Why do some valves have metal casing and some do not?
Just wondering...
Do you mean the cinched tube clamping the base as found for instance on the KT88? If so I think it is just for mechanical strength, that's a big ole glass envelope.Originally Posted by boyhowdy
If you mean all metal valves such as the 6V6 variant (6L6?) I believe it was for super rugged milspec' purposes. I have read that metal enveloped valves are bog usless for audio purposes!
Then there was the "50" base range of valves, sort of mini octal. Horrible things, the pins used to oxidise badly. Very common was the EF50 often found in V old tellies and oscilloscopes.
Dave.
If you mean actual metal-cased valves as Dave described for the 6V6 and 6L6, don't try them in a guitar amp - they're all old original-spec valves, not the later A/B/C variants (eg 6V6GTA or 6L6GC - the G stands for Glass), which have higher voltage ratings. There's a good chance they will simply fail, and possibly damage the amp in the process. I know some people who have tried it and not had anything bad happen, but I seem to remember no-one thought they sounded very good.
Or do you mean the metal screening can that clamps over a preamp valve? They're to shield the valve from external electrical noise, and to physically hold it in place. Some people think they affect the tone - due to the small extra capacitance to ground that they add - they possibly can in some amps, but it's very debatable as to whether it's really noticeable. Some also claim they make the valves overheat, which is bollocks.
"Just because I don't care doesn't mean I don't understand" - Homer Simpson
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Yeah that's what I meant - thanks!
Shielding from EMI.
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