NOT-A-BOX POWER SUPPLY
Muscle for just about any amplifier.
by Bob Hutchinson, N5CNN

The heart of the matter is the power supply. This article is about combining two SB220 plate transformers to produce a versatile four kilowatt HV power source for Amplifiers equipped with bridge rectifiers and requiring 2,600 - 2,800 VDC B+ or 2,900 - 3,200 VDC B+ and, at the high range, 5,200 - 5,400 VDC

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By mating two of these fine transformers and connecting the primary windings in parallel and the secondary windings in series, the result is a healthy 4.000 watt AC power source for linear amplifier use. Follow me along as I put together this unusual, not-a-box AC power supply.

Well Proven Transformer
The SB220, SB221 and HL2200 from Heathkit use the same power supply configuration. It is a 2,000 watt full wave doubler rectifier/filter design, proven over the years to be ideal for Ham Radio SSB operations.

Seeking another outboard power supply for future use I decided to examine using two of these transformers with secondary windings connected in series. Folklore about transformer secondaries in series was that the windings need to be identical, otherwise they could eat each other or be inefficient. Turns out, this really is folklore. But, as we will see, the primaries can provide flexibility. With 235 VAC mains and two primary windings, the transformer secondary, no load, AC voltages available are  950 VAC  and 1,125 VAC.

With a previously removed transformer, I put both on the work bench and involved them in my thoughts for the next several days trying to devise a cheap way to house this power supply without a lot of metal work. I wanted it to be small enough to set on the Ham desk, after all the transformers are small and only weigh 23 lbs. each. Of course by the time they're both in a case with other stuff, it'll be over 50 lbs. That's the most today I want to lift and I'm sure the figure will get smaller, but I can handle fifty lbs. or a little over.

This Heathkit SB220 schematic, which is modified for clarity, shows two identical primary windings. The two are connected in series for 235 VAC mains, high and low taps available. (See picture.) I sorted all the wires just like the schematic and decided to use some terminal strips and hook up the primary windings in a manner similar to the schematic for each transformer to keep all this somewhat orderly in my mind.

Shoebox?
I kept thinking about a shoebox. A shoebox couldn't possibly be big enough for such a power supply, could it? Well, no but I came up with a better idea. Just screw the two transformers to a board, wire em-up and attach another board on top with spacers. This leaves the sides open to the breeze. Some might say it's ugly, but it looks like a power supply.

Red oak planks
The board is a nice piece of 1x6 red oak board fifteen inches long left over from a previous power supply adventure. I devised a platform fixture next to the transformers so I could wire it all up with the top board temporarily secured beside the transformers, upside down, and then flip it over and attach it when done.

Add circuit breakers, DPDT switches for tap selection, main switch, soft-start relay, pilot light, etc. The soft start is a husky 25 amp DPST 277 VAC relay with big 10 ohm 32mm Thermisters for surge protection. The 22mm Thermisters in the picture were replaced with bigger, higher rated ones. With the smaller Thermisters a short in the secondary will blow both Thermisters and then a fuse. With the higher current Thermisters, just the fuse will blow.

A Dayton cube, on-delay relay, with adjustment of .1 to 10 seconds, keeps the Thermisters in the circuit for one second while the filter capacitors are softly brought to voltage, then energizes the relay to short around the Thermisters.

I ended up with some metal work anyway in the form of front and rear 1/8" aluminum plates to help hold the boards together and for switch mounting. Most wiring ended up attached to the covers or the top board. I guess about here it made sense to leave the back end cover smooth in case I wanted to use this not-a-box standing on its butt end. Well, why not, the components won't know or care and the thing may be convenient to use like that, perhaps under the table or desk. I may install another relay, to replace the manual switch, just for remote start capability.

Rusky tube B+
Well, not being totally perfect with my planning, I decided not to use the two toggle switches and wire the primaries of each transformer in parallel direct from the soft start relay. With the secondary in series this will produce a secondary of 2,250 VAC and a B+ of 3,200 VDC or so unloaded. I expect the voltage to sag to 2,800 VDC with a 2,000 watt output. This power and voltage range will work fine for the metal ceramic tetrode Rusky types in use today, such as 4CX800 and 4CX1,600, etc. to produce legal ++ power.

Good ole 3-500Zs being driven through a full wave bridge rectifier/filter arrangement can benefit from this AC source. Although the voltage is not higher than that of most 3-500 amplifiers, it doesn't sag as much under load. This Not-A-Box boosted the Centurion right up to legal limit output. (See Lighten Up.) With a 240 VAC, 10A. to 15A. Variac capable of 280 volts, driving this power supply to the Centurion, it becomes a legal limit + amplifier.

Lower and higher
For Eimac metal ceramic tube such as 3CX800A7 the primaries would be wired for the lower HV voltage production of 1,900 VAC for a B+ of 2700 VDC. For the higher voltage requirements of some other types of tubes - maybe 5,000+ VDC, the primaries of each transformer can be wired parallel to produce 3,800 VAC for a B+ of about 5,300 VDC. Amplifier makers today don't use tubes that require voltages in excess of about 4.000 VDC.

More serious on the electrics
Heathkit used the same 14 gauge wire for both primary and secondary. I'm sure the wire is rated high enough for the 1,125 VAC secondary and high enough for the new 2,250 VAC secondary. However, I opted to add a margin by slipping clear PVC tubing over the lengths of secondary wiring outside of the transformer and used additional insulation and attention for the connection to the high voltage cabling. The two 30 KV wires I pulled through a length of 1/2" clear PVC tubing and slipped smaller tubing over the ends next to the Millen HV bulkhead connectors.

Does it run?
Well, the voltage is as expected so I'll hook it up to an amplifier and see if it truly is, muscle for almost any amplifier.

It does run and is muscle

No, I did not
I didn't leave the sides completely open. I covered the sides close to front with 1/16" aluminum from the hardware store to keep fingers from fondling the 235 VAC mains surprises. Someone asked again, "Why I left it open and showing the ugly transformers and wasn't it dangerous?" My response, "You and I are ugly, not these transformers. And, high voltage is not available to hands or fingers anywhere in the power supply and this is my personal piece of Ham Radio gear that looks like Ham Radio gear or something seriously powerful." It is truly a versatile AC source when used with a 15A. 240 Variac capable of 180 VAC to 280 VAC.

If you would like to publish an article here contact Bob Hutchinson, N5CNN.