I am building a replacement AC power supply for an old National NCX3 ham transceiver. The low voltage requirement is for 280 volts DC, but the transformer I have produces 430 volts after rectification. Can I put in a power resister to ground to bleed off the excess 150 volts? If so, how do I figure out what value I need? Or do I just need to look harder for the proper transformer?

Take a look at www.hammondmfg.com There are application notes for sizing the transformer depending on the filter type (LC), C and type of rectification (full, half).

Bob,

Unfortunately a shunt resistor is not the way to reduce a rectified supply voltage - you'd probably burn up the reisitor and/or transformer before you achieved the reduction you're looking for!

Your next thought might be a series resistor to drop the voltage (this makes more sense), but that also is unlikely to be satisfactory because the varying load current on the B+ supply (between receive and transmit modes, for example) would cause the B+ voltage to vary wildly (even with a further filter capacitor on the load side of the series resistor).

One potential (excuse the pun!) solution is a regulator tube after the series resistor. These are like a vacuum tube, except they are gas-filled, and maintain an almost constant voltage across them once 'struck'. A common type is the OA2, but its regulation voltage is only 155V, so not right for your transceiver, and I'm not aware of any shunt regulators with a voltage as high as 280V.

The next option would be a series or shunt regulator using a power tube, in much the same way as you'd design a low-voltage regulator using a series or shunt transistor. In the tube design, you'd still need something like an OA2 as the reference element. However we're starting to get into a complicated design for a simple problem, I'm guessing way beyond your design capabilities (no offence intended).

So I'd say your best bet is to follow Kiss's advice and find the right voltage transformer using the reference article he cited (http://www.hammondmfg.com/pdf/5c007.pdf) as a guide.

Keep in mind that the output voltage of the desired transformer must be specified at the maximum load current (and you'll need to find out what that is). The open circuit voltage you'll measure will be substantially higher due to the winding resistance of the high voltage secondary, and other losses.

There are companies that will wind custom transformers to your specification, once you know what you want, but I suspect Hammond (and others) will have something off-the-shelf to suit. E.g Hammond's 263AX or 167H200 (depending on your load current). Keep in mind that B+ voltages are usually not critical; something within ±15% of your target B+ voltage should adequate.

Might I ask why you're building a replacement PS rather than just fixing the existing one?