[ARC5] 10 meter BC-454 - more

[Geoff]

> On 23 Feb 2013 at 23:46, Dennis Monticelli wrote:
>
>> I guess I am missing something here.  If the amplitude of the
>> background noise is higher and the S/N is the same (can still hear the
>> weak signal with about the same readability , then the weak signal was
>> amplified too and by about the same amount.  Everything is just a
>> little louder so the gain is higher.  Same effect as just turning up
>> the gain control.
>
> I guess I am not making myself clear: to my ears, I am still hearing the 
> weak
> signal with "about" the same amplitude, but I have to "dig" a bit to hear 
> it
> through the noise contributed (obviously) by the tube.
>
> The transconductance (for one thing) of the 12SG7 is twice that of the
> 12SK7 under the same operating conditions. 4000 vs 2000.
>
>> If the background noise goes up and covers up part of the weak signal
>> (readability goes down), then additional noise was contributed that
>> reduced the S/N.
>
> Yes. That is what occurred.
>
>>  If that was accompanied by an overall increase in
>> amplitude of the desired signal then the gain went up too,
>
> It "appears" that the overall amplitude of the desired signal remains the
> same.
>
>> The 12SK7 is supposed to have less gain than the 12SG7, so something
>> is not adding up.  Maybe the 12SK7 is running more current in the
>> given bias arrangement.
>
> I don't think so, but can check that. One thing that does somewhat concern
> me is that the screen voltage, from that fixed voltage divider in the rear 
> of the
> receiver, is 75 volts. In the specifications that voltage is much higher 
> and is
> close to the same voltage as the plate voltage.



** No matter what tube is used Id suggest following the tube manual voltage 
and cathode resistor specs as the original use was not weak signal DXing.

Carl




>> I forgot which role this tube was playing.  Was it the RF stage or one
>> of the IF stages?
>
> RF amplifier. There really isn't much improvement by using a 12SG7 at an
> IF.
>
> From my observation here, the 12SK7 is contributing significant extra 
> noise,
> while the gain is remaining the same.
>
> Ken W7EKB
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