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Cathode follower circuit for Rf generator

Discussion in 'Radio and Wireless' started by Y2KEDDIE, Feb 20, 2015.

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  1. Y2KEDDIE

    Y2KEDDIE

    259
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    Sep 23, 2012
    I revived from my junk box, two military frequency meters: A CRR-740 (LM-13) and a BC221AA.
    I built Ac powered battery eliminators for each unit ( 180 VDC and filament source.) I’d like to build a cathode follower for each unit so I can pick off signals for my frequency counter and also use the output for driving a RF power amplifier tube, 6146 or similar, for RF experimenting.
    I’m open to experimenting but it would be nice to start with proven designs.
    Any suggestions on proven cathode follower circuits, tube or transistor?
     
  2. Arouse1973

    Arouse1973 Adam

    5,164
    1,080
    Dec 18, 2013
    I cant answer your question sorry but Hop might be able to. You out there Hop big buddy? @hevans1944
    Adam
     
  3. hevans1944

    hevans1944 Hop - AC8NS

    4,459
    2,074
    Jun 21, 2012
    I am sorry I can't help with this one. It has been many years since I have "played" with cathode followers or the 6146 power tube (which I designed into my homebrew novice transmitter) using information available to me in 1966. This was mainly the ARRL Amateur Radio Handbook. If you can find one of these in a library it would be a good starting point for experimentation using a vacuum tube cathode follower. A more modern edition would have information on emitter followers.

    The BC221AA will cover a range of about 125 kHz to 250 khz, and 2 MHz to 4 Mhz in two ranges, but anything above 4 MHz requires the use of harmonics of the fundamental frequencies. That means RF bandpass filtering will be required before attempting to excite a 6146 to ensure only one RF frequency at the amplified output.

    According to one of the BC221AA manuals I found on the web, there should be a binding post on the back of the unit that will allow you to connect your frequency meter. This was originally used to "excite" a receiver or sample (antenna style) a transmitter output for frequency calibration of same. Since you actually have one in hand, I suggest you either measure what is available there (with an oscilloscope) or try to drive your frequency meter with this signal.

    You mentioned in an earlier post that you used to have both amateur and commercial radio licenses and more than a little experience in this field. You should be able to throw together a cathode follower fairly easily. I don't know what RF level the BC221AA provides. You may need some amplification ahead of the cathode follower to adequately drive a 6146 as a common-cathode power amplifier. My favorite for this sort of thing was the 6SN7-GTA dual triode, which may still be available as "new old stock" on E-bay. If I were to get back to "playing" with tubes, I think I would prefer the octal based dual-triode tubes rather than the nine-pin based tubes, like the 12AU7 or 12AX7. More room to wire things up and easier on my old eyes.

    Please keep us posted on what you are doing, maybe upload some photos or schematics. I am going to be busy messin' with Arduino Uno and Zigbee modules for awhile. Huge learning curve there, but also a huge online community to get help from...
     
    Arouse1973 and KrisBlueNZ like this.
  4. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    Thanks for your input, as I always apprciate.

    I built 40 meter hombrew MOPA back in the 60's, and again when in tech school in the 70's. I remember the transmitters used a 6146 and were quite simple. I don't think there were any tuned circuits inbetween the oscillator and 6146 grid input; of course these were crystal controllled , single band transmitters.

    I was thinking of using the BC-221 as a VFO as well as a calibrated signal generator. I'm thinking the output connector on it is Hi-Z. If I could use a cathode follower to bring ot down , I could use my 50 ohm calibrated attenuator with it, along with oher devices and have plenty of output. Also, I have the calibration book for both units and hopefully will not pull off frequency.

    It's funny. I built and operated these circuits 40 years ago and took the designs, from magiznes and text books, for granted. Now, as I experiment, I can see a lot more thought went into designing than I was aware of.

    My career took a turn many years ago to the electrical power. My emphasis has been automation using AB PLC's, and VFD's , instrumentation and process control. It's still electronics but very little RF , and no tubes!
     
  5. hevans1944

    hevans1944 Hop - AC8NS

    4,459
    2,074
    Jun 21, 2012
    As you noted, no tuned circuits in grid necessary to excite the 6146, although the usual pi matching network is needed to transform the plate impedance down to something a little more useful. My novice xmtr was a crystal-controlled oscillator of course, with some fancy circuitry for the keying that has long since disappeared from what's left of my mind.

    IIRC all a cathode follower needs is AC coupling to the grid, a grid-leak bias resistor from grid to ground, a smallish (50Ω ?) cathode resistor, and maybe 100 to 200 VDC on the plate, perhaps isolated with an RFC and a bypass capacitor. You can capacitive couple the cathode output signal to whatever you want to drive. The grid bias needs to be negative enough to place the plate current somewhere near the middle of its nominal range from minimum to maximum, but I don't remember what is a recommended method to do that. Back in the day, I would have a separate 0A2 gas-tube regulated negative bias supply and a voltage divider to set the grid bias. I am sure there is a simpler solution.

    I found a web page that shows how to bias a cathode follower, being used there as an audio stage for a guitar or speaker application. Should work okay for RF though if the interelectrode capacitances are reasonably low and the frequency isn't too high. It's worth a try to see if it gets you in the ball park.

    If it weren't so darned cold here, you have almost inspired me to go out to the garage and try to dig out my old homebrew power supply and novice transmitter. OTOH, that power supply is a real boat anchor, so maybe it's best to let that sleeping dog lie and continue to work on stuff whose power supply I can carry in my coat pocket.

    Did you find schematics for your versions of the frequency meters? From your first post I gathered one was Navy and the other was Army, although both were very similar in construction. There is currently some historical interest in this type of World War II electronics surplus equipment, so the documentation is improving as old manuals are resurrected, scanned, and uploaded to the web. I found several on the web, but without knowing which one you have it's impossible to discuss possible additions.
     
  6. Y2KEDDIE

    Y2KEDDIE

    259
    15
    Sep 23, 2012
    Yes, I have manual for both units, I found on the WEB. I had battery eliminators so I was able to test and fire each unit up. I was suprised how smooth, and how much bandspread are on these units. You can really fine tune the output frequency.

    I bought some transformers, and all parts to build internal permanent supplies on the unit's chassis. I plan to use these so I don't think modifying them with internal supplies is going to hurt. I want to come up with a cathode follower/buffer circuit I can add at the same time; hopefully I can fit it all in the same box.

    I was looking at the HP606B. That unit would be nice to have with it's high output. I loooked at it's shematic to see how they got such a high bufferd output, but it looked a little more complex for me to reverse engineer. At $100 typical, its not a bad price for a good generator. I'll have to keep a look out at the flea markets.
    I vaugely remeber these generators from when I was in the Service.
    I worked on: T195, R392, R390, and GRC-106 which was state of the art i the early 70's.

    I love the "old" equipment with lettering I can read and knobs I can grip to turn. No softkeys and menues, LOL.

    I have a Tek 191 , and a Cushman CE-3 Service monitor for RF signal generators but I thought it would be fun to revive these old military oscillators for something useful and fun to build.
     
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