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I just bought a matched quad of EL84s (Amperex BB) and one of the four tubes doesn't glow as bright as the other three (probably less than half as bright.) Does this indicate a problem?
Follow Ups:
I bought a Cayin A-88T, one of the small tubes also didnt glow as bright as the others, I asked everywhere, and got the same answer, no problem...BUT, it make all kind of HUMMMs, so I returned it, the supplier found out the small tubes where defective, replaced them for better quality, and returned the amplifier back to me, no more problems, so far...
Antonio Melo Ribeiro
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The term Matched should refer to electrical measurements and not cosmetic appearance. The degree of brightness associated with filamemts is a factor of how much actual filament structure is visible outside of the enclosing cathode sleeve. IOW, if more (or less) of the filament is exposed outside of the cathode, the brighter (or dimmer) the filament will appear. But since a vacuum tube's purpose is different than that of a light bulb, the amount of filament glow seen is of no consequence to tube operation; the sole purpose of the filament is to heat the cathode to electron-emiting temperature - so a somewhat better evaluation method than looking at it would be to touch the tube & discover if it burns your finger.
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