[ARC5] R-23/ARC-5

[Richard Knoppow]

     The 6SG7 is not exactly an octal base 6BA6. If you look at 
the specs you will find the Gm of the 6SG7 will be cut off at a 
much smaller bias, around 20V where the 6BA6 takes about 40. The 
difference is in the design of the AVC circuits. The 6SG7 (or 
12SG7) certainly has better performance at high frequencies than 
tubes like the 6SK7. You can probably improve the sensitivity by 
reducing the amount of bias voltage at the tube.
    Gain is also dependent on the Q of the RF coils and 
dielectric losses elsewhere. I can't explain why the antenna 
trimmer was no longer effective, maybe enough difference in input 
capacitance or grid loading. Some high Gm tubes also have high 
grid loading so may make the RF less selective, something that is 
not desirable. This is true of some of the high Gm tubes designed 
for TV IF amps.

On 3/1/2019 7:37 AM, Kenneth G. Gordon wrote:
> On 1 Mar 2019 at 0:59, Robert Downs via ARC5 wrote:
> 
>> The 12SG7 (equivalent to a 12BA6 except for base and envelope) might have
>> helped the performance of the 10-meter conversion a little bit, but only if
>> some other component values were also changed.
> 
> I did try the 12SG7 in both the RF and 1st IF stages of my 10 meter conversion and the
> result was that the background noise level was reduced, but the overall sensitivity was not
> improved.
> 
> I suppose this means that the S+N/N ratio was improved, but the antenna trimmer was no
> longer effective either.
> 
> As you say, some circuit changes would have been needed in order to make full use of the
> improved signal to noise ratio.
> 
> I didn't think the improvement was worth the effort.
> 
> And using one in the 1st IF made no improvement whatever.
> 
> I WAS somewhat surprised at just how effective the 12SK7 was at 10 meters, though.
> 
> Ken W7EKB
> 
> 
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-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL