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

"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

"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

"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

Roger wrote:
The 380 volts is only used for the audio output tubes. If you
don't need the full 10 watts of audio, I'd just use the 270
volt supply.

Agreed, although you might lose a little linearity. Those 6F6s will last a
WHOLE lot longer though.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Sun, 10 Jun 2012 18:59:41 +0200, Antonio I0JX wrote:



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.


The tube manuals that I have declare that their ratings are "average
expected values" or something like that -- basically, they are numbers to
be followed when your set is plugged into the wall and the line voltage
is 117VAC (or 110, depending on the year), but they are not ratings which
will lead to immediate and catastrophic failure if they are exceeded.

Tube failure conditions to not have as sudden an onset as semiconductors
-- so industry practice was often to sneer at the tube ratings as you
passed them by, and accept a lower tube life in return for a less
expensive set.

Lowering the supply voltage to those tubes will reduce the maximum audio
power a bit and let the tubes run cooler, which will make them last
longer. Since you can no longer schlep down to the local five and dime
to get tubes cheap, that may not be a bad idea.

--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
Why am I not happy that they have found common ground?

Tim Wescott, Communications, Control, Circuits & Software
http://www.wescottdesign.com