SB-301 Frontend tubes...sensitivity

[talen]

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

Listserver Subscription:listserv at listserv.tempe.gov - "subscribe heath 'name' 'call'"
Listserver Submissions: heath at listserv.tempe.gov
Listserver Unsubscribe: listserv at listserv.tempe.gov - -"signoff heath"