[Patrick Turner]

Iain Churches wrote:

"Patrick Turner" wrote in message
...



I have never bothered to try to provide an ac balancing pot in any amps
because one could sit there swinging the balance pot while listening to
music and not hear
the slightest changes to the sound.
At low levels of 1 watt average from 20 watt AB amps, the ac balance
doesn't need to be perfect.
Its class A operation, and inherently its going to be OK.

I like to include one, though few people seem to be able to
hear the difference between 0.1% and 1% :-))

Iain

But the 2H which is present due to unmatched tubes
is usually much less than the 3H in a PP amp.

Having a balance adjustment means another darn thing that can go wrong,
and often in an anode circuit of a driver tube with dc flowing in a pot.
I have seen the results and had to repair them, and usually
its a case of removing the pot and replacing
it with fixed resistors to ensure equal balance with a given tube type
such as a 6SN7 in a LTP driver/phase splitter which does not have ccs
tail.

Subsequent replacements with other 6SN7 will all give very close
balance.

Even with most old output tubes, the 2H is low.

If there is an SE input tube before the LTP which has been balanced to
equality,
then the 2H of the input tube is included at the output.
often reversing the positions of output tubes will make the 2H less due
to cancelations.

But trying to unbalance the ac drive to have output stage 2H cancel
previous stage 2H seems mad.

Better to make the amp have low thd to begin with, and the mild NFB will
reduce all THD OK.

Patrick Turner.