[Chris Hornbeck]

On 29 Oct 2006 16:59:01 -0800, "
wrote:

5-there are two OUTPUT BAL controls near the tubes, one for each
channel, what are their purpose?

Based on the link that Arny posted, these are DC
balance trims. Unfortunately, the circuit provides
no convenient place to measure. There are a couple
of clumsy ways to do it, if you're bound and
determined:

1) You could measure the OPT's primary resistances
(cold, of course), measure voltage drops across them,
calculate plate currents, and adjust the currents to
be equal.

2) You could make a pair of plug/socket combo's
with cathode current sensing resistors built in.

3) You could permanently insert a small resistor in
each cathode lead, between pin 5 and the trimpot,
and measure across them.

The purpose of the controls is to balance standing
primary currents, which minimizes large-signal
distortion in the transformer. So you could also
4) Measure full power distortion at a low frequency,
maybe 40 Hz, and adjust for minimum. This often
proves to be fairly confusing, though, even with
the needed equipment ready to hand.


All good fortune,

Chris Hornbeck