[ARC5] 10 meter BC-454 - screen voltage

[Geoff]

Those are called space charge tubes and have been around in various forms 
since WW2. Also used in millions of auto radios.

Carl


----- Original Message ----- 
From: "john rose" <brokenthumb at live.com>
To: "arc5 mail list" <arc5 at mailman.qth.net>
Sent: Friday, March 08, 2013 10:50 PM
Subject: Re: [ARC5] 10 meter BC-454 - screen voltage


> 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?
>
> ----------------------------------------
>> 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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