[sdiy] cap autopsy

[Eric]

>I don't know about tubes (for sure) as they get older their
>emissivity goes down (they get weaker...). But I don't think
>that is over a two week period... its more a long term function.
>This applies mostly to power tubes... not usually the preamp stages.

Preamp tubes can easily last 20-30 years---EXCEPT for
cheap Chinese and Russian tubes of current manufacture.
Cathode materials purity is critical. NOS tubes usually
last a long, long time. I have seen 12AX7s in 40-year-old
hifi preamps that still tested like new....

Power tubes use up their cathodes faster. The output tubes
in a guitar amp can last only 1-2 years if the amp is played
loud all the time. Hi-fi amps are not as hard on power tubes--
figure 3-5 years for most types.

Always, ALWAYS replace filter capacitors in old
tube equipment. ALWAYS. Even tubes often outlast
filter caps.


Eric Barbour
METASONIX