Fw: SB-301 Frontend tubes...sensitivity

[talen]

That's right, Ed. But just like on the SB-104(A) testing, I tweaked and
peaked everything for the frequency under test. In this case the Ant
and RF slugs and the Preselector, of course. The 6GM6 and 6EW6
(especially the 6EW6) tuned up differently and showed more overall
gain (making me think initially it was a hotter frontend) so I reduced
the AF gain to maintain a 30mV Noise level. Turns out the gain was
both signal and noise and the (Signal+Noise)/(Noise) ratio didn't
change substantially from that of the 6BZ6 as indicated.

The thing that surprised me was how closely together they measure
on my TV-7 when set up for a 6BZ6. I don't get that........

I don't know what the effect of a "new" 6EW6 or 6GM6 (with really
high gm) would be because I don't have any.

Also, all these measurements have been at 14.250Mhz only. I intend
to take more data on 15m and 10m because 20/15/10 tend to be
where sensitivity causes the most problem.

73s  Kees K5BCQ

----------
From: Ed Richards <k6uuz at JUNO.COM>
To: HEATH at LISTSERV.TEMPE.GOV
Subject: Re: SB-301 Frontend tubes...sensitivity
Date: Wednesday, August 09, 2000 5:25 AM

Don't forget that when you substitute a tube with different
inter-electrode capacity, the tuned circuit is no longer resonant. You
need to peak the input and output circuits to re-establish resonance. You
do not say if you did this.
Regards;
Ed Richards

On Wed, 9 Aug 2000 00:52:32 -0500 talen <talen at INETPORT.COM> writes:
> little bit of a continuation of the previous SB-104A story..........
>
> Started out with the original 6BZ6 and tried a 6GM6 and 6EW6
> with much higher gm and lower capacitance. Should see an
> instant improvement of S/N radio....right ? The only problem is
> that the tubes are not new (may be your story too).
>
> Interesting data. My SB-301 is a little picky on levels so it's more
> difficult to set the Noise level because it tends to vary around
> 30mV
> ....all the way from 25mV to sometimes bouncing up to 40mV but I
> tried to get it as stable as possible. There is more variation than
> on
> the SB-104....maybe due to some marginal component...a leaky
> cap further down the line or a noisy switch on the SB-301, etc ?
>
> The results were about the same, varying from around 15.4db for the
> 6BZ6, to 14.8db for the 6GM6, to 15.5db for the 6EW6. I'd say all
> are
> +/- 0.5db considering the above.
>
> The key point is tube transconductance emissions is indicated by
> the tester and is directly related to tube gm. On the TV-7 the tubes
> test as follows:
>
> 6BZ6 "58" with "32" as the minimum acceptable
>      ........or 81% better than min
> 6GM6 "46" with "38" as the minimum acceptable
>      .......or 21% better than min
> 6EW6 "76" with "52" as the minimum acceptable
>      .......or 46% better than min
>
> You can see that this 6BZ6 is a pretty "hot" tube. Since we are
> using
> the same circuit, not optimized for the other tubes, test them all
> using
> the the 6BZ6 TV-7 setup to get "relative data" seems appropriate:
>
> 6BZ6   "58"
> 6GM6  "56"
> 6EW6  "56"
>
> To draw any conclusions, I need to test a "hot" 6GM6/6EW6 which I
> don't have but the answer may be to optimize the circuit for a
> particular
> tube or get the "hottest" one for a given application.
>
> 73s  Kees K5BCQ
>
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