[ARC5] Receiver Tube Substitution

[Richard Knoppow]

    While the 6SG7 will plug in place of a 6SK7 and work OK I am not so 
sure its noise will be any lower.  The bias for maximum gain is about 1 
volt while the 6SK7 needs 3 volts so in a receiver designed for the 6SK7 
the bias will reduce the gain and Gm of the 6SG7. I have not tested this 
but think the resulting noise would be the same.
     The 6SG7 BTW is a good tube. Used by RCA in the AR-88. The 6BA6 is 
a miniature equivalent but not identical. The 6SG7 actually has a 
slightly higher Gm but probably higher capacitances.
     For AVC use the 6SK7 needs about -40V bias to be essentially cut 
off while the 6SJ7 will cut off at about -7 volts. This will upset the 
gain structure of the AVC. The SJ has only slightly lower noise than the 
SJ so I agree that probably someone who didn't have a spare SK stuck it 
in because it worked.
     To see what must be done to replace a 6SK7 with a 6SG7 see the 
difference between the Hallicrafters S-20R and the S-40 or S-40A RF stages.

On 12/15/2016 9:26 AM, Kenneth G. Gordon wrote:
> On 15 Dec 2016 at 9:51, Rich Post wrote:
>
>> Plug-compatible but 12SJ7 is sharp cutoff versus 12SK7 being remote cutoff.  If in an AVC
>> circuit, remote cutoff is needed.
>
> Right. It was probably what the original user had in stock. The R-23 and the R-23A do have
> AGC, so using a 12SJ7 as the RF amp would make the receiver more prone to overload by
> strong signals.
>
> The effect probably wouldn't be noticed much in normal operation though.
>
> Another common sub was the 12SG7. This results in lower noise.
>
> At frequencies below about 20 MHz, there is no advantage to using them.
>
> Ken W7EKB
>
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL