feb 28th, 12:21

K0WUQ

Joined: Dec 3rd 2012, 11:13
Total Topics: 0
Total Posts: 0
One of my delusions of grandeur as a kid back in the 1960s was to build a 1KW push-pull amplifier for 40 meters. To this end, I bought a couple of used or surplus 4CX250Bs from the venerable Bob and Jack's Store For Hams in Des Moines. I also really splurged and ordered a couple of brand new power transformers to pump this dream rig.

Well, I still have these tubes and transformers, which have been sitting around in attics and garages for the last 55+ years, so I'm sure they're still in great shape to use. My desires are more modest now, though, so I'd like to design a simple amp using one tube and one transformer. Each tube (designed originally for VHF use) will handle 250 watts 24/7, so amateurs have always gotten 500 watts out of them with no sweat. Of course, they need forced cooling, but I also bought a little centrifugal fan for that at the time.

But what I want to do now is start 'small' using the tube at maybe 100 watts and gradually working the current and voltage up as I gain confidence, hopefully without starting a house fire in the shack. I don't really have money to throw around on a lot of new components, so I'd like to use one of these transformers in a low-power mode.

Now friends, one of these babies is not something you want to drop on your foot. The transformer has two primary windings that can be wired in series for 230VAC power, or in parallel for 115VAC. The output is a mere 2880VAC with center tap, rated at 250 mA. What I wonder is this:

Would there be anything wrong with using the primary coils in series but powering with 120VAC, so the output would be at half voltage? I know that some motors (for example) won't even start to run at half voltage, but I really don't see why a transformer (a much simpler device!) couldn't be used this way. Here are the calculations for what I should end up with:

The transformer run in its original 230V mode would have:
Maximum power: 2880V x .25A = 720 watts
Primary maximum current: 720W / 230V = 3.13 amps
Step-up ratio: 2880V / 230V = 12.52

For the half-voltage mode, I'll use 120VAC rather than 115 because it's a more up-to-date figure. I'll assume the step-up ratio will be exactly the same, since this should be solely determined by the turns ratio of the coils. I'll also assume that power will have to be limited by the current capacity of the primaries in series, exactly the same as in 230V operation. So:
Maximum power: 120V x 3.13A = 375.6 watts - we'll use 350W as a practical target maximum 'just to be safe'
Secondary voltage (from step-up ratio): 120V x 12.52 = 1502VAC
Secondary max. current (from allowable primary current and inverse of step-up ratio): 3.13A / 12.52 = .25A (we've kept the 250 mA limit in the original specs!)

Using a typical two-rectifier full-wave design, we only get the voltage from each side of the center tap. But, this is RMS voltage, so to get the maximum filtered DC voltage (before any regulation), we can multiply be the square root of 2: 1502V / 2 x 1.414 = 1062 VDC

So, we would basically have a nice 1000 V B+ supply that we could use up to 250 mA - up to 250 watts DC plate power. Some would roll their eyes, but for me that's HUGE capability. Man, I'd have to build myself a 40M low-pass filter and everything ;-)

All right, you engineers out there: Is there any valid technical reason NOT to do this?

Larry K0WUQ

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