Wednesday, November 11, 2009

Homebrew kW for the Big Guns

DIY, or homebrew is one of the most active and popular areas of the amateur radio hobby, particularly among the QRP crowd.. They have been designing, building and sharing circuits for the flea power fanatics for a very long time. In fact, approximately half of the all the Altoids sold in the US have the candies thrown away just for the tins to house a Powermite or other QRP rig.

Well it's time the Big Gun QRO crowd join the fun. The next edition of QRM!, the official journal of the Lost Island DX Society (LIDS) will feature an article on a homebrew kW amplifier. But this amp doesn't depend on a big fire bottle 3-500 to make it's heat, or even a ceramic 4CX800. This is a solid state kW.

There are several solid state kW amps on the market, but they all use fancy high power transistors, which carry comparable price tags. The price of a replacement set of finals for some of these amps is worth as much as your car. Well, that is, it's worth as much as MY car. YMMV, but the ole' Gremlin still gets around pretty good.

This exciting new high power amp design uses a power device that is dirt cheap and readily available - the 2N2222, the cockroach of solid state devices. The lowly 2N2222 is the unappreciated ugly step-sister with the hairlip of the transistor family.

By itself, the 2N2222 is only capable of generating 1W of power, but put 1,000 of them in a cascode/cascade design and you've got a full gallon of RF ready to pump into the ether. But a 1,000 transistors sounds awfully expensive, doesn't it? 2N2222's are dirt cheap. 1,000 of them will run you $435 from Digi-Key. That's less than fifty-cents a watt - a bargain compared to hollow-state technology.

There is another advantage of running a massive number of solid-state devices compared to tubes or high-powered transistors - redundancy. I like to call it putting all your eggs in one basket conundrum. With a big tube amp, you might have one, two, three, or even four tubes to generate that kW. If you pop a tube, you lose, at best case a quarter of your output power. Worst case, you're back to fighting it out with the QRP, excuse me, low power crowd running a 100W. Then you've got the big expense of buying replacement tubes. Ouch! See the comment above about the price of power devices and cars.

With this new homebrew kW design you have 1,000 transistors. So what if you lose 10, 20, even 50 of them? That drops you from a kW to 950W. That's less than 0.25 dB loss. And the replacement cost is less than a week's worth of Starbuck's.

Tube amps require high voltage power supplies, typically 2kV or more. You've got to be careful around that kind of voltage, lest you end up flying across the room from an errant screwdriver. It might be fun the first time or two, but after a while the twitches become annoying.

With 2N2222's you can build this amp to run off of 12V, a nice safe voltage. And the beefy 800A power supply can be used for spot welding in a pinch as well. Dual use technology - yet another advantage to this design.

It's tough to sandwich 1,000 transistors into an Altoids tin, even if you use surface mount. But we've found a source for a special breadbox size Altoids tin that's just perfect to hold a kilo of 2N2222's. (See photo at the top of the page)

The next issue of QRM! will be available as soon as the printer finishes the that batch of contest certificates for the World Wide LIDS QSO Party. If you're not a LIDS member, but would like a copy of the new issue of QRM! with the new amp design in it – just ask one of the local LIDS to borrow their copy!

1 comment:

  1. I have always found the tag line "Curiously Strong" on the Altoids tin to accurately express my feelings about certain very loud stations I hear. Some of them seem to be using amplifiers made up of 1000 devices, but more likely 6146s than 2N2222s.