>Rich said:
>
>>EUR to test the cooling on the filament seals, after about 5-min. of
>>standby, briefly touch one of the filament pins. If it's painfully hot,
>>the cooling is not wonderful.
>
>The classic problem comes with the heat reducing the spring tension which
>increases the resistance which increases the heat........
>
>
>Admittedly, not with 3-500s, but with QY4-250 and QY4-400, which have the
>same
>filament and similar external mechanical structure, I've seen the problem
>of the
>solder melting. As Rich says, the pins should ideally be cool enough to
>touch. I use a small fan blowing directly onto the socket (two tubes = two
fans) to
>keep the seals cool. As the full air flow isn't needed, I use two 220 volt
fans in
>series across 220 volts, which keeps the seals cool enough to touch, and a big
>fan 'upstairs' for the plate/envelope cooling, together with heat dissipating
>plate connectors.
>
? Orient the two sockets with the filament pins facing each other, as
Heath did in the SB-220, and one fan will cool all 4 filament-pins. If
it's a 15cm fan and a split chassis is used, a single fan will cool the
works.
>Incidentally, I've seen a number of military tx's in which there has been
>insufficient cooling for the seals on this type of tube; it would seem that
>limited life is accepted, even though it neeedn't have been. Although
>Philips/Mullard offered a full air system with chimney, I've never seen them
>used over here.
>
? After seeing the burned glass-marking ink on 3-500Zs that came
therefrom, it is my opinion that the design of the air-system/chimney
cool system was not well thought out. Tubes that came from a TL-922 and
SB-220 typically have bright, untoasted, glass-marking ink -- provided
one oils the fan bearings periodically so that the bearings do not seize.
>
cheers, Peter
- R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K,
www.vcnet.com/measures.
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