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Power for a grid dip oscillator.

Discussion in 'Electronic Basics' started by Peter Howard, Jul 19, 2013.

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  1. Peter Howard

    Peter Howard Guest

    This is a question about a piece of vacuum tube test equipment commonly used
    by amateur radio
    operators but as all the ham radio newsgroups I've looked at seem rather
    inactive I thought I'd ask it here. I live in a 240Vac mains country and I
    have several USA spec GDO's. They are tube types, a Millen, a Lafayette and
    Heathkit and are of course meant for 110Vac mains which is why I've only
    got around to doing something to make them usable on 240Vac. A step
    down transformer is not the answer. The one I tried put 8.8v on the 6.3V
    tube heater. Maybe because the transformer is marked 230V-110V to suit the
    nominal 230V Australian mains but in practice the local juice is anywhere
    between 240V and 250V.
    Each GDO is very similar with a small power transformer and a single half
    wave rectifier diode with two electrolytic caps and a resistor for
    filtering.. I
    replaced electrolytics where necessary and also a defunct selenium stack
    rectifier (shows how old the Millen is) and brought them all up on a
    borrowed variac to the point where the heaters had 6.3v on
    them. The B+ line on all is around 130Vdc, hardly surprising when two are
    designed around the same 6AF4 triode and the other a very similar 9002 type.

    I have it in mind to power these dippers from a step down power supply
    consisting of two identical back to back transformers in a box. They have
    multiple taps on their secondary windings and in practice I can achieve a
    wide range of AC voltages by selecting the connections between the tapped
    secondaries. When transformers with high voltage secondaries are almost
    unavailable new, this arrangement is often used by experimenters with tube
    equipment of modest current requirements. The heater supply will be from a
    separate 6.3V heater transformer in the same box

    I have two choices. One is to remove the transformers from the dippers,
    leave the rectifier and filter caps in place and bring in the heater supply
    and a higher voltage AC feed for the rectifier through a multiwire
    connecting cable. The connecting cable with plug and socket are suitably
    rated for the voltages and currents involved.
    The other is to do the rectifying and smoothing in the transformer box and
    make it a DC feed direct to the B+ line in each dipper.

    I'm trying to choose between these alternatives and both seem equally good.
    Can anyone think of a reason to prefer one method over the other?

  2. But if you'd posted in the most obvious, you would have got an answer.
    The reality is, a newsgroup thrives on people actually posting, and while
    some groups have completely died, the lack of postings doesn't
    automatically mean that, it just means nobody is posting to provide
    All GDOs were relativley low power, and used nothing special in the way of
    a transformer. You might consider just scrounging up new transformers and
    repalcing the old.

    Another consideration that has come up in the antique radio newsgroup is
    that line voltage used to be lower than it is now (all those "110vac"
    references has now become "120vac". So older equipment might run at a
    higher voltage now precisely because it's getting a higher voltage from
    the wall.

    Replacing the selenium rectifier may require the additon of a series
    resistor, because there is a loss of voltage through the selenium
    rectifier while a silicon diode will put out a higher voltage.
    The cheaper GDOs tended to have the power supply built in. But the better
    ones had the power supply separate. All with a separate power supply had
    the rectifier and filter capacitor in the box with the transformer.

    That said, leaving a solid state diode and filter capacitor (assuming they
    still work) inside the GDOs won't cause an issue if the external supply
    puts out DC. A slight voltage drop from the diode, but the filter
    capacitor will just be an extra filter capacitor.

  3. Phil Allison

    Phil Allison Guest

    "Peter Howard"

    ** The US mains supply is typically 125VAC at 60Hz.

    Back in the 1960s, it was 117VAC.

    Have to go back to the 1940s to see 110VAC.

    ** Of course it is.

    You can get them rated at 100V for Japanese market equipment.

    OR buy a transformer with twin 55V secondaries.

    ..... Phil
  4. default

    default Guest

    Another option is to get a low voltage (step down from 240) and employ
    the low voltage output to "buck" and in series with the isolation
    transformer's primary winding to lower the input from 240 to what
    gives you the right numbers. The buck transformer's low voltage
    winding is just sized to carry => the primary current, so it is not a
    large expensive transformer.
  5. Just find a transformer with the right voltage, and then the secondary can
    subtract from the primary. You end up without isolation, but since the
    GDOs have their own transformers inside, the equipment is isolated. This
    frees up a variac from adjusting the voltage.

    I made a big mistake a few months ago. I spot a stepdown transformer,
    nice and heavy, can't figure out what I'd use it for, so I leave it. But
    the box on the sidewalk had a nice electric sander
    (the newfangled ones with a sort of triangle at the front for the
    sandpaper), so I bring that home.

    Once I get home (it was dark out), I realize the sander is for overseas
    use, it has an odd plug and runs off 240vac. That explains the stepdown
    transformer, the two were used together. It was too late by then to go
    back, the garbage would have been picked up. So in not looking things
    over carefully, I ended up with a nice sander that can't be used, unless
    inside there's a means of jumpering it for 120vac.

  6. Phil Allison

    Phil Allison Guest

    ** Fraid that is wrong.

    The OP needs to get down to 110V from 250V.

    .... Phil
  7. That would work. It's not like the GDO requires a lot of power. And
    unlike some of the times I've seen a capacitor used to drop AC voltage,
    there's an actual transformer between the AC line and the circuit.

  8. Phil Allison

    Phil Allison Guest

    "John Fields"

    ** The load current for those dippers is gonna be nothing like a sine wave.

    Firstly, there is half wave rectifier in the DC supply - so the iron core
    of each AC transformer will be off set creating a highly asymmetrical
    current wave.

    Then there is the issue of 50Hz operation in lieu of 60Hz - driving the
    cores hard into saturation. Normally this produces a spiky current wave
    peaking around each zero crossing.

    The combination of the above currents will look just horrible on a scope.

    Putting a series cap in the primary circuit will not behave predictably.

    The simplest solution is a resistor in series with each heater winding, to
    bring the voltage close to 6.3.

    .... Phil
  9. Peter Howard

    Peter Howard Guest

    I've read a lot of interesting replies with gratitude but your idea of using
    a series capacitor sounds the most promising. When I come to think of it, I
    have seen LEDs powered direct from mains with a capacitor and a half wave
    So, here are some figures I took.
    Millen GDO. 115Vac transformer primary current 39mA
    Heathkit GDO 115Vac transformer primary current 44mA
    Do those sound about right? The figures were taken with the only AC ammeter
    I have, an analogue multimeter inherited from the Australian Post Master
    General's telephone branch. It was allegedly a good instrument in its day.

    In both cases 115Vac from the variac was the voltage that gave me heater
    voltage of around 6.3V. I suppose I have a bit of latitude when ancient tube
    manufacturers data sheets quote a heater supply tolerance of +/- 10 percent.
    So, whats the procedure for getting the capacitor value? And of course, what
    sort of capacitor?

    The other Lafayette GDO is already operating from AC delivered by cable from
    my back to back transformers in a box using the diippers own internal
    rectifier and filtering. Works fine. The transformers inna box don't even
    get warm. Not surprising considering the modest current requirments of a
    little triode.
    The Millen though is built like a tank. While I was able to renew the filter
    caps and rectifier, removing the power transformer will require some
    disassembly to get at it. On the plus side, the transformer windings passed
    the megger test okay so it seems a good candidate for the capacitor trick.

  10. The Millen is the best of the bunch. It cost more, but I think it had
    more to it than just the name. About 1972 there was an article in one of
    the ham magazines about their effort to solid state it, basically using
    the same tuning capacitor and coils (and box) and they put in a lot of
    effort to get the same sort of operation, so I assume they put effort into
    the original.

    The top of the line is surely the Measurement Corporation's Model 59, not
    likely to go cheap today. I had one for years, and it didn't false dip
    (which some of the lesser GDOs are supposed to, though the Model 59 is the
    only one I've had experience with).

    So you'd probably want to leave the Millen intact anyway, If you have a
    sudden need for money, that's something that probably can be sold fairly
    rapidly and at a decent price.

  11. Phil Allison

    Phil Allison Guest

    "Peter Howard"
    ** Not even faintly relevant.

    ** The idea of using a series cap in the AC supply is WRONG !!

    To make sure, I tried it with a small ( ie 8VA) 120V, 60Hz transformer
    today on the bench. With 120V input and using a mix of resistive and half
    wave rectified load with filter electro, it worked as expected with a
    current draw of 86mA.

    With a 2.2 uF, 250V film cap added in circuit, a BIG problem occurred due to
    *series resonance* with the transformer primary inductance. The primary
    voltage INCREASED by 20% and the current draw by 90% !!

    With a 1uF cap instead, it was not too much better.

    With 0.57uF (0.1 & 0.47) it became possible to apply 240VAC to the circuit
    BUT the secondary voltage wave was now *grossly* asymmetrical causing the
    DC voltage to drop by 45%.

    But the worst outcome was with no load (same as with valve removed from your
    dippers ) where the DC voltage on the filter electro rose by 80% and the
    0.57uF cap was showing 375VAC across it !!

    Substituting a 1.5kohm, 10W resistor in lieu of a film cap restored
    conditions close to normal.

    So FORGET the series cap idea COMPLETELY !!

    .... Phil
  12. Phil Allison

    Phil Allison Guest

    "John Fields"
    ** But that is exactly what it does.

    ** But Oz *is* a 50 Hz environment..


    ** But you could NOT predict the cap value !!!!!!

    That * carefully chosen on test * cap is *series resonating* with the
    primary inductance of the particular tranny and has double the AC voltage
    across it.

    This voltage will go dangerously high when the Oscillator switch ( S1) is
    open - using a 275VAC rated class X2 mains cap will be essential. A DC
    rated cap will fail short and kill the tranny.

    The internal DC voltage is gonna be well down too, scope the secondary
    voltage and see why.

    Massive asymmetry

    Bad idea.

    .... Phil
  13. Phil Allison

    Phil Allison Guest

    "Ian Field"
    ** Tube heaters in audio pre-amps are often fed with DC current from a
    constant current source.

    Warm up time is slower, but nothing strange happens.

    .... Phil
  14. Phil Allison

    Phil Allison Guest

    "John Fields"
    "Phil Allison"
    ** FFS the CORE is OFFSET by the DC component in the load current.

    So it saturates, once each cycle, all the time !!

    ** FYI small e-core transformers are commonly operated deep into
    saturation - at their RATED frequency. Dropping the supply frequency by 20%
    makes this MUCH worse, just as increasing the supply voltage by 20% would.

    ** FFS asshole !!!

    Do you READ YOU OWN POSTS ??

    .... Phil
  15. Phil Allison

    Phil Allison Guest

    John Fields, ASD fucked & Senile Septic ****

    ** Transformers do have a *LOT* of trouble passing DC current.

    You STINKING, AUTISTIC PILE of SHIT !!!!!!!!!!

    ** Take any transformer wall wart and shove it up your fucking ass.

    No, better still - try swallowing it instead.

    Watching a **** like you CHOKE would be such fun.

    Massive U-tube hit for sure.

    .... Phil
  16. Phil Allison

    Phil Allison Guest

    " John Fields, ASD fucked & Senile Septic ****"

    ** Transformers do have a *LOT* of trouble passing DC current.

    You STINKING, AUTISTIC PILE of SHIT !!!!!!!!!!

    Wanna know how small e-core trannies behave ?

    Take a transformer wall wart and shove it up your fucking ass.

    No, better still - try swallowing it instead.

    Watching a **** like you CHOKE would be such fun.

    Massive U-tube hit for sure.

    .... Phil
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