Folks,
Looking at the circuit for the power supply used with the Drake L7
amplifier, the primary of the mains transformer appears to consist of four
separately accessible circa 120 Volt windings. With the jumpers set for 240
Volt operation (i.e. for UK mains), all four windings seem to end up in
series across the 240 Volt supply when the amp. is set to the 'CW/Tune'
position, resulting in the lower of the two HV settings. With the amp. set
to the 'SSB' position, the 240 Volt line appears to energise only two out of
the four windings, so twice the primary Volts, giving the higher of the two
HV settings.
In contemplating the usual (essential) mods. for an amp. of this type, I'm
wondering if there is any case to be made for re-wiring the mains side of
the HV PSU to energise all four primary windings (two windings in series, in
parallel with the other two windings in series - paying proper attention to
the phasing of the windings, naturally) to give the HV transformer an easier
ride and possibly bolster HV regulation some? To preserve best linearity in
operation, the CW/SSB switch never moves from SSB here and serves only as a
reminder of the US '1 kW DC input' days.
If I followed through with the mod., I might re-deploy the CW/SSB switch to
apply some positive bias to the valve cathodes when on CW, to reduce the
standing dissipation, doing away with the bizarre original routing of the
mains power for the HV PSU into the amp., via the switch, and back out.
It's a pity there isn't just a little more HV available from the stock power
supply. The full HV on the L7 (not much more than 2.6 kV on mine if the
metering is to be believed) seems somewhat miserly to give a pair of 3-500
proper enthusiasm. I reckon another 500 - 1000 Volts might have improved
what is already a nice example of its kind.
Any experiences in this area out there?
Regards.
Mark.
*******************************************
Mark Hill - G4FPH
E-mail: g4fph@mjha.co.uk
Current web pages at: www.g4fph.net
Old web pages at: www.qsl.net/g4fph
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Dipoles resonant on 1940 / 3685 kHz
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Remember - SIDE for HV safety:
S witch off
I solate
D ump
E arth
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