[ARC5] R-23/ARC-5

[CARL HUETHER]

> On March 2, 2019 at 11:13 AM CARL HUETHER <k1uhy at comcast.net> wrote:
> 
> 
> The 6AB7 and 6AC7 were particularly noted for that and had a negative effect on image rejection such as in the SX-28 which initially used 6SK7's as it was a 1939 design. The effect was written up in QST prewar. I use 6SG7's in mine along with a 6SB7Y mixer which makes up for the poor efficiency RF circuit design.
> 
> Carl
> 
> 
> 
> > On March 1, 2019 at 11:06 AM Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:
> > 
> > 
> >      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
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