On Thu, 10 Jun 1999 12:18:25 +0100 Peter Chadwick
<Peter_Chadwick@mitel.com> writes:
>
>In the early 1970's there was a National Semiconductors app note on a
>modulated switching regulator, supposedly for use in powewring a high
>efficiency solid state HF linear. The idea was to vary the supply
voltage in
>sympathy with the envelope of the modulation, so maintaining efficiency.
>
>Does anyone know of any work done on modulating the plate supply of a
tube
>amp in sympathy with the envelope of the signal? When you consider that
a
>pair of 6146's were rated for 100 watts carrier, 100% modulated, which
is
>400 watts PEP in AM service, then some sort of plate voltage variation
with
>modulation would appear to give the posssibility of 400 watts PEP of
SSB.
>
>You might even get 1kW linear out of a pair of 4CX250Bs that way!
>
>And no, this is NOT a G2DAF.
>
>Could we see modulation transformers coming back into fashion?
>
>73
>
>Peter G3RZP
>
Hi Peter,
If you were thinking of National AN-8, that example was probably
somewhere around the 25 watt level. I suppose that was an early "Class
H" amplifier... though that term is more properly applied to the audio
world.
If efficiency (and low distortion) is the paradigm, then an unusual
design I have been kicking around may be of interest.
I was thinking around the first of the year that a class D or E
transistor amp with a switching power supply to provide envelope
restoration might be feasible with some of the modern switching FETS.
Some not pretty considerations started to show up when I calculated some
of the required circuit components. And trying to figure out how to
drive it from a typical transceiver was likewise not looking promising.
Then, I had the thought that if I employed a tube (something that would
look cute like a 4-400) and added some of the envelope following
circuitry I might have a concept. Again, the varying drive level
presented a problem.
Last month, I think I hit on an interesting solution. Allow the
class of operation to vary with the drive level!
The initial mode would be Class A operating at say 1000 V and 200 ma of
bias.
As the drive is increased the amplifier will be dynamically rebiased
into Class B (or H as noted before) at typically 2000 V and 300 ma plate
current.
And finally at the highest drive levels the unit will operate in Class
C with 4000 V on the plate. Here full envelope restoration will
necessarily be employed.
Note that the selected operating points all conform to a 4000 ohm load
line and simultaneously fit well with the available drive power. I would
anticipate a smooth variance of the operating point rather than a
stepwise transition as the whole scheme only needs to track the audio
frequency components of the SSB signal.
One final touch I added to the concept is to put a 3rd harmonic trap
directly in the plate circuit so as to allow the tube to actually
approach Class D operation at the highest drive levels.
FYI, the tank components are 500pf vv tune cap, a 32 uh PI roller coil,
a 4000pF vv load cap, an 8 uH L roller coil plus the 3rd harmonic trap
consisting of a 100pf vv and a 10uH roller coil. I am now trying to
figure out how to motor drive all of this sillyness. :-)
The power "supply" will consist of a 2KW PFC input stage operating from
220VAC boosting to 375 V. This will be PWM'ed at 100 KHz lowpassed at 10
KHZ and transformed by an H bridge with a 12-1 step-up transformer
operating at 100 KHz which also provides the AC line isolation.
At this point one must inquire: "Is this trip really necessary?" I
suppose the answer is no... but, the scenery is cute to look at.
73,
Marv WC6W
*
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