Good afternoon gentlemen,
I would like to re-open the discussion on the effective cooling of the
pins on valves like the 3-500 and 4-400 series. Looking back through
my old Eimac handbook, and ARRL handbooks, the required volume of air,
applied to the 3-500 valve, for example, is quoted as 13cfm, with a
back pressure of 0.082 inches. This seems quite precise, and from a
number of 250 hour continuous operation, 4-400 "pulls" I received in
the past, from BC transmitters, which probably had the specified
bases,chimneys and the correct, or greater flow of air through the
base and around the envelope of the valve, none showed any signs of
the overheating filament pin problem.
The charred pin/wire problem with a pair of 3-500 valves, I raised a
few days ago, was the result of my first experience looking into a
commercial amplifier, which uses 3-500 valves, and is built for the
amateur market. It would seem that the manufacturer relied on one
single fan, fitted in the back panel of the cabinet, to cool
everything inside. I was also suprised to see that inspite of all the
details, with regard to the operating conditions for power valves,
published over the years, by Eimac and the ARRL, present day
manufacturers of amplifiers for the amateur market appear to rely on a
mixture of convection and pressurised cabinet cooling for this type of
power valve, rather than the direct forced air cooling, specified by
the valve manufacturers.
The consequence of this, is that in the amplifier example I saw, the
original pair of 3-500 valves had been discarded, diagnosed as "low
emission" although never run at more than 500 watts, when in fact, the
problem was most likely to have been overheating and the melting, then
carbonising of the valve filament pin/wire joint, causing a high
resistance connection, and thus reducing the actual voltage across the
valve filament, inside the envelope. The first set of valves probably
still had many hours, maybe years, of useful life left in them, and
could have been rescued by cleaning and resoldering the pin/wire
joint, but ended their days prematurely in a crusher. Fortunately for
the amplifier owner, whom I was visiting, I was asked to look into his
box, prior to the present set of valves ending up the same way!!
I suppose my question to the group, is, that if you discover, the
expensive valve replacement way ! that your amplifier, does not have
the specified air flow, sockets and chimneys, to suit the valves
fitted by the amplifier manufacturer, has any group member,
effectively resolved the problem, without resorting to a "rebuild"
and, are there any "retro fit" components that can be used, or
adapted, to correct the cooling situation. My present thoughts are
around a separate blower box and a flexible hose to the amplifier
cabinet, but the "sticking point" is how do you effectively distribute
the air equally around the tube pins, without the special sockets
being fitted.
Although, having bought a commercial product,it is not unreasonable to
assume that all the appropriate assembled component parameters have
been considered and taken into account in the design process, then,
the final product, on sale to the public, in English law, should be
of "merchantable quality, and fit for the purpose intended". If this
amplifier were mine, I think I might well seek legal advice. I assume
that in the EU and USA there is a similar course for redress.
As for my own amplifier, it is home brew, to a design from the ARRL
handbook of many years ago, uses a pair of Eimac 3-500s, in Eimac
sockets / chimneys, and the air flow is 20cfm, and the valve filament
pins show no signs of heat stress. The valves are at least 30 years
old, for I bought them, with their fittings, new. They run at about
400 watts, and at that power, they should last for ever, but, I do
have a set of Eimac spares, just "in case".
Oh, for the word "valve", read "tube", no excuse other than I am
English, and somewhat over the age hill!! your advice and comments
would be much appreciated. sincerely John. G3JVC.
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