[R-390] Why 6DC6?

[rjb]

Phil

This question came up on this list several years ago. I do
not have the specifics (perhaps others can help here) but
the gist of it was that the 6BZ6 was considered in the
original design, but the 6DC6 was chosen instead, for
reasons relating to better gain distribution and less
vulnerability to front-end overload. If i recall correctly,
someone had a reference to a Collins internal document
discussing this issue - wish i had more, but others may be
able to help.

FWIW, i have an HQ-180 with bad cross-mod problems below
about 3MHz, thanks to an extended-band rap music station a
couple miles from my QTH - i tried a 6DC6 in place of its
6BZ6 - no difference. In this case the interference is
probably blasting right into the IF strip.

Bob



Philip B Atchley wrote:
> 
> 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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