[ARC5] Quick sensistivity comparison

[Geoff]

> On 13 Jun 2013 at 15:05, Geoff wrote:
>
>> No surprise at all when you compare a low performance remote cutoff
>> pentode with a sharp cutoff tube with 2X+ the transconductance.
>>
>> If you want an apples to apples comparison then use a 12SG7 making
>> socket changes as/if needed for them.
>
> The 12SH7 has only a little greater transconductance than the 12SG7. Both
> are about double the 12SK7's 2300 micromhos: 4700 at max for the 12SG7
> and 4900 at max for the 12SH7. However, the 12SG7 is a remote cutoff type
> while the 12SH7 is a sharp cutoff type.
>
>> I suspect there are other issues involved with that radio if you can
>> hear a difference on an antenna that isnt a low noise type.
>
> Ha! Low noise? You have got to be kidding: this antenna is so noisy it 
> isn't
> even funny! It is a low (8 feet at the far end) end-fed "long" wire, about 
> 50
> feet long, running east and west, connected to the receiver through a 
> series
> of clip-leads that are draped over my computer and monitor. "Noisy" is too
> kind a word to describe it.
>
> But, what is more important to me, what "other issues" might you suggest
> would be the cause of this observed reduction in noise?

** Dunno, what have you done in the rebuild process, I havent been following 
along.
The usual culprits for starters and volyage checks.


>
> And, BTW, disconnecting the HP-8640B connection to the receiver antenna
> post, yet leaving everything else as it is, results in absolutely nothing 
> heard in
> the receiver until I have cranked the 8640B up about as far as it will go.
>
> Then, with the signal generator cable within an inch or so of the receiver
> antenna connector, I can JUST hear a signal.

** That is not the leakage I mean. Hook the cable to the radio, ground both 
the genny and radio together well and then test the genny. Without an 
external step attenuator available turn the attenuator to the lowest range 
and crank the level down to zero. It should be a linear decrease and end 
with completely no signal.  If it goes down to a certain point and stays 
there then there is leakage....somewhere and could even be common mode.


>
> Furthermore, when connected to the receiver, I can crank the 8640B's
> attenuator down one more "notch" and can hear nothing in the receiver
> either.

** OK forget most of what I said (-; I was thinking of another thread with a 
rather "low" sensitivity report.

I also have all the BNC caps on where they should be. All the original
> covers are on the signal genny and are all screwed down properly, and it 
> is
> grounded.
>
> I really doubt if I am getting much leakage, but what do I know? I could
> definitely be wrong. I have been wrong before and will be again.
>
> Cables (Pomona's) are all new, and the 50 ohm terminator is in a box. It 
> is
> one of those which were ordinarily included with the URM-25 with
> male-BNCs on both ends.
>
> Ken W7EKB
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