[R-390] 3TF7 replacement...newbie on first 390a

[Todd, KA1KAQ]

On Fri, Apr 8, 2016 at 10:21 AM, djed1--- via R-390 <r-390 at mailman.qth.net>
wrote:

>
>  I've been using a resistor in my R-390A for 30 years with no discernible
> effect on stability (replacement tubes were hard to find back then).  I've
> got a couple of spare 3TF7s, but have found no need to use them.
>

I've used the resistor approach with no issues over the years, too. 47 ohm,
2 (or 5?) watt IIRC. In fact, one A model that I sold to a list member had
a very nice, heavy duty version installed with its own heat sink, which was
mounted on the side of the chassis. Much cheaper and more reliable than the
ballast tube. Easy to install - stuff a resistor into the socket between
the correct pins (2 and 7 I think?), completely and quickly reversible if
the Originality Police show up for a surprise inspection.

Interesting tidbit: most know that the R-390A is the cost-reduced/updated
version of the original R-390. Less known is that the ballast tube was one
of the original features that was supposed to be dropped from the overall
design as it was deemed unnecessary at that point. Somehow it remained.

Had my first 3TF7 burn out last year. Including the one I stepped on a few
years back, that's only 2 in 3+ decades.

de Todd/'Boomer'  KA1KAQ/4