[R-390] Ballast Tubes in Shipment

[Bob Camp]

Hi

Battery powered gear is *very* different than line powered stuff. I 
used to run "portable" FM stuff that ran on batteries. It was amazing 
to me that the filament D cells would die as fast as they did. The 
voltage starts out at 1.5 volts but does not  stay there for long. It 
gets down to about a volt fairly quickly. Without *something* to 
stabilize the oscillator tubes you would need the Energizer bunny and 
an 18 wheeler to keep your portable radio up and running.

Since you always were on the verge of nuking the filament batteries 
most of the little 1.25 volt tubes didn't have as much margin built 
into the emissions side of their design. It would be very interesting 
to see if we can come up with some real transductance versus filament 
voltage data on some of these tubes. I actually had do do a lab on it 
back in High School physics (tubes had just been invented ....) . For 
some odd reason I seem to have misplaced that log book ....

Since we're dragging everybody back into this .... Chuck Rippel had 
some stuff on the performance of an real ballast tube up on hid site 
for a while. If it's still there I can't find it with a quick look. The 
net result of what he found was that the ballast tube was not all that 
great a regulator in the first place. It was far better than nothing at 
all but it still wasn't great.

As best anybody can figure the ballast tubes are built by selecting a 
pretty darn small diameter wire and balancing it against a weird back 
fill gas mix at partial vacuum levels. Given how much fun it is to get 
a batch of number 51 wire done up I suspect that on occasion they might 
have used number 50 or 48 instead. When they did that the extra wire 
had to go somewhere. Net result was the odd wire running all over the 
place construction.

One very simple explanation for the parts getting fragile would be 
simple evaporation of the metal from hot spots. It's not a very elegant 
explanation but it has to happen to some extent. Flexing as the wire 
heats up and cools down can't help much either. There may be some 
change in the wire with time but if there is it's not very obvious. A 
final possibility would be air getting into the poor little thing. With 
no gettering the oxygen would head straight for the iron wire. Looking 
at about 90% of the sutf sitting out in the yard here suggests that 
Iron reacts with  oxygen in a predictable way.

Bottom line - they used ballast tubes in the portables because they had 
to. The shock and vibration issues were secondary ....

	Take Care!

		Bob Camp
		KB8TQ



On Jul 4, 2004, at 5:30 AM, Barry Hauser wrote:

> Hi James
>
> Yes ... there have been reports of that happening from time to time 
> and when
> I've had '390's shipped to me, I've asked that the 3TF7 be removed,
> bubblewrapped and stuffed in somewhere.  Way, way back, someone posted 
> that
> their ballast tubes would seem to last forever, but a failure, when it
> occurred, tended to be after the receiver was moved around -- onto the 
> bench
> or whatever.  Others have sort of dismissed all that.
>
> What's curious is that the last model series of the Zenith Tranoceanic 
> tube
> portables (600 series) and the military version of the 500 series 
> (R-520)
> all have 50A1 ballast tubes.  The construction of the tube is the same 
> -- a
> 9-pin envelope with a long iron filament strung around almost 
> "nonchalantly"
> over the mica disks, like a poorly decorated Christmas tree.  These are
> portable, luggage style radios with no shockmounting whatsoever.  
> There were
> numerous clones and also mil portable gear with ballasts -- and some 
> tube
> testers.   So -- I dunno.  If they were prone to vibration damage, 
> would the
> mfr's go with them in portable gear?  Not sure, but I doubt ballast 
> tubes
> were ever cheap relatively.  Nowadays, it's a horse race as to which 
> would
> cost you more -- a 50A1 vs 3TF7.  Of course, those T/O's also have the 
> very
> pricey 1L6.
>
> As for vibration/shock hot vs. cold -- the T/O's are battery 
> portables, so
> might very well be in motion while running -- and included the famous
> removable "wavemagnet" with suction cups so you could attach it to the 
> train
> or car window while traveling.
>
> I suspect failure-proneness may have to do with aging through use --
> filament becomes more brittle with more heating and cooling cycles?
>
> Barry
>
>
> ----- Original Message -----
> From: "JamesMiller" <jmiller1706 at cfl.rr.com>
> To: <r-390 at mailman.qth.net>
> Sent: Saturday, July 03, 2004 9:48 PM
> Subject: [R-390] Ballast Tubes in Shipment
>
>
>> One or two 390a's I have bought in the past arrived with bad ballast
> tubes.
>> But they were presumably working before shipment.  The only reason I 
>> can
>> think of is excessive vibration and breakage of the flimsy filament 
>> during
>> shipment.    Solution?  Before shipping a 390, remove the ballast 
>> tube and
>> wrap in soft foam and in a separate box inside the shipping carton.
>>
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