SET amplifier Q
"flipper"
"Phil Allison"
UBT-1 is rated at 160mA on the primary...
** Yep - the total idle current is 110mA.
Did those 300Bs develope a dead short?
** According to the owner/builder - there was a flash / bang / wallop
event
and then a dead channel.
One dud 300B draws no bias current while the other draws waaaay to much,
the
UBT-1 has an open deep inside but shows no sign of shorted turns.
What has me intrigued, however, is the low THD figures at and near full
power PLUS the fact that 3H dominates.
The test results are far more like a PP class A triode amp than a SE one.
The 6SN7 is running at full gain ( about 18 times per half ) and there is
definitely NO negative feed back - in fact the two speaker windings have
been left floating.
Sounds like by either intent or serendipity the 300B and 6SN7 2'nd
harmonics are canceling.
** Bingo !!
Seems an eccentric Russian gent was responsible for the circuit design.
The owner has agreed to purchase another UBT-1 from the USA and new 300Bs
and send them to me.
Then I can hook the beast up to my own speakers ( 10 inch 3 ways plus active
sub ) for a listen.
...... Phil
|