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Frequently Asked Questions / Tech Talk
Q. When replacing output valves - should the bias be checked?
A. Most definitely YES. The new valves are unlikely to draw
the same idle current as the old set. If the bias is set too high
for the new valves, they will not survive long.
Q. Should new output valves be a matched set?
A. Yes. The output stage will work at peak efficiency when
all valves are drawing similar currents and amplifying the signal
to similar levels. Mis-matched output valves are a common cause
of hum - audible even with volume/master pots turned down.
Q. Can my amp be altered to have the 'Mullard' sound?
A. Only if you are prepared to pay for Mullard valves. Nothing
else sounds quite like a Mullard.
Q. Why do the two EL84 output valves in my vintage British
20 watter keep blowing?
A. Usually because modern production valves are not good
when it comes to surviving the high currents and voltages present
in some older amp designs. Few modern EL84's will last when subjected
to anode voltages of 330 volts or more.
Q. Do you sell valves?
A. No, we only supply and fit valves to equipment during
a workshop repair or service.
Q. It is often said that the voltages present in an amp
can kill, is this true?
A. Yes, none more so than 240v AC mains voltage. In our
workshop a mains isolation transformer is always used when dealing
with a live amplifier.
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