This project was a bit more complex than most of the stuff I do. You ask why? Well, I had a bunch of working modules on a wooden board that had to be refitted into a homebrew enclosure and still work after reinstallation. I needed a special power supply that had to be designed and built, and I also had to design a vacuum tube final amplifier.
It is one thing to 1st determine a size of an enclosure and then create the electronic modules to fit that real estate envelope. The other way around is to have all the modules that are spread out on a wooden board and then try to mash that "blob" into a reasonably sized enclosure. This latter approach is more difficult.
Nearly Completed Mostly Vacuum Tube Wireless
Proof of Life!
I am pleased at how the rig looks, but it took a lot more noodling to figure that out. I can see some wasted space but for the most part all of the modules were already built, and this drove the final configuration. I tried to place the most frequently used controls so that they could be easily accessed. With my FFS, (Fat Finger Syndrome) Big Knobs was the order of the day.
I did a recheck of the power supply and especially the power plug wiring. Accidentally putting 500VDC on the Filaments will kill the project, so the wiring has to be right!
This recheck did uncover a small wiring error in the BIAS supply. It would not have been catastrophic but would result in too high of a value. The power transformer has one other winding other that 520VCT and that is 6.3 VAC at 3 amps. The filaments are wired for 12VDC so that winding was initially unused.
But then I had to have a -60 VDC bias for the rig. So, a lightbulb went on as I bought a 12.6 VAC small filament transformer. By pumping 6.3VAC (power transformer) into the 12.6VAC side of the filament transformer the primary side is now the secondary and we have 60VAC which can be rectified to deliver -60VDC.
The 12.6VAC side has a center tap and that was the wrong connection. I was pumping 6.3VAC into 1/2 of the filament so the secondary voltage was 120VAC. A simple wire change now produces -60VDC for the BIAS and cutoff supply. Always a good thing to recheck wiring!
[Later when I was testing the transmitter the -60VDC was not enough to set the bias level so back to the original wiring and -120VDC. Now I can set the idling cathode current to the proper level (25 ma).]
Bias Supply Wiring Change
The power plug wiring on the main rig chassis was checked and rechecked with the intent of no HV pumped into the filament circuit. The plan is to initially not connect the plate supply of the 12GE5 into the power socket and to remove the 12GE5 from its socket as 280 VDC is relay switched to the screen grid on transmit.
The initial Big Smoke Test will essentially check all of the circuitry that was formerly installed on the wooden board. The output of the LPF from the IRF510 in normal operation is connected to Pin 3 on the 12GE5.
For the initial test I will connect a 50 Ohm non-inductive 15-watt dummy load to Pin #3 to do all of the initial transmit and receive tests. Caddock makes the non-inductive resistors for the dummy load available at Digi Key and Mouser. The Proof of Life video passed the test and no smoke, but I only looked at the receiver.
I need to do some more work before hitting the Transmit button. Stay tuned! Who Can Wait... so check out the video below. With a bit of circuit peaking I get 25-30 Watts PEP to the antenna.
I am a happy camper and still going through shakedown but encouraged with the results so far. Even though I used a choke input filter there is a pretty substantial sag in the plate supply to the 12GE5. No load there is about 550 VDC on the plate of the 12GE5. That drops by a hundred volts key down. With a consistent 600VDC (and more current) on the plate I suspect we would see 40 watts out. The Hammond Transformer is 520VCT at 104 milliamps. I am seeing a cathode current of 150 Ma. on tune up and voice peaks. Simply stated I need a bigger transformer!
Them that know, really do know!
73's
Pete N6QW