[R-390] Why 6DC6?

[Philip B Atchley]

Hello all.

No, this is not to try to "second guess" Art Collins or degrade our
beloved R-390A receivers.  I wouldn't dare do that, especially on this
list 8^)

But I have a question.  I had a very severe IMD problem that generated
nothing but a hash of stations at the top end of the BCB band, where we
have two locals at 1480 & 1580kHz. Replacing the 6DC6 RF amplifier
eliminated this severe IMD.  However, I have noticed that the receiver
does still have what is apparently 'some' IMD as I can hear some "mixing"
of one of the above local stations with a couple stations lower in
frequency.  Not bad like it was before, but it is there.

When I first started designing and building some of my own receivers
(some quite elaborate) I was "taught" from various sources that for AGC
controlled stages you definitely want to use remote cutoff or (possibly)
semi-remote cutoff Pentodes for those stages.  A sharp cutoff pentode
tends to go into a nonlinear region much faster with increasing bias.

Yet here, in the R-390A we see a "Sharp cutoff" tube used in the RF
amplifier stage and it has AGC applied.  It seems to me that something
like a 6BZ6 etc would have been a better choice.  Again, I'm not
criticizing "Arts" design, but inquiring minds want to know.  Perhaps
others on the list might also be curious.  

Yes, I do have some 6BZ6 tubes here and two separate identical Marconi
signal generators but I really don't have all the test fixtures and
additional equipment to run a total analysis comparing both tubes against
each other for IMD, gain etc. Hence I'm leaving the 6DC6 in at this time.
 Especially as to get optimum results one would have to adjust the
cathode bias, screen resistors etc to get a fair comparison of tubes and
I don't feel like cutting into my R-390A (that would be blasphemy).
       
73 de Phil  KO6BB
Loving home provided for wayward Boatanchor Receivers
ko6bb@juno.com
Merced, Central California
37.18N  120.29W  CM97sh

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