[R-390] 6BJ6/6BH6 sub

[Tim Shoppa]

> I surmise the radio will not operate with good response to strong
signals if
> 6BH6's are sub'd for all the i.f. 6BJ6's,  since a 6BJ6 is remote
cutoff and
> the 6BH6 is sharp cutoff.  But will one replacement as I have done
really
> matter much?

In both the R-390 and 390A, there are so many stages of conversion and
IF gain that in general the properties of any one stage don't dominate
the
whole gain factor.

Compare with much thriftier designs where there's a RF amp, a mixer,
then
two or maybe three stages of IF gain.

Depending on what stage you subbed your 6BH6 into you might see
some distortion especially as AGC action kicks in and it gets driven to
cutoff.

>   I'll do one more search for a 6BJ6 today and can probably get
> some at a hamfest in a couple of weeks.  I seem to recall depleting my
> supply when I rejuvenated the 390 about a year ago and I seem to have
a lot
> of 6BH6's.  Dan. 

A previous posting by Dave Wise talked about subs for 6DC6, which
has the same pinout as the 6BJ6. By the tabulation it looks like
6BH6 for 6BJ6 is a pretty good match except for cutoff characteristic.

The tabulation was:


7CM pinout (same as 6DC6):

6AS6 (way over max ratings)
6BH6 (low gm, too sharp)
6BJ6 (low gm, too remote)
6BZ6 (8000, 19V=3D50umho.  Runner-up to 6DC6 in Cost Reduction Report.)
6CB6 (8000, 6.5V=3D20uA)
6CF6 same as 6CB6
6DE6 (8000, 9V=3D20uA)
6DK6 (9800, 6.5V=3D20uA.  Used in large numbers in Tek 500-series =
scopes.)
8136 same as 6DK6
6EW6 (14000, 3.5V=3D20uA)
6GM6 (13000, 15V=3D60umho)
6JH6 (8000, 19V=3D50umho)
6JK6 (15000)

7EN pinout (same except internal shield connected to cathode instead of
=
G3):

6DT6
6GX6
6GY6
6HZ6

These tubes are designed so G3 can be used as a second control grid.  =
All
have lower gm than the 6DC6.  I didn't write down their cutoffs.  I =
think they're
all sharp.