[R-390] 5814A/ 6C4 tube placement

[Tim Shoppa]

Scott Bauer <odyslim at comcast.net> wrote:
> I wonder if there is any particular circuit in the 390 or 390-A
> that would benefit from using a triple mica / dual support rod
> version of the 5814A? 
> I have read that 6C4's can be very micro-phonic and a triple
> mica type would be very helpful in the R390 and 390-A.
> Also, has anybody noticed any difference using 6189's rather
> than 5814a's or 12AU7's?

The one place where tube quality can make the radio be lousy in
these tubes is if there is excessive grid leakage current, which
can mess up AGC action in really mystifying ways. Several 6C4's
and 5814A's in the 390A fall into this category. While I have
destroyed many leaky tubes with a vengeance I cannot say that
I have seen a pattern among 6C4's/5814A-alikes. It is possible
that supports/mica/etc. details can contribute to gassy tubes
in some circumstances I suppose.

My preference is to use sturdier examples rather than flimsier
examples and I suppose that more supports inside the tube make
them on the sturdier side. I have shared my experience
with E European 12AU7-alikes being much much less than sturdy in
the past here.

Nor have I noticed microphonics being bothersome. Usually it's
the 390A causing microphonics by being dropped, rather than it
being a victim :-).

Tim.