Tuesday, August 23, 2016

Homebrew Germanium PNP Transistor 20M SSB Transceiver

UHF Germanium PNP Transistor SSB Transceiver


Whilst I await some parts for the LDMOS amplifier and Arduino Control system, I began to search my large box of projects that "sorta worked" , worked only once or never worked looking for something I could fix in short spurts of time. Somehow those who possess the "knack" for not having a project work or work properly are always drawn back to the bench in hope of finding the magic pill to bring the "pile of parts" back to life.
 
Mention of Germanium PNP  transistors must seem odd today. For some it must seem like a cruel April Fools joke suggested by Bill, N2CQR. Yet for other old timers who really do know about such devices invariably must think N6QW has passed into senility.  Yes there really are (or were) Germanium transistors (the CK722 was one of the first) and most early transistors were of the PNP variety. One of the early RF grade Germanium Transistors was the Philco SBT-100.
 
Interestingly enough today audiophile homebrewers and DIY musicians love germanium transistors as applied to "fuzz boxes".
 
Today by and large most transistors are silicon and NPN. Some 60 years ago most transistors were limited in power and frequency range; but today cheap UHF silicon NPN  transistors can be had for pennies.
 
In about 2011 I had a grand idea to use Germanium PNP  transistors in a SSB transceiver. I was not the first, as Ben Vester, W3TLN in a 1963 QST article did just that, and it was a small radio, roughly 5X7X2 inches. His rig was on 20M and ran about 1/4 watt. W3TLN's article is what prompted me to build the now "Juliano Blue" shirt pocket SSB transceiver. My project was 2X2X4 inches. In preparation for that project which ultimately used no Germanium PNP  transistors, I did purchase a 100 piece box of Russian Germanium PNP UHF transistors. At the time I paid about 10 cents a piece for them. These transistors were weird looking and the in line pinouts were BCE.
 
In 2013 after moving to Southern California, I decided to attempt to build the Germanium PNP UHF transistor SSB transceiver using the Sideband Engineers Model SBE-33 as a template. You might be surprised to know this radio vintage 1963 used bilateral circuitry. I did build the radio and the results were less than spectacular --so into the large box it went. Fast forward to today and I found that radio and cannibalized the parts. Using Vester's approach I built a new Germanium PNP UHF Transistor SSB transceiver. There are still lots of bugs but I am on the path to success. If you look closely in the video you will see some of the strange Russian transistors.
 
 
 
Some changes I made to Vester's deign include the following:
 
  1. Vester used a tuned Germanium transistor RF Amp Stage that fed a Germanium Transistor Mixer Stage for the front end. Using a 4 Pole Slide switch whereby using one set of the poles, he switched that front end output to the Crystal Filter on Receive and on Transmit fed the Balanced Modulator output  into the filter. In my scheme on receive I have a broadband Germanium transistor RF amplifier feeding into an SBL-1 Mixer stage. Utilizing a pair of ultra miniature relays ahead and following the SBL-1 on transmit the Microphone amplifier (a Germanium PNP audio transistor 2N996) is switched in place of the RF amp. On receive the LO (an AD9850) is fed into Pin 8 of the SBL-1; but on transmit the BFO is now switched into Pin 8. Cheating a bit? I don't think so --just using what I have.
  2. Following the SBL-1 is a post mixer amplifier (Germanium UHF PNP transistor) and that feeds the 9.0 MHz commercial crystal filter. Both the broad band amp and the post mixer amp are simply my standard 2N3904 general purpose amp stage that has been modified for the Germanium PNP transistors. The output is adjustable so we don't overdrive the follow on stages. Works perfectly!
  3. In W3TLN's design he has a two stage IF amp chain following the filter and that feeds a diode ring. On receive it is used as the product detector and as the transmit mixer on transmit. Another small relay will switch the BFO/LO signals into this diode ring.
  4. I currently have some dreaded feedback/oscillation issues in the IF amp stage which I hope to cure. Shielding and grounding are key no matter what kind of transistors are used.
  5. The video was made in the early evening and 20M signals were not too strong. The radio does have AGC (one of the few NPN devices --and not Germanium) so it does have some frills.
Stay tuned.
 
73's
Pete N6QW

4 comments:

  1. At one time I lusted for an HRO-500 receiver. The HRO-500 is an all discrete transistor general coverage receiver that is built mostly from PNP transistors. I was fond of it due to that wonderful HRO dial and that it was implemented with all discrete transistors and diodes. So - amazing things can be done with PNP transistors!

    I bid up to $700 dollars for one on ebay, but got cold feet and sweaty palms - for that much money I already had other receivers that worked great. A little later I bought an R390A, Collins built, much cheaper and in good working order. The R-390A is another radio I am still fascinated by, a real electromechanical wonder.

    Good luck with your project and thanks for sharing your efforts on the net.

    73's
    Jim N8CAH

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    Replies
    1. Hi Jim,

      Thanks for your post --many of us lusted after an HRO-500 --almost the same for a Hammarlund HQ-215 --same idea. But alas they are expensive and substitutes can be found for a lot less money and perhaps even better performance. Regrettably I smoked something on one of the PNP boards --so now it is back in the troubleshooting mode. Fun nonetheless.

      73's
      Pete N6QW

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  2. Hi Pete,

    I liked the video! It reminded me of one of those shots where they pan the camera up the side of a Saturn V rocket!

    I am really kicking myself, I was at a hamfest a few weeks ago and found some germanium transistors at one of the tables. I set them down as I was looking at some variable caps at the same table. I got talking with the guy and paid for the caps went on my merry way. Three hours later when I got home, I realized that I had forgotten about the germanium transistors! Ughh!

    The upside is that the same transistors will probably be there next year :-)

    BTW... I'm thinking it might be the time to form the GLA (Germanium Liberation Army)!

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