[ARC5] 10 meter BC-454 - screen voltage

[Dennis Monticelli]

Ken,

I think raising the screen voltage is a good idea because it will raise
gain and that may improve the S/N ratio.  Lowering the plate will reduce
dissipation and perhaps noise as well.  Your swing will be more limited but
only the 12A6 is going to see a big swing anyway.  I'd say, go for it.

Dennis AE6C

On Thu, Mar 7, 2013 at 1:14 PM, Kenneth G. Gordon
<kgordon2006 at frontier.com>wrote:

> Looking at the tube tables, I see that the screen voltage to the 12SK7s is
> 150 VDC when the
> plate voltage is 250 VDC. When the plate voltage is 100 VDC, the screen
> voltage should also
> be 100 VDC, according to those tables.
>
> Both ratings are for when the tube is operated in Class A.
>
> This receiver is still using the original screen-voltage voltage-divider
> (the two black wire-
> wound resistors standing up in the back). Screen voltage is 75 VDC to all
> the tubes in the
> receiver, except the 12A6. Plate voltage, with the power supply I am
> presently using, is 275
> VDC.
>
> I wish to reduce the plate voltage to no more than 180 VDC. I plan to use
> a VR-75 and a VR-
> 105 in series for this.
>
> What is the consensus concerning screen voltage to the RF and IF amps?
>
> My plan is to raise it to at least 100 VDC via a resistive voltage divider
> across the 180 VDC
> regulated input.
>
> In your opinion, what might be the result, both pro and con?
>
> In my opinion, this will result in greater gain (due to the higher
> relative SG voltage) and lower
> noise (due to the reduced plate voltage) at the RF and IF amps.
>
> Ken W7EKB
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