Check those Bias caps!

[Roger A. McCarty]

FYI, for anyone refurbishing vintage Heathkit tube type amplifiers. Check
those Bias caps, or better yet, it may be a good precautionary measure to
replace them as a lot.

I discovered the hard way by destroying two brand new 6l6's. These caps are
often bad after the long years they have been in use. This is probably old
news for OT's.

An easy way to check them is to measure the bias voltage at the tube socket
with the tubes removed. If the negative bias voltage is anything less (more
positive) than that which appears at the bias bleeder resistor in the power
supply, the cap is bad. The only place for current to flow under these
conditions is through the capacitor which of course with a good cap, it will
not do. If you see minor variances on the different tube sockets, it is an
indication that they are becoming leaky. Again, the voltage should measure
EXACTLY the same as it appears on the bias bleeder resistor on every tube
socket. For 6L6's this should be real close to -37v and for 7591's real
close to -16V.

In many Heath amps, .1uf at 400V tubulars are used.

FWIW

Roger KD6CC


Roger A. McCarty ARS KD6CC
http://www.qsl.net/kd6cc

Caution!  The 2nd Law of Thermodynamics is at work on all things at all
times.
Mankind and Human nature are not exempt from entropy. The only solution is
to decide on a higher standard, and commit yourself to it.

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