Vacuum Tube Direct Conversion Receiver / QRP transmitter projects

Discussion in 'Homebrew and Kit Projects' started by WB0RIO, Mar 20, 2012.

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  1. WB0RIO

    WB0RIO Ham Member QRZ Page

    You all might be interested in this pair of vacuum tube hacks I put together this winter: The House Finch Direct Conversion Tube Receiver
    and The Little Chickadee 6U8A QRP Transmitter.

    Both projects are built around common 6U8A tubes, the receiver also has a 6BA6 RF amp and a 6C4 audio preamp.
    Before building the receiver, I searched the net for tube-based direct conversion receivers and did not find any
    previous work.

    6U8DCRphoto1.jpg 6U8QRP.jpg
  2. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Those are really nice.
    How well does the receiver work?
  3. WB0RIO

    WB0RIO Ham Member QRZ Page

    I've copied CW, RTTY, WSPR and SSB signals with the receiver, and that's with a marginal 40 meter diople that sits on
    the shingles of my roof. As with most Direct Conversion designs, there's a tendency to pick up 60Hz hum, no AGC and reception
    of both sidebands, so don't expect it to be much better than a solid-state DC receiver. The audio filter is selective enough for
    decent CW reception.
    The VFO mechanics could be improved a lot, but it *is* a first generation prototype. It's basically a proof of concept design.
  4. AF6LJ

    AF6LJ Premium Subscriber QRZ Page

    Well that sounds good.
  5. KC8YLT

    KC8YLT Platinum Subscriber QRZ Page

    I like how you constructed everything! Take your time youll get the bugs worked out. Looks like you have something to be proud of! KC8YLT.
  6. W1VT

    W1VT Ham Member QRZ Page

    That is a very good idea--electrical boxes are not only cheap, but a very good choice for direct conversion receivers in that they are very sturdy (less microphonics) and made of steel (magnetic shielding).
  7. K5UOS

    K5UOS Premium Subscriber QRZ Page

    Homebrew tube receivers are rare. Thanks for showing your project.

    Have you measured/calculated the overall gain of the receiver?

  8. AB9LZ

    AB9LZ Ham Member QRZ Page

    Mostly because folks build regens instead... which is in essence a direct conversion receiver with the added ability to detect AM signals. Since not too many folks do AM anymore, the direct conversion idea is a good one.... especially of one cheats a tad and uses a diode ring mixer (still use tubes for the osc and af amp).

    73 m/4
  9. WB0RIO

    WB0RIO Ham Member QRZ Page

    I just did some quick tests, the results aren't super-accurate. And I don't claim to know how to do this test ;-)
    With a 15mV P-P input signal (50 ohm source) tuned in to produce the peak frequency
    of the audio filter, the output is 5V P-P into the 8 ohm speaker, the volume is set to about 2 out of 10.
    Dropping the input down to around 1mV P-P, the output is 4V P-P with the volume set to 10.
    That's as low as my signal generator goes and the scope (200Mhz Tek mainframe) mostly shows noise at 1mV.
  10. KO6WB

    KO6WB Premium Subscriber QRZ Page

    I use to actually use a military surplus frequency meter to act as a receiver. It worked okay and it was a direct conversion reception type of receiver. It didn't have a tuned input circuit and I was sending the signal it generated into the antenna. This means I was radiating a signal, not much of one but it was there none the less. I don't remember it having hum problems and there were no microphonics. It worked but not as well as a dedicated receiver. Take a look at;
  11. N2EY

    N2EY Ham Member QRZ Page

    You can see quite a few here:

    I know of at least two hollow-state direct-conversion receivers in QST:

    May, 1961 (more than 50 years ago!) W2WBI described a "T.R.F. Receiver" for 80 and 40 with a balanced detector. RF stage, pair of pentagrids, VFO, audio amp. Nice job, but for that many tubes you could have a small band-imaging superhet. Although the term "direct conversion" is not used in the article, it is clearly that kind of receiver.

    July, 1973, shows "The 80 Meter Pebble Pulverizer" by W1CER and W1NPG. A complete QRP CW transceiver for 80 using tubes. 6GX6 is the detector.

    73 de Jim, N2EY
  12. K5UOS

    K5UOS Premium Subscriber QRZ Page

    Hi Jim,

    I subscribe to the HBR site and the email reflector, too. It is a fine site for anyone interested in researching tube receiver design. Kee's has most of the HBR circuits available for download.
    I also remember your tuning mechanism made with a bowl.

    That said, I would not build a tube receiver based on the HBR series for obvious reasons. It is not a very good performer above 40 and 80M.
    But I have used circuits like the audio stages and IF stages from the HBR. The HBR14 is a good candidate for a DDS (like Pat, WA6MHZ is designing) but the RX's still need improvement in selectivity.

    I knew there were other DC tube receivers. There are some others on the web and I know we have discussed ideas about them on the glowbugs site.
    But Galen, WB0RIO, has a unique design, all his own. It is pretty cool! There are not many folks scratch building tube receivers (or any type receiver) and a unique design is nice to see.

    There were reasons the tube DC receiver didn't catch on. You mention one above. For all the work you could build a really great entry level superhet. There are lots of other caveats.
    Since effectively all the gain is at audio (often > 100dB) the difficulty increases in tubes over solid state.

    I like pointing out your station: It is unique, too. Tubes are fun.

    73, K5UOS

  13. AB9LZ

    AB9LZ Ham Member QRZ Page

    My (older) brother and i built the pebble pulverizer ( thats actually a stretch, he did most of it, i just made a mess) i do remember that the thing was really prone to microphonics (an understatment) and common mode overload, wls came in like gangbusters everywhere and all of the time. And the selectivity was pretty bad. The regen he built didn't suffer from any of that stuff.... at least not as badly anyway.

    40 years later i've forgotten all of that ;) and think that a tube DC rx is a good idea.

    73 m/4
  14. KL7AJ

    KL7AJ Ham Member QRZ Page

    That's really cool. I LOVE the cabinets!

  15. KA4DPO

    KA4DPO Ham Member QRZ Page

    I second Erics statement Galen, very clever use of the electrical boxes as chassis. I'm curious, why a DC receiver and not a regen?
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