[Milsurplus] Technical assessment

Tue, 29 Jul 2003 23:32:01 -0700

Hello Pete & the Group,
    In my BC-669-C, many of the tubes (Even the Original Military 'Metal'
tubes)
didn't meet spec on the tube tester, yet performed for many years before
slowly
going out. Unlike a light bulb, they USUALLY don't go 'Flash-Bang' (
Although
they CAN do that), Yet in my BC-348-E, tubes when below spec immediately
show a poorer performance. I *THINK* (means Don't quote me, Mate) that it
depends on how the radio circuits are designed as to how quickly the loss of
performance
will show up dramatically.
    For example: a circuit that uses the 'average' of multiple tubes (Let's
say 4 for
argument's sake) and you have a high (100) and a low (25) and the
other two are middle (50) your mean average would be the 50 +/-. If two were
at 25,
you would wind up with 50 again, even though more tubes are weaker. If the
radio
is a single tube for a specific purpose, 25 becomes quickly more
noticable....
    Transmitter tubes seem to show it much quicker as well, because of the
much wider
swings in Plate voltage/amperage... Receive and Amplification tubes seem to
be more
'stabilized' since they usually are in their 'normal' state at all times the
set is on. But that
is the opinion of a 'kid' who grew up building/fixing tube radio sets and
televisions and
has had the BC-669-C since I was 15... Got it free of all things, as the
local Radar
Station was closing and the New one 50 miles up the coast had all new Racal
gear,
and didn't want the 'Old' stuff around..... So the Air Force NCOIC for their
logistics
asked me if I wanted it (I was friends with his Grandson), otherwise he
would demil
it with a 20 lb. sledge...... So I said "sure", having grown up around 2
Hams and a Navy
Radio/Radarman and Naval Intel family members....

73 OM de John AAA0OR OR/AAA0ID ID/ W6HNC/7
US Army MARS SMD Oregon/Idaho, USA

----- Original Message -----
From: "pete williams" <[email protected]>
To: "milsurplus" <[email protected]>
Sent: 29 July, 2003 18:25
Subject: [Milsurplus] Technical assessment


>
>
>  G'day,
> I've been waiting for a break in the ... er ... weather, to ask what
> might be a fundamental question on tube (valve) behaviour.
> I had occasion to check a number of 12 SG7 on my TV7-B tester and
> found that a large number had readings of between 10 and 30 when the
> listed value was 40 as a minimum.
>  Thinking that the test set might be defective, I then tested  a
> couple known to have readings of 100--- they came from a U S supplier
> and were marked as such. My readings  of these 2 agreed with  theirs.
>  Strangely, the tubes I would have discarded performed OK in the radio
> with no discernable difference between them and the  high reading  "u
> beaut" tube. Tubes were part of a  IF strip.
> Question ... why is it so ? I am aware that the readings are a view of
> the transconductance and I have  some - not enough ?- knowledge of
> what it means.
> Having gone thru' tubes over the years and  trashed those that didn't
> measure up, am I derelict in duty of care and common sense not to have
> put them to the service they were intended for ?
> May be the cumulative effect of low reading tubes would have shown up
> in ultimate performance of the radio as there are  3 in the IF and a
> couple in the RF stages.
> Comment pse.?
>
> ---
> Pete Williams
> [email protected]
> METUNG AUSTRALIA 3904
>
>
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