[R-390] 6BJ6/6BH6 sub

[n4buq at knology.net]

Okay, now it's time to educate me on this issue.  If the 6BH6 is a remote
cutoff tube, then it won't go into cutoff as "early" as the sharp cutoff
6BJ6, right?  If that's the case, then why will you see distortion on strong
signals with the remote cutoff tube?

Barry - N4BUQ

On Tue, 28 Feb 2006 13:45:20 -0500, "Tim  Shoppa" <tshoppa at wmata.com> 
wrote :

> > 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.
> 
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