After a couple of days scratching my head and asking around trying to figure out why my TZ-20 amplifier wasn't working up to expectations I finally did what I should have done first. I
checked the wiring.
One TZ-20 wasn't connected to the final tank coil. I was running with only half an amplifier and a poorly designed one at that. It worked but not well. Once I fixed the wiring error power levels jumped to the 50-60 watt range. Neat, but now I needed to deal with typical amplifier problems.
TPTG feedback -The first problem I had was that the amplifier took off on it's own (without any drive) when the input/grid circuit was peaked for maximum output. This was solved by removing C3, the TZ-20 grid tuning capacitor, from the circuit. C3 in the amplifier and C8 in the final tank circuit of my 1934 transmitter interacted anyway so this eliminated one pesky control. Problem solved but now the drive level was
way down...one solution led to another problem.
Drive level - With no tuned circuit on the grids of my TZ-20s they were not getting the drive needed to get anywhere close to full output. This one was solved with a new coil for L1/L2. More turns for L2 got the power transfer up to the point that now I'm easily driving the amplifier to 75 watts input.
Bias - This all started with me wondering if I has the bias set right. The advantage of the TZ-20s is that they do not need an external bias supply to protect them when the drive goes away. With no drive (and no bias) plate current drops to almost zero. For the TZ-20 a simple resistor from the grid to ground is all that's needed to produce needed bias....but what value? It turns out that the 1939 Taylor tube manual has a formula for calculating recommended Class C CW grid bias for a given plate voltage, in my case 500V. Bias came out approximately 20 volts. I'm driving the grids to about 30mA ... 20V/30mA = 666 ohms. I had a 750 ohm resistor, close enough.
One question that had to be dealt with was how best to couple the output to the antenna. It turns out that, for my 40 turn tank coil, a four turn link gives me maximum output into my 50 ohm dummy load. I am set up, though, so that I can add a loading capacitor in series with the link to cover real world antenna conditions.
Now I have an amplifier for
my 1934 transmitter that takes about 6 watts of drive to run 75 watts input (45-50 watts output) on 80. With a couple of new coils I should be able to put it on 40.
Look for me during the
AWA Linc Cundall OT CW contest next week.
I've now finished up the transmitter power supply so I'll soon be testing it out on the air. The transformer is rated for 560V at 270mA so should certainly easily handle my 50-70W transmitter. At 31 pounds, mostly iron, I don't plan to move this one around much.
After Thanksgiving Dinner this year I took the opportunity to make progress on the power supply for my 1939 transmitter.
I completed the 500 VDC and 7.5 VAC sections needed for the amplifier. PSU Designer II indicated that I needed to allow for almost 2000 volts across each of the diodes in the bridge rectifier. I ended up using two 1000 PIV diodes in series for each leg of the bridge. This is a boat anchor power supply with silicon hidden under the chassis. It needed some glow. I added a type 80 rectifier connected only to 5 VAC so that it would look right. After all, real radios need to glow in the dark.
Between 80 and 40 I managed twelve contacts using my National HRO Sr and TZ-20 transmitter with a low hung inverted L antenna. By contesting standards not a lot but still better than any of my LC QSO counts going back 20 years.
I worked nothing to the west but I did manage Qs to Canada, the east coast and southwest. I heard N2BE a lot. Too bad I could only work him once per band. I was surprised to not hear many of my MN friends on the air. Of the eight I worked in the BK I only heard three on for the LC.
when one of my friends commented that it certainly was wireless, just look at the one rotatory switch with no wires attached to it. This reminded me that I had one last update to post on this project.
loading adjustment. With SW3 I can switch out the series capacitor all together or add in additional capacitance to give me a total range from about 20pF to 900pF. Tune up becomes very much like a pi network "dip and load". With SW3 and C5 set for minimum capacitance the plate current is dipped using C4. After dipping, the loading capacitance is increased using C5, C6 and C7 into the range of operating plate current and then the current redipped using C4. C4, C5, C6 and C7 interact so this process must be repeated until the last C4 adjustment puts the plate current where it needs to be for full output...in my case about 150 mA. 500 VDC on the TZ-20 plates at 150 mA gives me 75 watts input.
checked the wiring.
way down...one solution led to another problem.
I expected.
Taylor Tube manual I expected to see 50-75 watts input with 35-50 watts output, instead I'm seeing 20-25 watts input and only 8-10 watts output. I do have three TZ-20s. Any two of them behave the same. What an I doing wrong...or is it working as designed?
waiting for some sort of rack to mount it above my 1934 transmitter. I finally got some help from Skip, K7YOO, to cut and bend a piece of 16 gauge aluminum to form the shelf. 1" angle aluminum is used for the vertical legs and the shelf supports.
a few coats of paint, probably 
black wrinkle to match the rest of the station.
