[ARC5] BC453 on 24V - screen voltage

[Dennis Monticelli]

Actually, floating the ground lug leaves the screens hard connected to B+
via the top resistor in the divider.  That's OK.

I will add here that a string of 9V batteries works well for reduced B+
operation and has the added advantage of less supply coupled noise.  If for
example a string of 7 nine volt alkaline batteries are used, the net B+
current is only 5mA (with the bottom lug of the screen divider lifted) ,
which translates into 100 hrs of operations.  I have run my BC-453 this way
and even taken it into the field (gel cells for the heaters).

As for the minimum practical B+ voltage, I have found that the first
circuit to collapse is the LO.  Sometimes running down to 28V kills that
oscillator, sometimes it doesn't.  Obviously the gain will be way down.  I
have found that around 60V gives decent operation.  More of course would be
better for gain reasons but S/N doesn't improve much further, which is what
we mostly care about.

Dennis AE6C

On Tue, Dec 10, 2019 at 9:30 AM Kenneth G. Gordon <kgordon2006 at frontier.com>
wrote:

> On 10 Dec 2019 at 15:44, J Mcvey wrote:
>
> >
> > Apples and oranges, folks!  I'm doing this testing with 25 and 50 volt B+
> > which DOES impact performance. This radio did perform as Gordon
> > demonstrated running at normal B+ with the dynamotor supply.
> >
> > Lifting the ground lug on the bottom screen voltage divider resistor
> helped quite a
> > bit at 25V , but still not back to "normal".
>
> In my experiments, I came to the conclusion that the overall best lower
> voltage for our
> receivers was around 170 VDC, with the screen voltage at the same point.
>
> Reviewing the specs for the 6/12SK7s, we find that the max, and "design
> center" values for
> plate and screen voltage is 300 VDC, and for a plate voltage of 250 or
> lower, the screen
> voltage should be around 100 V. From the specs, it appears that allowing
> the screen
> voltage to equal the plate voltage is just fine. The lower plate voltage
> keeps dissipation
> lower. The tubes are rated for 4 watts plate dissipationm which IMHO is
> high for a receiving
> tube..
>
> I also prefer to connect the screen voltage feed to the hot end of the
> divider resistors, since
> it seems to me that allowing it to "float" is not a good idea. I like
> voltages to be as steady as
> possible.
>
> Ken W7EKB
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