[Hallicrafters]V23#15- Ballast Tube(s)

[Phil Barnes-Roberts WA6DZS]

hallicrafters-request at mailman.qth.net wrote:
{snip}
> Subject: [Hallicrafters] SX-73 Ballast Tube
> From: "Barry H" <barry_hauser at juno.com>
> Date: Fri, 9 Dec 2005 21:46:35 GMT
> To: hallicrafters at mailman.qth.net
> 
> Hi Gang
> 
> Anybody know what the number is for the ballast is in the SX-73?  I think I asked this before, but don't recall getting an answer.  It is only referred to as "R80" on the schematic and there's a Halli part number, which is of no use.  The number is rubbed off on this particular one I have.
> 
> Also -- is there a known tube sub.  While searching, I came across some posts about using a 6V6 in place of the 4H4 ballast in the SX-88 and that it was factory recommended and works well.
> 
> Barry
> 
> ------------------------------------------------------------------------
> 
> Subject: Re: [Hallicrafters] SX-73 Ballast Tube
> From: W4AWM at aol.com
> Date: Fri, 9 Dec 2005 17:09:07 EST
> To: barry_hauser at juno.com
> CC: hallicrafters at mailman.qth.net
> 
> 
> Amperite 6 4. That could read 6-4 but nearly unreadable.  The 6-4 is in the 
> catalogue, the 6 4 is not listed so I think that 6-4 is correct.
> 
> 73 and Holiday Greetings,
> 
> 
> John,  W4AWM

'--O=o=O--'
I went looking for lore on the wily Ballast Tube as well, looking for 
one for our SX-88 (#150.)  When we got the radio, there was a weak old 
6SK7 in that socket.  A weak old 6V6 didn't improve it, really.  There's 
a long series about ballasts in the Boatanchors archives, from around 
9/97 to 9/04; see starting at < 
http://www.ibiblio.org/pub/academic/agriculture/agronomy/ham/GLOWBUGS/ATL/Vol01_100_to_149/v01_n112.glo.html 
 >.  National recommended the 6V6, since it didn't seem to make all that 
much difference in drift, as the 4H4 and 4H4C got harder to find.

The Amperite ballasts are available; Amperite is still in business, and 
there's a party who sells them directly or on ebaY, _but_ 4H4's are 
Buy-It-Now for $39.95 IIRC.  < http://www.onlinecomponents.com > will 
send you a new 4H4C for only $170.00 (!)  Amperite also has a Website < 
http://www.amperite.com >, with some good info.  See < 
http://www.amperite.com/Uploads/Ballasts.pdf >(includes availability.)

*Ballast 101*
A ballast tube is a resistor with a positive temperature coefficient; a 
tapered iron wire, suspended in a non-oxidizing (but heat-conductive) 
atmosphere (H2 or He2), such that it glows dimly and acts as a current 
regulator over a fairly wide voltage range at its rated current, 60mA to 
some at up to 3A.  In our favorite toys that glow in the dark, it's 
(usually) used to keep one or more oscillator tubes at a constant 
filament current, thus constant temperature, to reduce drift.  The 
tolerance of this technique is - moderate, shall we say - but pretty 
good for the time.

Some ballasts were also used in battery farm radios to keep operation 
fairly constant over the power-life curve of the batteries.  See < 
http://www.peacockcollectibles.com/radios2/radios3/5e1.htm >.

I found a very long PDF (172pp's) archive thread on the R-390A reflector 
about it, including discussion of a (*gasp*) solid-state replacement for 
their (now unobtanium by that #, but a Mil-Spec replacement _is_ 
available) 3TF7 ballast tubes, using a common LM317T and a couple of 
caps and a resistor, either in an aluminum box or with an angle heatsink 
- to keep it cool.  It's wired as a current regulator, depending on the 
resistor to set the current value, and the little decoupling caps to 
keep it from breaking into song.

That's all placed in the middle of a four-diode rectifying bridge 
(1N4001's are fine) to work with AC or DC heaters, and mounted in a tube 
base or plug to suit (thus an eminently swappable, non-permanent, 
improved replacement.)  It could even be put in a pretty aluminum 
sleeve/tube to look right at home.  Some even just wire it to the 
original socket underneath, and skip the tube; I'd maybe paste a 
reminder label over the socket, to keep crud - and other tubes - out. 
At least one vendor will sell you such a module, to wire into the 
under-chassis power wiring.

*An SS Advantage*
Many common 6.3V tube heaters draw 300mA, some 450mA.  When cold, their 
resistance is _much_ lower than when hot, and they draw a significant 
inrush surge current.  Either the ballast, or the SS regulator, holds 
the current constant, allowing for a gentler startup for the regulated 
tube(s), and probably longer life, thus fewer realignments when 
replacing critical oscillators.  But the ballasts have a limited life of 
a few thousand hours, shortened with frequent power on-off cycles.

If you want a more stable radio, and aren't that concerned about it 
being 100% authentic NOS throughout (past re-stuffing electrolytics, and 
Black Beauty/BumbleBee paper caps with modern innards, etc.- and leaving 
a label under there to remind a new owner it's been done) try the modern 
trick.

The R-390A folk have seen production mods like that over the many years 
they were made, as the state of the art changed.  The Aero-1A 
fire-control radars I worked on in the Phantom F-4B was 'hybrid' - about 
half-tubes.  "Prepare Now for an Exciting Career in Electronics", says 
the matchbook.  It's been right!

-- 
73, Phil Barnes-Roberts WA6DZS mailto:wa6dzs at arrl dot net
All else aside, RF to the other fellow's antenna is what counts.