Experiments With A 6AR8 AM/SSB Modulator

Discussion in 'Homebrew and Kit Projects' started by W0RIO, Sep 9, 2019.

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  1. W0RIO

    W0RIO Ham Member QRZ Page

    I've been reading about some of the unusual types of vacuum tubes such as the 6BN6 limiter/descriminator
    and the 6AR8 (and 7360 and others) balanced modulator and decided to run a 6AR8 through its paces.
    Here's a circuit that I came up with that works nicely as a DSB or AM modulator depending
    on the bias on the deflector plates.

    6AR8DSB1.png

    You can read about all of the gory details of my experiments here:
    http://www.solorb.com/elect/hamcirc/6AR8modu1/index.html

    Do any of you old-timers have ideas on why there seems to be a built-in imbalance in
    the tubes? First, I made sure the output coils and caps were balanced, then
    I tried two different RCA NOS tubes, tried swapping the deflectors and
    plates, did some resistive balancing of the input transformer windings, and even
    tried demagnetitzing the tubes to no avail. Adding a large capacitive imbalance to
    one side of the output circuit did help move the DSB null point to the center of the
    balance pot range, but it doesn't seem logical that it would be that far off.
     
  2. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    There was a DSB transmitter project in the June 1962 issue of Popular Electronics that used the 6AR8 tube in the balanced modulator. The circuit is considerably different from your circuit. Have you seen that transmitter?

    I actually built the transmitter the summer between my senior year in high school and my freshman year in college. It was my very first suppressed carrier transmitter. Using a WRL 755 VFO instead of a crystal, I could cover the entire 75-meter band.

    If you don't have the information, I can forward a PDF of the entire issue. Contact me via E-Mail at

    k9sth (at) sbcglobal (dot) net

    and I will forward the information to you. The file is 8.96 MB in size.

    Glen, K9STH

     
  3. W0RIO

    W0RIO Ham Member QRZ Page

    Yes, I have seen the article, I believe it was called the "Simple Sidebander" although I did not use that as a starting point.
    I think I'll read it again though, the article is also available here:
    https://americanradiohistory.com/Archive-Poptronics/60s/62/Pop-1962-06.pdf

    It's interesting that in the Simple Sidebander, the deflector biases are positive and are derived from each corresponding
    plate, not from a steady source like B+.

    I built my circuit up using info derived from the 6AR8 data sheet and chapter 1 of the GE Ham News sideband handbook:
    http://n4trb.com/AmateurRadio/GE_HamNews/sideband_handbook.htm
    thanks to N4TRB for putting that online.

    I notice that many applications for the 6AR8 use a positive deflector plate bias even though
    the data sheet indicates that the deflectors are most sensitive when biased at -8V, which is what I used.
    I did notice a slight improvement in the deflector sensitivity when biased around -8V.
    Perhaps that may have something to do with the assymetric behavior I am seeing.

    The prize for getting the most functionality out of a sheet beam tube (6ME8) goes to R. Weaver:
    http://electronbunker.ca/eb/OneTubeSuper.html
     
    AH7I likes this.
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    From various Internet sources, I have been able to acquire a virtually complete library of all Popular Electronics issues. I do have all the parts, in stock, to build another version of this transmitter and am seriously considering doing so. Why? Just to do it! Maybe my justification is because I have been able to re-create all of my primary stations starting when I was a Novice Class and having another example of the transmitter would add to the collection.

    It has been over 55-years since I built the first DSB transmitter. However, I do remember that it worked very well. At least most of the stations that I worked never noticed the extra sideband!

    Glen, K9STH
     
    K6CLS likes this.
  5. KX4OM

    KX4OM Ham Member QRZ Page

    Have you looked at the 7360 design in the March 1960 pages 33-38? It appears to balance the carrier amplitude for the balanced modulator application a bit differently. Might be some other info useful for you as well.

    Ted, KX4OM
     
  6. W0RIO

    W0RIO Ham Member QRZ Page

    I haven't seen that article yet, the GE Ham News SSB book mentions a few articles on early SSB/DSB:
    July 1956 CQ magazine: SSB Exciter
    March 1960 QST: Sheet Beam Tube apps
    May 1961 CQ: Command Set DSB Xmiter conversion.

    These guys have the CQ archives but you have to pay for access:
    https://hamcall.net/cq
    archive.org has some pre-1940 issues of QST but nothing newer.
    ARRL has the QST archives, but you need to be a member, I've been less inclined to re-join since they
    split the technical articles off to QEX.
     
  7. K6BSU

    K6BSU Ham Member QRZ Page

    Years back, I built a DSB transmitter based on the RCA 7360 balanced tube. Worked OK, but carrier suppression wasn't very good. Could have been due to chassis layout and signal leakage.
     
  8. KL7AJ

    KL7AJ Ham Member QRZ Page

    The 7360 was used in a couple of early FM stereo receivers for the stereo demodulator!
     
  9. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    RIO:

    From various Internet sites I have been able to acquire the CQ archives from 1945 (first issue) until 2005. Quasi legal! However, considering all the money that they owe me from when I was the first FM Editor of CQ (January 1971 until August 1973), I guess I could consider the acquisition a small payment towards the amount owed plus interest!

    Glen, K9STH
     
    W0RIO likes this.
  10. W0RIO

    W0RIO Ham Member QRZ Page

    It was also used in some Collins SSB gear and shows up in both a phasing and filter SSB transmitter in the 1964 ARRL handbook.
    Apparently the Collins guys have kept the 7360 price high. The GE Ham News SSB book also mentions a 7763 tube which could be
    run at VHF frequencies. The 7763 had a screen grid, whereas most of the others just had an accelerator.

    The TV-grade color demodulator tubes that I've found include the 6AR8, 6HW8, 6JH8 and 6ME8, those are plentiful and inexpensive.

    It's interesting to see how many types of beam deflection tubes were made, too bad tube development came to a screeching halt right around the
    time this technology was being worked on.

    In my search for articles on beam deflection tubes, I ran into some very interesting beam tubes including the
    Raytheon QK-329 "square law" tube which had a parabolic function cut out in the beam path for use as a very fast multiplier,
    Analog to digital converter tubes with grey-code encoded outputs and the Selectron, which was a vacuum tube memory:
    https://en.wikipedia.org/wiki/Selectron_tube
    They should have called that a T-RAM (Thermionic) ;-)
     
    Last edited: Sep 11, 2019 at 7:46 PM

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