[R-390] Random tube sub of the day: 6AU6 for 6BA6

[Drew Papanek]

Tim Shoppa wrote:


>One tube sub I tested: 6AU6 for 6BA6. They are both 7BK base pentodes
>but the 6BA6 is remote cutoff (more appropriate for AGC action)
>and the 6AU6 is sharp cutoff.

>Sticking the 6AU6 into various places in the IF strip resulted in
>changes in AGC action and carrier level meter. The changes were
>noticable but not horrendously bad. I did not tweak the GAIN ADJ
>for each sub. I only subbed one tube at a time.

Since there are a multiplicity of gain-controlled stages, losing control in 
just one stage (as by sustituting a sharp cutoff tube for the original 
variable mu tube) would have mostly minor effect.

AGC is applied to the R-390A's mixer stages; some have written on the topic 
of the receiver's oscillator frequencies being pulled by reflected 
capacitance changes at the mixer cathodes induced by  AGC.
If such be the case, less pulling could be had by removing AGC from the 
mixers.  Makes me wonder if less pulling combined with less AGC control 
would be a worthwhile tradeoff.

The Final Engineering Report (Cost Reduction version) mentions that AGC was 
added to the R-390A's mixer stages to compensate for  ACG control lost  by 
having one less RFstage and a couple less IF stages (as part of transforming 
the R-390 into the R-390A).  Is all that AGC control range necessary in our 
applications?

'Tis interesting that a few folks have found R-390A's with the third mixer 
stage modified to use a 6BE6. The 6BE6 pentagrid mixer would offer superior 
isolation from AGC-induced capacitance changes as compared to the original 
6C4 triode mixer.  The 6BE6 would also be vastly inferior to the 6C4 in 
terms of resistance to overload, and would be far, far noisier.  (The 
accounts I have read all mentioned reversing the modification).

>What was more surprising was action in the PTO and BFO sockets: no
>oscillation at all! I had expected the cutoff details to be
>unimportant there, but I suppose it's possible that the 6AU6
>is biased deep into cutoff where the 6BA6 isn't.

Interesting. One SSB conversion I read about recommends swapping the 6BA6 
BFO tube with a 6AU6 to obtain increased BFO signal amplitude.  Maybe your 
6AU6 has low transconductance?

Drew