On Oct 9, 2005, at 5:06 AM, John E.Cleeve wrote:
> 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.
Who was the mfg of said amplifier?
> 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,
Power level has nothing to do with emissive life. Filament-potential
does and the relationship is (E1 / E2)^23.4.
> 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,...
3-500Z air system sockets and chimneys do not provide adequate cooling
unless a high pressure centrifugal blower is used. Such blowers are
quite noisy and I know of no Ham type amplifier that uses such. In my
experiences, the transverse-air, >150mm slow-speed fan cooling system
used on the Heath SB-220 and Kenwood TL-922 result in lower
glass-envelope and filament-pin temperatures than those temperatures in
similar amplifiers that use air system sockets
Richard L. Measures, AG6K, 805.386.3734. www.somis.org
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