[ARC5] Receiver Voltages.

[Dennis Monticelli]

I hear you, Neil, but it still seems excessive to me.  The screen current
is very small and the bouncing of the supply voltage creates far more
variation (via the divider) than the screen currents themselves.  Besides,
a simple neon bulb stack could provide better regulation at a fraction of
the current.   At 250V, that resistor chain pulls 18mA and dissipates 4.5W.
  I respect the original designers, but this is still somewhat of a mystery
to me.  I'm sure they had a good reason.

Dennis AE6C

On Thu, Apr 30, 2015 at 5:19 AM, AKLDGUY . <neilb0627 at gmail.com> wrote:

> The "power hungry" nature of the screen divider resistors is probably
> by design and intended to secure better regulation of the screen
> voltage to all stages.
>
> I understand that the ARC-5 receivers had AVC, while the ARA and
> SCR-274-N receivers had manually controlled gain. The screen currents
> of the controlled tubes therefore vary with gain setting.
>
> Although the variation is only a fraction of the set's total screen
> current (since most of the tubes aren't gain-variable at all), there
> would be a tendency for the screen voltage at the junction of the
> divider to vary with the gain setting. This is undesirable, since it
> affects all stages. To offset that effect, the current through the
> divider seems to have been made significantly greater than the screen
> current itself so that variations in the latter are small by
> comparison. To put it more simply, the "bottom" 7K resistor is about
> half the resistance presented by the total screen current draw, based
> on your figures.
>
> The voltage therefore remains "stiffer" than if a divider with higher
> value resistors were used.
>
> 73 de Neil ZL1ANM
>
>
> On 4/30/15, Dennis Monticelli <dennis.monticelli at gmail.com> wrote:
> > I find it interesting that those screen bias resistors are so power
> > hungry.  For example, at 60V the B+ draw is 10mA with >40% of that from
> > those resistors.  The tubes are only pulling about 6mA, and a lot of that
> > low draw is due to the very low screen voltages.
> >
> > Dennis AE6C
> >
> > On Wed, Apr 29, 2015 at 10:44 PM, AKLDGUY . <neilb0627 at gmail.com> wrote:
> >
> >> > Well, as it is, with 250 VDC HV, the screen voltage on all tubes is
> 125
> >> VDC.
> >> > At 100 VDC, the screen voltage would be 50 VDC.
> >>
> >> A little lower in both cases I think, since although the divider is
> >> 7K/7K there is a small screen current drawn through the "top" 7K.
> >>
> >> Otherwise, I totally agree.
> >>
> >> 73 de Neil ZL1ANM
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