Amplifier progress

I realized that I haven't written an update about the amplifier project in a couple of months. Some good progress has been made.

To recap, I've decided to home-brew an HF amplifier by taking two other kits (the Communication Concepts AN-762 amplifier and the HF Projects LPF kit) and combining them in my own enclosure, with a heatsink and a keying relay circuit. Once I have the basic amplifier working, I would like to home-brew some additional features, like a built in SWR meter, automatic bandswitching, etc.

Phase 1's goal has been to get the basic amplifier working with a manual band selector, etc. I harvested power transistors for the amp from a donor EB-63 kit that Nick's boss gave me, so that saved me some cash. Rob helped me get started populating the SMT parts on the board, and then I finished the through-hole stuff at home. I built the LPF in two evenings with no difficulty.

Choosing an enclosure was a real headache. I wanted something that would look nice and relatively modern, but finding the right thing in the right size (6"W x 5"D x 3"H or so) was next to impossible. I finally settled on a Ten Tec box with aluminum end panels and a steel top and bottom. I would have preferred all aluminum but that's life.

Finding a heatsink was another chore. Again I wanted something about 5" x 6", and it was just very difficult to find anything like that in quantity 1. I found lots of smaller stuff, and I found shops that would produce extrusions on demand (with minimum orders anywhere from 10 feet to 150 pounds). I also found surplus stuff, but with no thermal impedance information. Finally I found heatsinkusa.com, who sell a handful of extrusion widths in custom lengths for a good price. I got a 5" piece for about $12 plus shipping.

Just as an aside, I have to say that Google really let me down on both of these searches. The heatsink company was about 3 pages in to the search results. I didn't even find Ten Tec enclosures on Google, I spotted their ad in the back of QST. I seriously must have looked at 20+ enclosure companies and 10+ heatsink companies before I found what I wanted.

I took the completed amplifier to Nick's house to perform some basic smoke tests on it, since he has an oscilloscope and a signal generator. On the whole, it definitely seems to amplify, so that's good! But the output signal was not very clean looking. Nick suspects this is either due to his ancient janky bench power supply and/or the insane amount of RF present at his apartment due to his proximity to Sutro Tower. It didn't catch fire, so that's a good start. He said we should get it mounted to the real heatsink I want to use before continuing to test, so that's up next.

I went to McMaster-Carr and ordered 4-40 screws, a 4-40 tap kit and wrench, spacers, and standoffs. Mouser supplied UHF panel mount jacks, a powerpole chassis mount plate, female header blocks (to attach to the LPF control headers), and RCA jacks for the keying interface to the K2. At Harbor Freight I got two step drill bits, a center punch, some small files, and a digital caliper. I think this should be everything I need to do the mechanical work (plus a drill press and a Dremel; I have access to the former and already own the latter).

The amp PCB is 5.25" x 3". Since the enclosure top is steel, which doesn't conduct heat as well as aluminum, I think I'm going to cut a hole in it big enough for the PCB, and mount the heatsink to the enclosure over that hole. Then the amp can mate directly with the heatsink. Nick is going to give me some SMA connectors and semi-rigid coax, so I should be able to connecterize everything inside the box and not have anything hard-wired. I'd rather do that for ease of modifications later.

I'll post some pictures of the various components soon.

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