[ARC5] Receiver Voltages.

[AKLDGUY .]

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
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>