Manley Stingray iTube - Manual (Page 15)

What do I do? The amp should be well warmed up (30 min.) and no music playing. Set the meter to read DC
volts. The meter has two test leads, red and black. Put the black lead in the black terminal on the top marked TPG
(ground) or touch the metal point to the steel chassis (but not a painted or anodized part). Put the red lead in the
first red terminal marked TP1. The meter should read either .25 V or 250 mV(same thing). If it doesn’t you need to
adjust BIAS 1 trimmer until it does. It doesn’t need to be exact - .23 to .27 will do. Now move the red lead to the
red test point labelled TP2. Same thing, if it is not .25 adjust BIAS 2 so that its close. Continue till all 8 points have
been done. If you trimmed a few, you should re-check because there is a little interaction and you may want to fine
tune a bit.
What if the reading is drifting up and down? Some drift is normal. It is caused by the AC mains changing and by
a little very low frequency noise in the tube. If one drifts above .3 or below .2 then you may be seeing a tube begin
to go bad. Watch it for a while or check it every few weeks noting changes. This tube should be replaced probably.
With mains changes all the tubes drift together and you may see more long term drift. Another cause is that you
forgot to stop the music.
What if I can’t trim a tube into range? If all you get is zero or near zero on one terminal it means either the tube
or 10 ohm resistor is dead and needs replacing. If it is too high then it probably is the tube. If you look at the tubes,
that one will probably be cherry red. Get a new tube. If all the terminals read zero on one channel, then the B+ fuse
is likely blown on that side. If all the terminals read zero on both channels but the amp still plays, then you have a
problem with the meter.
Are there any advantages to biasing higher or lower than 250mV? This is the optimum trade-off between tube
life and cross-over distortion. If you bias a little higher you may lower distortion a bit but shorten tube life a lot. If
you bias lower the distortion climbs fast and the tube life will lengthen - partially because you won’t want to listen
as often or as long.
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