"Antonio I0JX" wrote in message
. ..
"Richard Knoppow" ha scritto nel messaggio
m...
"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
The handbook for the SP-400 indicates +380V on both
the power supply schematic and on the tube chart. There are
two handbooks on BAMA and a military handbook for the three
versions of the SP-200, also for the SP-110. Its been a long
time since I had my BC-779 out of storage but seem to
remember the voltage seem high to me. The RCA tube handbook
does indeed give 375V as the maximum plate voltage for the
6F6.
I think my copy of the military handbook was on an
archive drive that crashed, at any rate I can't find it to
check the earlier values. The tubes _do_ run hot.
--
--
Richard Knoppow
Los Angeles
WB6KBL