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

[Richard Knoppow]

     Look at the published data for these tubes, part of the 
explanation is there.  The 6SG7 is a semi-remote cut off 
tube which will give good AVC performance with relativily 
low AVC voltages, say around -20 volts.  The 6SH7 is a sharp 
cut-off tube which will completely cut off with only 
about -4 volts on the grid.  The Gm, plate resistance, and 
interelectrode capacity of the two is about the same. The 
6SK7 has about half the Gm and slightly lower plate 
resistance.  With 250V on the plate and 150 on the screen 
the 6SG7 wants a minimum grid voltage of somehere around -2 
volts where the 6SK7 wants negative three.  Increasing the 
grid bias on the 6SG7 will lower its Gm.  Gain is 
approximately proportional to Gm and noise is approximately 
inversely proportional to Gm.
     The 6SB7Y was a late pentagrid converter tube with 
relatively low noise for the type intended for FM radios.  I 
seems to work as a straight across plug in for the 6SA7 but 
again the bias is different so it may not show any advantage 
over a 6SA7 without circuit modifications even though the 
6SA7 is a rather noisy tube.
     The articles on converter and mixer circuits in the RCA 
collection at http://www.tubebooks.org  suggests that none 
of the older multi-grid mixers was wonderful.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "John Hutchins" <jphutch60bj at gmail.com>; 
<arc5 at mailman.qth.net>
Sent: Sunday, June 16, 2013 6:44 PM
Subject: Re: [ARC5] S+N/N ratio results..more.


> On 16 Jun 2013 at 8:17, John Hutchins wrote:
>
>> Ken -
>>
>> So what I am reading is that in the end a slight 
>> Ham-i-fication to
>> improve receiver performance, sensitivity and S/N ratio 
>> is to change
>> out the first RF tube to a -(12,6)-SG7Y
>
> Yes.
>
>> or -(12,6)-SH7
>
> No. The reasons being that 1) one can no longer peak the 
> input, and 2) the
> gain appears to be so much higher than either of the other 
> two tubes that the
> BFO voltage level is no longer "in balance", and one has 
> to turn the RF gain
> down in order to get a clean signal on both CW and SSB. 
> These effects are
> minimal with the 12SG7Y.
>
>> to improve the
>> front end sensitivity and S/N ratio.
>
> Yes, but it appears that at 2 MHz, the advantage to doing 
> this is minimal.
>
>> I assume in your tests that the AVC line is still active 
>> in the AM
>> mode to the 1st RF amp?
>
> Yes, as much of an AVC as there is in the early model 
> "ARC-5" receivers.
>
> The ARA 46104 design is very close to the early BC-454 
> design: the AVC
> circuit is very minimal, IMHO. This is why the later 
> AN/ARC-5 models used a
> 12SF7 as the seond IF tube instead of a 12SK7.
>
> The 12SF7 has a diode included in the tube and this was 
> used for a much
> better AVC circuit. That AVC circuit also works with the 
> BFO on whereas the
> earlier one did not. In fact, the AVC circuit in the 
> earlier receivers is so
> minimal that they don't even bother to turn it off when 
> the BFO is activated..
>
> From what I can tell so far, there is almost no difference 
> between the circuits
> of the ARA receivers and the SCR-274N receivers.
>
> Ken W7EKB
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