[ARC5] S+N/N ratio results..more.

[Brian Clarke]

Hello Ken,

One thing I learned very early on in electronics design of low noise 
amplifiers, is that the first stage is the most important - this applies for 
AF and RF. So, as you are presently planning, I would concentrate on the RF 
amplifier alone - don't bother playing with the IF amplifiers. After that, 
you may find you can get less noise if you can find something quieter than 
the 12K8 - say a dual triode with separate cathodes, like the 12SN7 or one 
of the later Philips / Mullard ECC series - use one triode as the oscillator 
and the other as the mixer / first IF amplifier.

I applaud what you are doing. I only wish I was 'between contracts' so I 
could replicate your 'experiments'.

73 de Brian, VK2GCE.

On Sunday, June 16, 2013 3:43 PM, you said:


> On 16 Jun 2013 at 13:55, Brian Clarke wrote:
>
>> Hello Ken,
>>
>> What adjustments did you make for the different G1 operating points?
>
> Absolutely none.
>
>> Gain is
>> related to the operating point - it is not an absolute independent of
>> operating point.
>
> Yes.
>
> The first thing I am trying to learn is exactly what advantage there is, 
> if any, to simply
> substituting a tube with a higher transconductance into the radio without 
> doing anything to the
> circuit.
>
> So far, it doesn't look there there is much, if any.
>
>> As I read the circuit, the Align Input capacitor compensates for
>> inter-electrode capacitance to achieve resonance with the RF input coil.
>
> Correct.
>
> Which is why I cannot peak the 12SH7 the way the receiver RF amp grid coil 
> is aligned at
> present.
>
> Even with the 12SK7, the Align Input peaks with the capacitor almost all 
> the way open. It
> should be 1/2 open at the high end of the band, and isn't.
>
> I'll have to adjust the slug in the RF amp grid coil before proceeding.
>
> The next step (after the above) is to make G-1 bias adjustable and set it 
> according to the
> manuals to see what, if any, improvement there is when substituting tubes.
>
> Another problem has cropped up: if I substitute the 12SK7 in the first IF 
> with either of the
> other two tubes, the stage will break into strong oscillation at certain 
> settings of the IF amp
> grid trimmer.
>
> I'm going to "operate" on the RF amp first and leave all the rest of the 
> circuit strictly alone for
> the time being.
>
> Ken W7EKB