[Ham-Computers] More info on Scott LK-72 biasing

[Phil]

Hello all
As some of you know, I'm working on a Scott LK-72 amplifier here.  I'd
recapped the paper caps and then, 'hoping' the 'lytic cans were good spent
over a week slowly re-forming them in with no tubes, just a silicon
rectifier.  The caps appear ok now as none blew up in my face at slightly
below rated Voltage.  I then put the rectifier and pre-amp tubes in it and
set it to just slightly below rated line Voltage (to compensate for lack of
output tube current) and let run again.  No smoke or flames <grin>.

All this was taking place while I was waiting for the copy of the service
info to get here so I could 'properly' set up the bias on the output tubes.
As some of you may recall, this amplifier has a slightly different output
circuit with two chassis mounted RCA jacks and a switch labeled Bias and I
wasn't sure how to set it up.

Well, after the manual arrived I discovered that there was no mention of
this 'mod' in the service instructions.  However, there were two, slightly
different  schematics of the unit.  Come to find out that although the front
panel says LK-72, this Scott is actually a LK-72B with a schematic drawing
dated 1963.  (the plain Jane LK-72 is dated 1960).  The schematic shows the
jacks and switch.  Apparently, Scott added a 3.3 Ohm resistor in each of the
two output section cathode circuits and a bias pot for each channel in
addition to the DC balance control (4 pots total).  So this allows you to
adjust both bias and balance of each channel.

Now, looking at the schematic, those jacks don't appear to actually be in
the cathode of the tubes  but elsewhere, though one side of the switch does
switch between the cathodes of the two channels.  Either that or there is an
error in the drawing!  I'm going to dig a little further into the actual
wiring of the Amplifier,comparing it to the drawing.

But, the way things are, I can just drop my  Voltmeter across the 3.3
cathode Ohm resistor in each channel and set the bias that way for rated
current and the DC balance for minimum hum as the 'manual' says.  Not pay
any attention to the switch and RCA jacks.  Those were probably installed so
that one could set the bias without removing the bottom cover.

Of course in the paperwork that also came, Scott said the amplifier used
"fixed non Adjustable bias and the tubes were very conservatively run for
long life"  8^)

This brings up another question.  In the RCA Tube manual RC-29 the limited
specifications say that for Class AB1 Pushpull running at 450 Volts plate
Voltage the nominal plate current (for two tubes) should be 66 mA.  The
Voltage in this Amp is slightly higher, Scott says 463 (though I won't know
for sure till tubes are installed).

QUESTION:  Considering the high cost of 7591's, should I bias them slightly
"cool" for long life?  I know that among guitarists, some like to bias cool
for longer life while others run them somewhat hotter for that "special"
sound.  I don't have a scope and Oscillator to check crossover distortion.

73 de Phil  KO6BB