I have never owned or operated one of the early Heathkit transmitters such as the AT-1, DX-20, DX-35, DX-40 or DX-60.
But now seeing some of these on eBay there is the great temptation to buy one just to fiddle with it. My early rig was a Johnson Adventurer which I thought was a bit more uptown than the Heath gear.
I now see some of these early Heathkit rigs on eBay in the $50-to-70-dollar range and so I keep looking.
Now I do have a homebrew vintage transmitter which uses a 6AG7 to drive a Glass version of the 6L6 but with the 350 VDC homebrew supply about the most I could coax out of that jewel was 10 watts. I knew the 6L6 could do a lot more damage. Avoid buying what is the counter intuitive metal version of the 6L6 as it has lesser ratings on power output and plate dissipation.
I turned to copilot and asked what the max is I could see from a 6L6 driven by a 6AG7. The answer was 20 to 25 watts. But on the critical path for that greater output level was higher plate voltage and a well-designed Pi Network.
The power supply was the second task, but I will cover that 1st. With a full wave rectifier, (two diodes) and a capacitor input (100Ufd) a 10 Henry Choke and a 2nd 100Ufd following the choke on key down I saw 350 VDC from the Homebrew supply.
Changing over to a full wave bridge choke input topology only needed the addition of two silicon diodes (1N4007) and changing over to the choke input and taking the former input capacitor and putting it in series with the second capacitor yields a cap rated at 50 MFD at 900 VDC. The key down DC voltage is now much higher on the plate of the 6L6. You must also unground the secondary high voltage center tap and just connect it to an unused terminal on a terminal strip.
The Pi Network was revised so that the Tune Cap is 100pF and the Load Cap was determined experimentally and consists of a 600pF fixed Silver Mica Cap (two 1200pF in series) in parallel with a 100pF Trimmer. The tank inductor is 26T of #20 on a T-68-2 core.
With these two changes the power output is better than 20 watts and I was heard on the RBN at close to 1750 miles away. It worked!
One other addition was to build a 40M W3NQN LPF installed in a metal box which is in line following the SWR BRIDGE. That certainly would address any harmonics of the 7MHz signal from slipping through to the antenna. Early CW transmitters like the Heathkit's did have that problem.
In essence what I have sitting on my bench is not unlike the Heathkit AT-1 and I didn't have to spend $130 plus shipping. My crystal is on 7030 and replacing the Band Pass Coupler (a plug in), the Pi Network and the W3NQN filter could put this rig on 20M at 14.060Mhz.
BTW the AT-1 is advertised as a 35-watt input transmitter. Do the math with 70% efficiency the best you can get is slightly less than 25 watts output. You have to love those marketing guys!
This transmitter built about 25 years ago does have a unique feature with the band pass coupler in the 6AG7 stage. This enables you to have a broad band output from the 6AG7 without the need for a Grid Tuning capacitor for the 6L6. I installed a 4-pin socket and thus band changing is simply plug in a new coupler for the operating band. You don't see that too often in homebrew (or commercial) rigs. I saw this used in an old RCA Ham Tips or GE Ham News in a design from the 1940's ~ 1950's. These publications can be found on the N4TRB's website.
Still happy with the choice you made last November. Now the beef prices look like they may escalate, and the talking heads are suggesting we move to lamb. How long do you think it will take before lamb prices increase based on an increased demand. Economics 101. For the majority you own it.
Before you hit the send button, the red pilot light with the ON label is to show you have filament voltage. The panel meter is not a Plate or Cathode current meter but instead is a meter that samples the RF output and with a diode becomes a power output meter. So, you tune for max smoke.
The green pilot light is a hole filler that formerly housed a Load capacitor. But I turned it into another visual indicator as there is a one turn loop connected through the T-68-2 core to the 6-volt pilot lamp. It glows brightly with max smoke and follows the keying. Cheesy I know!
Them that know can make things go.
73's
Pete N6QW