[ARC5] 10 meter BC-454 - screen voltage

[Brian Clarke]

Hello Ken,

The tube characteristics written in tube manuals (whose?) are for 
establishing other characteristics, like life, gain and emission. They are 
not set in stone.

If you want a quiet receiver, I would leave the Voltage regulator tubes 
out - they rely on ionisation of the enclosed gas = very noisy. I know 'coz 
I tried it once - with a BC-454.

If you want to experiment with tube element Voltages, set up an octal socket 
on a breadboard, and measure noise and distortion vs Voltage. Trying to do 
that inside a built receiver makes it very difficult to discover what's 
causing what.

73 de Brian, VK2GCE.

On Friday, March 08, 2013 8:14 AM, you said:


> 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