SB-301 Frontend tubes...sensitivity

[Ed Richards]

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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