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RE: Tube TC

Original poster: Sparktron01@xxxxxxxxxxx 

Jim

Tubes can absolutely be made to operate as a push-pull power oscillator.
I have built both types (Armstrong, Hartley), and recently (see below) posted a
URL concerning a 50kW (!!!!) Colpits push-pull unit used for a linear accelerator
HVDC multipler stack.

http://www.veccal.ernet.in/~vecpage/inpac2005/CD/Contributory%20paper/C-027.pdf

NOTES
I. There is a schematic error in the above URL, can anyone catch it??? (and it is
NOT minor... :^D )
II. The above URL may or may not work, try "push pull colpits" as key words in GOOGLE
then look for title:
_CONSTRUCTION & TESTING OF 50kW/120kHz OSCILLATOR FOR 3MeV, 30kW DC ..._

The difference between the three oscillator types are:
Colpitts uses a series capacitor voltage divider network to establish grid drive power.
Hartley uses a tapped inductor (dual of Colpitts) to derive grid drive power.
Armstrong uses a grid feedback (tickler) coil to establish 180 deg phase shift and
derive grid drive power. There is a now with Armstrong (3) degrees of magnetic
freedom in oscillator (1. Plate Tank to GFB, 2. Plate Tank to Sec., 3. GFB to Sec.)

I personally like the Hartley (or Colpits) over an Armstrong because...
1. Grid drive is derived directly from the tank circuit, rather then magnetic
coupling from a tickelr coil. It's just one less "fiddle factor" to deal with.
2. Push-pull was used in early induction heating due to lack of high
power tubes, and there is significant history and references (IEEE, IRE)
concerning use, design, etc.
3. Double the voltage swing and half the current would equate to ~1/4
i^2r heating loss in tank circuit, versus a parallel connected circuit of equal
power. Tank caps have to handle 2X the tank voltage, however.
(TINSTAAFL- There is no such thing as a free lunch).
4. My experience with PP suggests significantly improved stability, and less
likelihood of the oscillator to pull off frequency, even with significant loading.
5. I've not tried a Colpitts oscillator (as above URL), however, with HV vacuum
tuning caps (feedback/drive control) and a MMC array to handle HV and
high RMS currents, may have considerable design advantage over a Hartley
PPO at HV, high power.
6. An excellent power oscillator tube for a VTTC would be a air cooled 3CX2500H3
or water cooled Amperex 6960. Alex Tanjsek and I have culled some
6960's with socket and water jacket hardware from boneyard high power
LEPEL IH units. Fils are intact (12V, 30A), have not built up a constant current
test fixture to determine operational parameters.
7. Staccato mode power control can allow significant pulse power capability out of
tubes without overheating; I believe I would now use high power IGBT's instead
of SCR's for cathode modulation control. Much higher voltage, pulse current
rating in a cheaper package (IMO).

Regards
Dave Sharpe, TCBOR/HEAS
Chesterfield, VA. USA


> Original poster: "Jim Mora" <jmora@xxxxxxxxxxx>
>
> John or Dan or ?, I have 4 833a's with original Johnson Sockets and have
> wondered if way down the road they could be made to Push Pull. I haven't
> studied Tubes since 74. Can you elaborate on the Culpits vrs. The Armstrong
> oscillators. One thing I was reading stated that 2 Triodes in parallel cause
> Capacitance problems but there were work arounds? Any Ideas?
>
> They are not in my theory or family of curves text books.
>
> I net:
>
> "Colpitts oscillators are somewhat similar to the shunt fed Hartley circuit
> except the Colpitts oscillator, instead of having a tapped inductor,
> utilises two series capacitors in its LC circuit. With the Colpitts
> oscillator the connection between these two capacitors is used as the centre
> tap for the circuit.
>
> Regards,
> Jim Mora
> Ojai Ca.
>
>
> -----Original Message-----
> From: Tesla list [mailto:tesla@xxxxxxxxxx]
> Sent: Wednesday, October 12, 2005 8:16 PM
> To: tesla@xxxxxxxxxx
> Subject: Re: Tube TC
>
> Original poster: FutureT@xxxxxxx
>
> In a message dated 10/12/05 8:14:26 PM Eastern Daylight Time,
> tesla@xxxxxxxxxx writes:
>
>
> >All:
> >
> >I have been looking at different VTTC's, and noticed that most all of
> >them are variations of the Armstrong tuned grid oscillator. Nowhere
> >is it explained the merits of using such an oscillator. Why isn't the
> >Colpitts, for example, used? what is the advantage? or drawback? has
> >anyone built a VTTC that is wired differently?
> >
> >It would seem to me that a colpitts oscillator would give you a tuned
> >primary; since the DRSSTC coil works so much better on account of
> >having the primary tank tuned, wouldn't a tuned primary benefit a VTTC
> also?
> >
> >By the way, I just bought a 833a tube on ebay, and plan on building a
> >coil around it.
> >
> >Dan
>
>
> Dan,
>
> The traditional modified armstrong VTTC's also use a tuned primary.
> A number of folks have built Hartley VTTC's but I never saw any actual
> efficiency advantage over the armstrong despite theoretical advantages.
> I believe oscillation instabilities of the armstrong may actually be
> an advantage
> but I'm not sure.
>
> The 833A is a good tube for a VTTC and can give 21" sparks
> or so without staccaco (pulsed) operation. Longer sparks can
> be obtained with staccato operation of course with suitable
> decreases in tank impedance.
>
> John
>
>