[Johnson] 'brew xmtr about to be built/need tech comments

[kwylow zinjanthropus]

All:

Did a review of some circuits I had lying around in several books. Between the 1953 ARRL HB, 1962 ARRL HB, some ER articles, and my Ranger manual, I'm pretty close to a "firing solution" with regards to my home brew transmitter. Expected range would be from 160, 80, 40, 30, and 10 (with some other bands to be decided, especially if the touted 5 Mcs band became reality). Watts out should be 40-50 watts with AM/CW capabily. No coils to change out. All bandswitched, xtal controlled or VFO input.

It's going to be like a Ranger but scaled back a few items. No internal VFO (I will use a spare Heath VF1 I have lying around). It will have plate modulation (I'm temporarily borrowing an xfmr from another parts Ranger), using the following audio chain: 6SJ7 preamp to 6J5 interstage to the p-p 6L6 modulators. RF chain will consist of a 6AG7 into a 6146 final. 

Did some taking apart of that Bendix Skipper Marine transmitter and it yielded up some pretty good parts! Two Air Dux coils (one that looks to be around 108 uH and the smaller at around 50uH), the rf choke, blocking capacitor, and anode cap for the 6146, the tube socket, and a large RF 6 position switch (for the tank and driver circuits). 

About this time, I was getting excited! I remembered a large capacitor I picked up a few years back so I retrieved it from it's dusty lair: a large Cardwell transmitting cap with about 0.015" spacing between the plates. I couldn't tell what it's capacitance was so I did some rough calculations from the ARRL HB: it looks around 240 uuF. I also dusted off an untouched large Hammond metal cabinet and perforated cover from an aborted audio amp project from 1998. Cool!

I also dusted off a 10H, 300 mA Stancor swinging choke I had lying around plus 10 new medium sized Dakaware knobs found on the Collins A-line series. A look in another box revealed a new Triplett 250 mA meter (like the ones found on the 32V series xmtrs), and a multiple position switch that could handle up to 7 different positions (depending on the stop tabs you broke). This will be used for the xmtr's multimeter switch. 

Well, I had about 60% of a transmitter so I bought the following parts from Antique ES: a triple broadcast 500 pf (per section) for the loading cap (which I didn't have), a 3:1 interstage xfmr (equivalent to a Stancor A53-C), a 100 pf variable cap, and 5 Fender Amp type pilot lamps with 5 glass jewels (2 red 1 green, 1 amber, and 1 purple). The parts should be here sometimes next week!

I also have two 360-0-360 VAC transfomers rated at 250 mA that was given to me by Gene, KB4YST (SK).  

Here are some questions:

1) I would like to have about 50 watts or less coming from the 6146. I don't want to run to the full 250 mA rating of the tube and would like to loaf it along a little less. I'm thinking that maybe I should run the plate voltage of this around 400-500 volts (may take out my selection of my transformers). Or, I could use a voltage doubler on one of the xfrms and simply detune the input to get at the lower power of the 6146.

2) I still could delete the swinging choke and go full cap input on the supply. That's fine, right? Since I'm talking about it at this point, one xfmr could be used to power the modulator and driver stage while the other transformer runs the 6146. I'm used to running supplies using one xfmr. 

3) The 6AG7 to 6146 interface. Should I use some sort of tank circuit to tune the 6AG7 osc to the 6146 or just let it pipe in via a plain ol' cap (like some other circuits I've seen). 

4)  Since I'm on the 6AG7 circuit, can I control the power out of that tube by varying the screen grid resistor? The current resistor is at 22K at 1 watt. Shouldn't be too hard to sub in a variable resistor.

If all goes as planned, I may put this together with my Collins 75A1 rx (might as well mention that since I'm posting to a Collins listserv). 

Let me know your comments.

Ca, N6KYR



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