[R-390] Re: IERC tube shields and tube temps...a field expedient...

Tue, 22 Apr 2003 21:01:33 -0400

Hi,

Enormous chunk of wasted bandwidth follows ..... 1200 baud modem users
proceed at your own risk :)

This gets a little complex. On the thermometer question it depends on what
you want to measure. An IR thermometer will measure the temperature of the
inside of the tube IF the glass used is transparent at IR AND you know the
emisitivity of the thing you are pointing at. Since the thermometer responds
to light you have to know how much gets through the glass and "what color"
the thing you are looking at is. You can check both things out, you just
have to break a couple of tubes to do it.  If you want to measure the pin to
glass seal temperature then you need a nice little thermocouple.

Now for the complex part. Tubes wear out for a bunch of reasons:

1) Filaments go open circuit
2) Gas
3) Filaments loose emission
4) A grid goes open or short.

In receiving tubes (rather than rectifiers or power amp tubes) filament
emission is not usually a big deal. The tube will work pretty well even with
a well worn filament.

Gas can come from the stuff inside the tube, but in modern (post WWII) tubes
usually if it's gassy it's because it leaked.

Grid open and shorts usually are from shock and vibration.

Filament open's are a function of how often the tube is turned on, and
exactly how power is applied. A fast power on *may* actually be better for
some tubes than a slow power on.

Still with us .....

High temperature affects outgassing from stuff in the tube, and leaks in the
glass to metal seal. Unless it is combined with shock and vibration it
should not affect grid problems. This assumes the grid is not glowing orange
....

Filament opens and cathode emission don't seem to correlate well with a
modest change in the tube temperature. The filament is running in a fashion
that it self regulates it's temperature. Once it is at temperature it will
run there for a good long time. Again provided that there is little or no
shock and vibration.

All the yack above about filaments assumes that you run them to the same
voltage each time *AND* that it is roughly what it should be. The filament
has a bit of a memory affect so varying the filament voltage all over the
place does some weird things.

Finally the tube shields (at last ...)

Tube shields will affect the temperature of the upper glass bulb and of the
plate. Indirectly the grids may be cooled a little. The temperature at the
glass to metal seals may not change much at all with black versus shiny
shields. A full contact shield might reduce the temperature a bit, but it
could also raise it a little. Air flow would have an affect on the outcome
here.

But what about the data you ask ....

Yup the data is out there and it's correct. The tube shields do have an
affect on the life of the tubes in an R-390. Good Navy data and no reason to
doubt it. A couple of questions:

1) Does you radio see as much shock and vibration as the shipboard radios
the Navy ran the tests on?

2) Did they toss the tubes when they failed on a tube tester or failed in
the radio?

3) Do the tubes you use fail at the rate that the Navy tubes did?

4) Did the radios have tube or solid state rectifiers?

The last question is the most interesting. Even in 24/7 type service I don't
see anything close to the Navy failure numbers. I also do not run tube
rectifiers so maybe all their failures were rectifier tubes.

What's more to the point, back in the 1980's your government and mine went
out and bought tubes for "essential equipment" to keep it running for
something like another 20 years. The good old R-390(both A and not A) *must*
have been on the list. The mountains of tubes we see on eBay are a result of
them surplussing these tubes. You can find all the tubes except two - the
ballast tube and the rectifiers. I can understand the ballast tube, it's a
different category and probably didn't get covered in the order to buy the
vacuum tubes. The rectifier may have been left off since the solid state
conversion was an approved modification. If that's the case then they didn't
go through anything close to the number of tubes they thought they would. I
suspect that most of us on the list have more than ten sets of spares. Heck
I'm probably not that odd in having a couple hundred of some of them. Hall
of fame land may be in the multiple hundred sets range. We're all out
grabbing these tubes and they are still dirt cheap. There have to be an
ocean of them out there. We don't run as many radios as the Navy did, but
then they didn't plan on a 500 year supply of tubes either.

Bottom line - fancy tube shields are neat and they do look slick. For a
truly presentation grade radio they are the only way to go. If you put a
couple hundred bucks into the front panel then they are cheap by comparison.
For a use it every day and I don't care what it looks like radio, save your
money ....

    Take Care

        Bob Camp
        KB8TQ

----- Original Message -----
From: "Darryl Jones" <[email protected]>
To: "Kenneth G. Gordon" <[email protected]>; "R-390 List"
<[email protected]>
Sent: Monday, April 21, 2003 7:18 PM
Subject: Re: [R-390] Re: IERC tube shields and tube temps...a field
expedient...

> Well, I'm gonna give it a try and find out. Whats the worst that
> can happen? I lose a couple of tubes, of which with one or two
> exceptions, I have bucketloads :)
>
> Does anyone have a suggestion of how I can measure tube temps
> accurately?
>
> Darryl
> ----- Original Message -----
> From: "Kenneth G. Gordon" <[email protected]>
> To: <[email protected]>
> Sent: Monday, April 21, 2003 1:28 PM
> Subject: [R-390] Re: IERC tube shields and tube temps...a field
expedient...
>
>
> > I have had pretty good luck with simply spray painting the
> > inside and outside of a shiny tube shield with flat black paint
> > after masking off the lower part which would make electrical
> > contact with the grounding ring on the socket. My idea was that
> > the shiny interior of the tube shield was reflecting some of the
> > heat back into the tube, and by painting it black, some of that
> > reflected heat was absorbed instead by the shield and
> > transferred to the outside.
> >
> > Temperatures were noticeably lower for tubes with which I
> > tried this, although I didn't actually measure them.
> >
> > I have not yet tried to cut ventilation slots in them.
> >
> > I suppose I should try some sort of heatsink material around the
> > outside where it might fit.
> >
> > Ken W7EKB
> > _______________________________________________
> > R-390 mailing list
> > [email protected]
> > http://mailman.qth.net/mailman/listinfo/r-390
>
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