EMRFD Message Archive 6385

Message Date From Subject
6385 2011-06-16 06:36:48 kx4om 12BY7A driver to Ten-Tec 6m transverter
Hi all,
I have a Ten-Tec model 1208 6 meter converter that I'd like to use to get on 6m. Not having a QRP 14 MHz CW-SSB exciter/IF rig completed, I would like to use the transverter output of my Tempo 2020.

The 1208 accepts an input of 2 to 10 watts, (or less, I think, selected by a resistor). The 2020's transverter output is from the 12BY7A driver's output picked off at the grids of the 6146B PA stage (which has the heaters turned off when using the Xverter out.)

Now, once I have my QRP 20m rig built, it will feed the 50 ohm input to the transverter with no problem...I can figure out *that* matching issue :)

The question is, how do I figure out the match from the 12BY7A driver to get to 50 ohms? My math skills have slipped to the point where I'd have to start with the Pythagorean Theorem and work up from there. I haven't done Thevenin and Norton calculations in a few decades. My next step is to measure the peak-to-peak voltage at the transverter output with a scope, but what should I assume as the load I'm seeing? It looks like from the schematic that the only "load" that the Xverter output (with nothing plugged into the output jack) would see is the driver's LC network for 20m, DC isolated through the capacitor to the PA stage grids.

Once I know the driver output load impedance, I can calculate the impedance transformation circuit (if any) needed to get to 50 ohms for the 6m transverter.

Tnx in advance,
Ted, KX4OM
6386 2011-06-16 07:40:47 wb8yyy_curt Re: 12BY7A driver to Ten-Tec 6m transverter
I am much more familiar with the 1208 than the nuances of working with the 12BYA7 (although I may have replaced a few as a kid when working at a TV shop).

The 6m transverter is rather picky -- so do monitor its output power while doing experiments -- that is AFTER you have attempted to measure what is coming from your exciter. Too much power and you will smoke some of the 1208's control diodes (been there, done that twice). On my unit around 2-3 watts was the sweet spot - as more would introduce some distortion (presumably in the upconverter mixer). I would rely myself in evaluating power coming from your 20m source, if you have a power sensor you trust at the ~2 watt level. If you do not have a way to adjust 20m RF power coming in I would be very concerned you cannot use this transverter to put a clean signal on the air. Too little and your 6m output is low, way too much is smoke, too much is distortion around your signal disturbing others on the band. So unless your rig with the filaments can realiably give around 2 watts +/- ~1dB I sense you don't have a good 'match' (pun intended). Delivered power is a bit more important than impedance,but of course a great impedance mismatch would make hitting the target input power even more difficult.

Not exactly comforting, but likely you will have great distortion at a lower level than what will smoke the input diodes or overheat the attenuator resistors. TenTec put a leveling loop on the 2m transverter (which I still use BTW) to avoid this matter.

One more caution -- I found my 1980's rig I used with the transverter did not produce the same output power on SSB as CW (I had to reduce the drive on SSB, which was a real annoyance since using both modes is very common
6387 2011-06-16 10:05:31 kx4om Re: 12BY7A driver to Ten-Tec 6m transverter
6388 2011-06-16 10:23:21 kx4om Re: 12BY7A driver to Ten-Tec 6m transverter
I wonder if this would work:

o Tune the Tempo 2020 for normal 100W output on 14.100 using dummy load

o Switch the 2020 to transverter output (turns off PA filaments) and connects output jack to output of driver to PA coupling cap.

o Use a T-match tuner (my MFJ-969) hooked up backwards...

--connect a 50 ohm resistance Caddock 30w) at the tuner *input*

--connect the tuner output to the 2020 transverter output

o Transmit with the 2020 and adjust the tuner for minimum SWR

o measure the L of the inductor and the C of both capacitors

o build the t-match network in a box

It probably won't work for a variety of reasons; after all, I thought of it.

Ted, KX4OM


My original post truncated for brevity
6389 2011-06-16 13:19:50 wb8yyy_curt Re: 12BY7A driver to Ten-Tec 6m transverter

I should have thought deeper about the problem - in fact a helpful fact is lurking: the TT1208 begins with a lot of attenuation! These are actually resistive networks designed for 50 ohms, that feed a passive mixer circuit for the conversion to 50 MHz. If your exciter has a different impedance, all this 'padding' to go from about 3 watts to a few mW should address any impedance mis-match present at the rig's transverter output. If you have the stuff, you could construct an attenuator similar to the first stage attenuator in the TT1208 (I believe there are two of them). Then using information in the TT1208 manual, calculate the drive power that is required at the output of this first attenuator network, and convert it to voltage if you are using a scope instead of a power meter. By measuring what your rig's transverter output provides into an attenuator network, you can discern if you have too much or too little energy from your exciter. The presence of attenuators in this network can be used to convert this to a dB counting exercise if you make your measurements with an attenuator loading your transverter output, and you relate to the attenuators inside the TT1208. Avoid smoke testing the 1208 before you have good measurements, as it has some PIN diodes that may not be in your junkbox (although there is ONE diode that can be replaced with a common P-N diode part.

As you may have discerned, particularly if there is no level adjustment
6391 2011-06-16 14:02:46 kb1gmx Re: 12BY7A driver to Ten-Tec 6m transverter
6394 2011-06-16 19:30:48 kx4om Re: 12BY7A driver to Ten-Tec 6m transverter
6401 2011-06-18 15:54:30 davidpnewkirk Re: 12BY7A driver to Ten-Tec 6m transverter
6402 2011-06-18 19:09:22 davidpnewkirk Re: 12BY7A driver to Ten-Tec 6m transverter
6403 2011-06-19 08:10:46 kb1gmx Re: 12BY7A driver to Ten-Tec 6m transverter