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

[Geoff]

At 2mc I doubt if the tube input C difference has anything to do with it 
since it is such a small amount. OTOH the 12SH7 input admittance may be 
loading down the input coil as happens when its predecessor, the 6AC7, is 
swapped in place of the SK7, SG7, etc.

At 2mc a 6AB7 might prove useful.

Carl


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "Brian Clarke" <brianclarke01 at optusnet.com.au>
Cc: "ARC5" <arc5 at mailman.qth.net>
Sent: Sunday, June 16, 2013 1:43 AM
Subject: Re: [ARC5] S+N/N ratio results..more.


> 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
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