About,
"Today I was running my SB-220 with the case off, and noticed, to my
surprise, that the filaments of the two tubes show quite different
brightness reflected off the desk under the amplifier, even on standby.
Key-down, the brighter of the two tubes also appears to be taking the
lion's share of current, judging by the extent to which the plate of
that tube brightens up when key down, compared with the other one.
(etc.)"
Poor fil pin contact in the socket can cause all of this. Clean and
inspect the pins' condition and socket contactors' condition. They may
be discolored due to heating effects and lose tension, like a vicious
cycle. Swap the tubes' locations with each other and try again.
Luck & 73, Roy K6XK
P.S... I mounted a large mirror beside my 3-500Z amp in order to keep
an eye on the operating tubes.
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