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
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.
Red oak planks
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+
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
serious on the electrics
Does it run?
It does run and is muscle.
I did not
Bob Hutchinson, N5CNN
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