[ARC5] 10 meter BC-454 - screen voltage

[Geoff]

I doubt if zener noise will be an issue in this application, Ive yet to hear 
any indication in the dozens of HF sets Ive switched to zeners.

The 3 terminal regulators can be very noisy.

Carl


----- Original Message ----- 
From: "Dennis Monticelli" <dennis.monticelli at gmail.com>
To: "Brian Clarke" <brianclarke01 at optusnet.com.au>
Cc: <Arc5 at mailman.qth.net>
Sent: Friday, March 08, 2013 9:53 PM
Subject: Re: [ARC5] 10 meter BC-454 - screen voltage


> Zeners are often even noisier than an ionized gas tube.  But the standard
> fix is easy; an RC feeding a load.
>
> Dennis AE6C
>
> On Fri, Mar 8, 2013 at 3:56 PM, Brian Clarke
> <brianclarke01 at optusnet.com.au>wrote:
>
>> 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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