Don't forget that amp power supplies are not regulated. The bias
voltage follow faithfully the mains variations and so, the idle
current inthe output stage varies continually. Just measure to verify
it. Note it is recommended to verify the mains voltage each time you
adjust the idle current of an amp. If you do accurate measurements,
or if you test tubes on a lampmeter, you will notice that mains
varitions have an influence upon the tube heating voltage. All this
brings about accuracy and measurement reproduction problems. In all
of these cases, it's imperative to stabilise the mains.
To resolve simply this problem, you have only to recover an old mains regulator. Used in the 60's to stabilize the image of the tube TVs, this type of regulator weighs 2 kg, consumes 100W, but gives excellent results.
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