by Bryan Bergeron, Editor by Bryan Bergeron, Editor
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It's easy to forget that no two components or devices are exactly
alike. This is fairly obvious with
common leaded resistors, given the
tolerance marking is hard to miss —
gold 5%, silver 10%, none 20% for
four-band resistors. In other cases,
it's less obvious.
For example, in working with
batches of Arduino microcontrollers,
I've noticed small but significant
differences in the analog-to-digital
conversion accuracy from one
microcontroller to another. This may
be due to differences in the ATmega
chip but, more likely, it's due to
variations in the crystals or other
external discrete components.
resistors and my lab-quality Fluke
DMM and identified a matching set.
I only wish it were this easy with
Take vacuum tubes. I own
several vacuum tube amps — one
DIY and a couple vintage guitar
amps. One guitar amp uses a
matched pair of 6V6 output tubes,
and the other a quad set or 'quartet'
of 6L6 tubes.
In each case, the amplifier
designs assume that the tubes are
matched — in terms of
transconductance — which is roughly
the amount of amplification
provided by a tube. It's actually the
change in plate current divided by
the corresponding change in grid
voltage/plate voltage held constant.
The problem with tubes is that