Two weeks before the AM Rally, after yet another year of procrastinating and unfinished projects, I remembered that I really wanted to have a home-brew rig up and running for the Rally. I've got lots of stuff in various stages of completeness, but nothing close enough for any possibility of completion in time for the Rally. What to do?
The clock is running; time for a Hail Mary pass! This could be fun, or at the very least, there’s the irony of a guy who knows absolutely nothing about football making football analogies (that is a football analogy, right?)
I’ve got this little home-brew 75 and 40 meter CW transmitter that I picked up at a hamfest a while back; maybe I can transform this into a workable AM rig in time for the Rally. I’ve already got an external modulator that I built a while back, so part of the project is done.
http://amfone.net/Amforum/index.php?topic=47922.0 This little CW rig was built from plans in the Dec. 1958 issue of QST, according to the information thoughtfully marked on the inside of the chassis bottom plate by its original builder. It’s got two tubes, a 6146 as a keyed crystal oscillator, and a 5U4 rectifier.
Drilling and blasting went as follows:
I solid-stated the power supply, and used the 5U4’s octal socket for a 6AG7 driver, the tuned output circuit of which is visible on the top of the chassis.
I added a second octal socket, for another 6AG7, as a buffer for the external VFO. The output of this buffer is resonated for 40m via a slug-tuned inductor, typical of a DX40, DX60, Eico 720, and a host of others, but where those guys left the buffer un-tuned on 75m (on 75m the inductor appears as an RF choke in those rigs), I installed a disc capacitor switched in parallel with the inductor for 75m. I got a stronger, cleaner output from the buffer that way. I had to install a clumsy toggle switch on the back of the chassis for that, but since I was starting from an existing build, I had little freedom with the layout.
I changed the B+ supply from a capacitor input to choke input, to get better regulation under load and to lower the B+ to 350v, since all I wanted was around 20-25 watts out, to drive my amp.
I built a mosfet regulator to provide 300v for the 6AG7’s (the perf-board is visible in the picture), added a little 120-150vac transformer, on top of the chassis, and built another perf-board with a zener regulator to provide negative bias for the 6146, as well as -180v for grid-block keying to stifle everything when not transmitting.
I’ve got a couple of DX60’s and their HG-10 VFO’s, so I added an octal socket at the rear of the chassis, to supply all the needed voltages: 6.3vac for the heaters, 300v for the HV, and negative 180v to grid-block it. Unfortunately, that necessitated moving the RF output connector, leaving the SO-239 to make a rather undignified exit through the top of the chassis, but hey, this was never intended to be pretty.
I installed a barrier between the final PA section and the buffer-driver section, made from double-sided copper-clad, with acrylic plastic insulated feed-throughs (courtesy of my lathe), with bypass caps.
I decided to provide grid bias to the 6146 with a combination of grid-leak and fixed bias, at a roughly 50/50 ratio; that way I don’t have to mess with clamper tubes or switch the B+. The only downside to that, I found, was that with only the -51v output of my bias regulator, at key-up, the idle current of the 6146 was higher than I’d have thought, like 60ma, so I added some connections to my PTT relay to apply additional grid-block voltage to the 6146 when not transmitting, which settles the tube down to 30ma at idle.
All of this is pretty straightforward stuff, but I got a bit more experimental when it came time for applying screen voltage to the 6146, referring to this excellent dissertation by Dean, WA1KNX:
http://www.amfone.net/AMPX/71.htm. Following his example (see his link to the schematics and formulas) I used his recommended 60/40 ratio of un-modulated to modulated screen voltages, through the appropriate resistances arrived at through his calculations. It worked out quite well; in my next post I’ll include some pictures of output waveforms under modulation.
The story, though, didn’t have quite the happy ending that I’d planned. The rig is now working very well, and it’s as mechanically ugly as I’d hoped, but it never got onto the AM Rally. It had some last-minute glitches that drove me crazy, and I put it aside at the start of the Rally, and ran with my old reliable DX60. While it would transmit a fine carrier, it was very unstable, grid-drive jumping around spastically, both driver and final tuned circuits showing multiple nulls and peaks, and some very weird looking stuff on the spectrum analyzer. Come to find out that, in my haste, I’d entirely forgotten to bypass my heater connections, or ground one side of the heaters. I’d put bypass caps to ground where the heater voltage entered the final PA compartment, at the feed-through insulators, but nowhere else. Once I grounded one side of the heater terminals at the sockets, and bypassed the other side, it worked perfectly.
I still want to install a metal barrier, on top, to shield the driver tuning section from the 6146, and also some sort of housing to protect inattentive fingers from the 300v on the driver tuning air-variable. The driver’s tank circuit is in series with the plate voltage supply, and isolated from the chassis with a nylon block and fiberglass shaft coupler, so it’s hot, as my left pinky-finger will readily attest.
Check out the cute little horizontal grid current meter; found that in one of my mystery boxes in my shop.
Some pictures of the butt-ugly but functional result:
