"Richard Knoppow" ha scritto nel messaggio
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"Antonio I0JX" wrote in message
. ..
I am in the process of building a power supply for my Superpro SP-400-X,
and I am bit surprised of the voltages I read on the original power supply
schematic diagram.

The voltage feeding the audio amplifier (two 6F6s in push-pull) is 380V,
which looks quite high to me.

Does someone know whether the voltages shown on the diagram are
open-circuit or under load?

Thanks and 73

Tony I0JX
Rome, Italy

I have a BC-779 and maybe a slightly different supply.
The voltages under load should be the ones in the tube
voltage chart in the receiver handbook. The supply will be
higher without load. Be careful of the voltmeter used, the
voltages in the handbook were probably taken with a 1000 ohm
per volt meter and will read high when using a modern 20k
ohm/volt meter or an electronic voltmeter (except for grid
bias which were measured with a VTVM). 380V sounds about
right to me.
These are very good receivers. The only modification I
suggest is adding a voltage regulator tube for the local
oscillator plate. A VR-150 mounted under the chassis will
work. change the 12K plate resistor on the oscillator to
about 8K and connect the VR tube at the tube plate. This
will eliminate the pulling of the LO when the RF gain is
changed.
I don't know if Hammarlund improved the temperature
compensation on the SP-400 but the earlier ones need several
hours to stabilize. Once they stop drifting they are quite
stable.

--
Richard Knoppow
Los Angeles
WB6KBL



The tube manual reports that the absolute maximum plate voltage for 6F6s is
375V. So, designing a receiver with 380V on the plates does not seem to me a
good engineering practice. I guess that the 6F6s will become very hot, but
evidently they shall survive.

Thanks for the tips on stability.

73

Tony I0JX
Rome, Italy