[R-390] 6C4-The Final Horse Beating?

[David Wise]

That would be me.  Twice.

A few years ago I perfected the 3DW7A (proudly injecting my initials),
which switches the incoming AC on and off in the manner of a high-priced
reverse-phase-control dimmer to maintain constant RMS current as measured
by a neat little chip from Linear Technologies.

That thing can be packed into a tubester - I did it - but it's not
for the faint of heart.  Looking to shrink the parts list, I worked up
the 3DW7D (D for digital), which regulates constant RMS by passing
or blocking individual AC cycles using an A/D-equipped microcontroller
to calculate the RMS.  I puttered with this, on and off, for another
few years and just recently felt I had brought it to its peak.
A few people asked for schematics, which I supplied off-list.  I assembled
the prototype on a bit of vectorboard and did not have to resort to
1/16W resistors.  Works fine and has an LED on top that simulates
the variable glow of a real 3TF7.  It's not orange though, not efficient
enough.  Pity.

I don't have a Zenith T-O schematic in my head at the moment.  If the
low side of the 50A1 includes a filter cap, it might be possible to
do a series switcher in a similar fashion.  It would dissipate a bit
more heat than my thing because of the higher voltage it has
to drop for its internal power needs.  I don't think a linear-reg
tubester could be done.  The guy who told you a resistor wouldn't
work was a perfectionist.  (Yeah I know, pot, meet kettle.  But
Zenith used a resistor in more T-O's than the entire R-390A production.)

73,
Dave Wise (SWL in Portland OR)

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Les Locklear
> 
> I believe that somebody on this list has prototyped a solid 
> state (run for 
> the hills, he is a witch) plug in. I can't recall what his 
> name was, old age 
> ya know......................
> 
> ----- Original Message ----- 
> From: "Barry Hauser" <barry at hausernet.com>
>
> > Well there ... you've gone and done it.  The fabled 
> BallastHorse never 
> > dies, only goes into hibernation until it's time for the 
> next roundup.

[More wacky BH humor snipped]
 Now, ya' see I have an actual ballast tube question, as follows:

> > They make a solid state replacement for the 50A1 ballast 
> used in the 600 
> > series of Zenith Transoceanics and clones thereof.  AES 
> sells them for 
> > about $18, versus the $40 or so for an NOS 50A1, and nearly 
> that much for 
> > a used one -- so they're in the same price and availabilty 
> category as the 
> > 3TF7's. These plug-in replacements (also 9 pin) look like 
> some heavy 
> > potted white plastic or maybe even ceramic.  Until a couple 
> of days ago, I 
> > had assumed that they just contained a wirewound resistor.  
> Someone who 
> > rebuilds and restores Transoceanics (for over 25 years now) 
> assured me 
> > there was more to them -- couple of transistors and some 
> other parts.  He 
> > said that just a resistor wouldn't do well in the circuit, 
> but I don't 
> > know.
> >
> > If that's so, then can similar plugin be made for the 3TF7 socket?