An Easy To Build (Hollow State) VFO

Discussion in 'Homebrew and Kit Projects' started by N2EY, Jan 3, 2014.

ad: L-HROutlet
ad: l-rl
ad: abrind-2
ad: L-Geochron
ad: l-BCInc
ad: Left-3
ad: L-MFJ
ad: Left-2
  1. SM0AOM

    SM0AOM Ham Member QRZ Page

    The Vackar 1949 paper is attached.

    In 1950, SM5VL(SK) made the Vackar oscillator popular,
    and in his article simplified design equations were derived from those
    in the original paper. They are suited to spreadsheet solving,
    and a worked example for a 3.5 MHz VFO is attached.


    Attached Files:

    G3EDM and N2EY like this.
  2. K6BSU

    K6BSU Ham Member QRZ Page

    A practical Vackar VFO circuit operating in the 4 MHz range is described in my construction article published in QST magazine, September 1978 entitled "Meet the Remarkable but Little-Known Vackar VFO!" This long and pretentious title was invented by the magazine editors, and not me!
    G3EDM and W7UUU like this.
  3. G3EDM

    G3EDM Ham Member QRZ Page

    I love Jackson components. I recently managed to obtain a paper version of their original catalogue, which is not just a catalogue but really a complete set of specification sheets complete with templates and detailed measurements. Both of my receivers use Jackson dials and I have several more hoarded.

    The company still exists, sort-of. There is a UK-based eBay store where you can buy quite a lot of their core product range.

    I did not know about -- seems like a good resource! Edited to add: but pricey.... Although some items are good value including that Jackson Bros. capacitor.

    Plus, they are a good source for point-to-point terminal strips. Until now my favourite was the H.H. Smith range (now called Abbatron) which is still being manufactured. But they recently changed the style of the strips to a strange design with a metal clip at the bottom, running the entire length of the strip. (I assume there was something fiddly about the old manufacturing process and they found a new way.) The has the equivalent "old style" Belling-Lee range NOS, at prices that are not totally outrageous.

    (You can find old-style NOS H.H. Smith/Abbatron on eBay but supply is patchy.)

    73 de Martin, G3EDM
    Last edited: Oct 8, 2021
  4. N2EY

    N2EY Ham Member QRZ Page

    Looking at the W2YM circuit and comparing to the Vackar, the W2YM circuit may actually be a Vackar! In any event, a big part of the stability improvement is the high capacitances across the tube elements, so that tube capacitance changes are effectively "swamped" by the large capacitors in parallel.

    73 de Jim, N2EY
  5. N2EY

    N2EY Ham Member QRZ Page

    True - but compactness can come at a big "cost". The "cost" is lack of flexibility if parts need to be replaced or a modification is desired, and only a specific part can be used, or there's no room for a modification.

    Whether such a coil is more stable than Miniductor-on-perspex is a good question.

    I would say that price does not always equal quality in variable capacitors. The capacitor you pictured is an excellent example of a high-quality variable for a VFO, yet it's only 8 pounds from the source mentioned!

    It appears to me to be silver-plated brass (even better than plain brass), dual ball bearings, double rotor wipers, ceramic insulation, rigid frame - very high quality. Also has a shaft out each end, which makes it practical to gang 2 or more together.

    Some other notes:

    - If the capacitance range of a variable is more than needed for a particular application, the reduction in range can be accomplished by fixed capacitors in series or series parallel. This method can be used to linearize the frequency range covered, too.

    - Temperature compensation can be used to reduce thermal drift - BUT - the correct approach is to build as stable a VFO as possible before any temperature compensation is applied.

    - Some American tube types (12AU7, for example) were made in an "A" version (12AU7A) which has a controlled-warmup heater. It turns out that such tubes are often more stable oscillators than the plain version. However, check the tube data sheets, because the "A" suffix simply means "improved version" and may or may not be better. Similarly, the "W" suffix means "ruggedized".

    73 de Jim, N2EY
  6. SM0AOM

    SM0AOM Ham Member QRZ Page

    Certainly - my own preferred way of building power supplies is the 19" drawer system with lots of space around the components, but if you for one reason or another is cramped for space other ways need to be devised.

    In a "bout" of construction activity about 10 years ago, a few "utility" power supplies were built, using whatever 80-100 VA transformers the "junk-box" yielded.

    Their common characteristic are the 188x130 mm "footprints" from Eddystone die-cast boxes, and that they also contain a small audio amplifier with the ECL82. All have octal sockets for voltage output, and "floating" 6.3 V heater windings.

    The intention behind was to provide power sources for home-built or surplus gear that lacks internal supplies without tying up one or more series-regulated supplies.

    By using bridge-rectifier and "economy" designs, output voltages
    of 700, 350, 150 V and zener-regulated -100 V bias , as well as 300,210
    and 105 V outputs are available.

    One problem today is the cost of quality iron-cored components.
    The British-made Parmeko oil-insulated C-core mains transformers that once were cheap and plentiful as surplus, now cost about £ 150 on Ebay, and the prices have gone through the roof due to audiophools...

    The Jackson/Polar variable capacitor pictured certainly is a "bargain".
    For £ 8 as surplus, a capacitor with very high quality may be obtained.
    I have no idea about current production prices for precision capacitors, but I suspect that such an unit in one-off quantities is priced "North of" £ 100...

    N2EY likes this.
  7. G3EDM

    G3EDM Ham Member QRZ Page

    I bought one. Perhaps I should have bought several. Slightly odd that it has a plastic base.

    IIRC it is 56 pF so some gymnastics needed if keeping the tuning range to 150 kHz or less. (I am thinking that on 40m it should at least include the 7120 kHz watering hole for SKCC and so forth.)

    Somewhere, I have the Radiotron manual which is an excellent source for figuring out bandwidth (and linearity) of tuned circuits.

    Of course if using frequency multiplication (rather than heterodyning) then the frequency range doubles for each even harmonic....

    73 de Martin, G3EDM
  8. W9BRD

    W9BRD Ham Member QRZ Page

    Because crystals are fun, like frequencies trapped in amber.
    AC0OB likes this.
  9. W9BRD

    W9BRD Ham Member QRZ Page

    What's ironic about this is that a cathode follower doesn't provide as much isolation as a triode is capable of providing, and loses a bit of voltage in the process. Better to drive the second-triode cathode (via the smallest feasible coupling C, with normal class-A cathode bias R for the second triode), ground its grid, and then take the VFO output from the plate of that second triode. I recommend a resistive load for the buffer triode -- RC coupling to the pentode-amp grid.

    The result will be better buffering by means of the second triode's interposed grounded grid, and some voltage gain.
    Last edited: Oct 9, 2021
  10. W9BRD

    W9BRD Ham Member QRZ Page

    Consider also using a low-value secondary tuning capacitor for fine adjustment. We ask a lot from 6:1 planetary drives in expecting that they can be reliably set to within a few tens of hertz. Get in the neighborhood with "bandset," and then zero in with "bandspread." :)

Share This Page