[ARC5] 10 meter BC-454 - screen voltage

[john rose]

Case in point that has had me scratching my head for years. I have a R-392/URR, the compact, even svelte, lightweight (55 pounds or so) version of the R-390/URR (not A). It is designed to run on 24 volt B+ and is loaded with tubes designed to run at this level (26A6, 26C6, 26D6 etc). Every characteristic chart I have ever seen, and I've done a lot of looking, graphs out to 200 v or more. Will the tube really run at that B+? Are the 26 series really 6/12 v tubes with a different filament?

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> From: brianclarke01 at optusnet.com.au
> To: kgordon2006 at frontier.com; Arc5 at mailman.qth.net
> Date: Sat, 9 Mar 2013 10:56:49 +1100
> Subject: Re: [ARC5] 10 meter BC-454 - screen voltage
>
> 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
>
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