[Hammarlund] new tubes and SP-600 noise floor

Fri, 7 May 2004 08:56:39 -0500

Hi Craig and Group,

        I need to look at the schematic and see how the "Send" switch
functions in the 600 but my thoughts are that the new tube may have a good
bit more gain than the old one...amplifying the noise from the RF amp
section better than the old one.  Probably a good thing but you will need to
work to chase the noise out of the prior stages.  Unless of course the send
switch disables the front end in which case the RF gain control setting
shouldn't matter.

I do know that there is a suggested replacement for the first IF tube that
is a lower noise higher gain tube...but I can't remember what it
is...something like a 6DC6 or such....I'll have to look it up if one of the
other SP-600 drivers don't chime in and get us up to date...

My Thoughts...

Cecil...
----- Original Message ----- 
From: "Craig Roberts" <[email protected]>
To: <[email protected]>
Sent: Thursday, May 06, 2004 2:02 PM
Subject: [Hammarlund] new tubes and SP-600 noise floor

Calling all SP-600 jocks --

Just for fun, I'm incrementally re-tubing my SP-600 with NOS JAN tubes.
Unpleasant experiences and long nights have taught me to approach this sort
of thing one-tube-at-a-time (if at all).

While working on the IF string of 6BA6s, I noted that the internal noise
floor of the receiver was raised considerably by the installation of a 5749W
(a 6BA6 military equivalent) at the IF driver position V11. Re-insertion of
the original Sylvania 6BA6/EF94 JAN tube brought the noise level back down.
In fact, the Sylvania tube was a bit quieter in this socket than the old RCA
6BA6s used originally at other points in the receiver.

Has anyone else noted a particular noise sensitivity at V11?  For that
matter, how much noise do you experience with the mode switch in the "Send"
position and the RF Gain fully clockwise, and, like mine, does your internal
noise increase at the 8 KC selectivity position?

My receiver was especially "crackly" when I got it, but the scrubbing out of
every tube socket pin in the set with DeOxit and interdental brushes toned
it down considerably.

Thanks and 73,
Craig
W3CRR

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