Hello DK1IS,
Sunday, November 15, 2015
Awesome reply Tom, some great information for me to dwell on there.
Insulation breakdown is indeed a worry I have. A GS35B needs 3kV on
the plate to really get going, so that's a fair amount of voltage.
Perhaps i would be better looking at air cored coils ansd accepting
it's going to be BIG! ;)
The stuff on your QRZ page is a real credit to you and an inspiration
to many of us builders! You have certainly thrown body and soul into
building that LF setup, quite stunning building standards. Thank you
once again!
> Hello Chris and group,
> I failed to store all the postings of this thread - so sorry if some
> aspects of the following comments should have been already discussed or
> could be boaring to all of you succesful users of semiconductor-minded
> D- and E-designs.
> Chris, your problem seems to be typical when trying to use real TX tubes
> on LF/MF: the Vp/Ip ratio is rather high resulting in high impedances
> and bulky air cored coils for the tuned circuits. If you want to
> overcome this by using ferrite cores you run into new trouble: the high
> voltages and currents in resonance mode due to the necessary Q require
> large core cross-sections and can cause the danger of insulation
> breakdown between the windings or between windings and core.
> Single-ended circuits will cause detrimental DC premagnetization. The
> fact that the relative permeability of ferrite depends strongly on its
> temperature will force you into retuning the selective circuit again and
> again as it gets warm by the dissipation.
> About three years ago after some bad experiences with MOSFET-PAs I was
> sick and tired of those nervous semiconductor parts and started to
> construct a reliable and good-natured tube PA from scratch. Wanting a
> linear PA and keeping in mind the above-mentioned stumbling blocks I
> chose an alternative approach: broadband push pull class B without grid
> current and with transformer coupling like an audio amplifier using 4
> paralled (8 in total) TV line tubes (PL 519) in each branch. So I got a
> low plate-to-plate resistance (about 1 kOhm) at a plate voltage of 600 V
> DC and RF voltages about 1 kV between the two groups of tubes. Using a
> stack of 4 recycled colour TV line output cores with a total
> cross-section area of 610 mm² I got by on 2 times 28 turns primary
> between the plates and a secondary winding of 14 turns with switchable
> taps and contra windings for adjusting the load. To improve coupling and
> reduce stray inductivity the windings were twisted and interleaved
> similar to good quality audio transformers. For the windings I used RF
> Litzwire TEX-ELZ 7x 0,3 mm from Furukawa with a dielectric breakthrough
> voltage of 10 kV. At the end of the day I came out with a linear PA
> giving a continous output of 800 W RF maximum on LF and MF at 50 Ohms
> load with a plate efficiency of 73% on LF and 69% on MF. Without any
> further filtering the antenna (and dummy load) current is purely sinus
> shaped. A little disadvantage of the concept is the need for individual
> bias to be set for each tube. Via the voltage drop at an 1-Ohm-shunt in
> every cathode line I adjusted the individul cathode current to 250 mA DC
> at full load - the setting kept stable since the beginning. Because the
> ferrite core is used as transformer only with ample allowance there is
> no problem due to variations of permeability when the temperature rises:
> the transformation ratio keeps constant. Class B allows to form rise and
> fall of the keyed CW signal according to its speed to avoid key clicks
> in adjacent channels and can be used for future linear modes. Pictures
> can be seen on my QRZ site www.qrz.com/db/dk1is.
> If there are data for audio or RF push pull class B for your tubes or
> you can deduce them from the caracteristic curves you will certainly
> find higher plate-to-plate resistances as with my TV line tubes. But it
> should be possible to make a suitable ferrite output transformer anyway
> as long as you don´t use it as resonant tank circuit. Take care not to
> run into self resonance due to the higher primary inductivity. Perhaps
> you have to omit twisting and interleaving of the windings to keep the
> self capacity low. At higher impedances the stray inductivity wouldn´t
> be so important. The concept works with push pull only, single ended
> circuits will cause problems due to the DC component magnetizing the
> ferrite core.
> 73 es gl,
> Tom, DK1IS
--
Best regards,
Chris 2E0ILY mailto:[email protected]
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