Steve: 

If the last resitor shunt to
ground is 56K, try to substitute
a series string of a 50K
multi-turn trim pot and a 47K
fixed resistor to ground. That
is, full voltage to top of pot,
47K from bottom of pot to gnd,
with wiper of pot to existing
bias feed resistor junction
(where the top of the 56K
resistor goes now). 

If the supply feed from the
rectifier bridge does indeed
come through a capacitor from
the high voltage supply, this
capacitor's value has a great
deal to do with setting the bias
supply's maximum available
voltage. Don't change it unless
you know what you're doing. 

The reason for splitting the
total resistance into part fixed
& part variable is to prohibit
an "accidental adjustment" in
an unsafe high current range.
With the 47K being a little
smaller than the original 56K,
you could increase the idling
current somewhat above stock.
If it doesn't allow adjustment
as low as you need it, drop
down a little at a time (39K,
33K, etc.) until the range is
right. 

Doc

Steve: 

Don't try to fit all this on the
pcb. If you have to experiment
with circuit values the copper
traces may not last through all
the soldering. 

What I do is find a relatively
blank spot on the chassis and
drill holes, one for a bias adj.
pot and two others for cathode
current test point pin jacks. On
the underside (inside the
chassis) mount a terminal lug
strip near the pot & pin jacks
to mount 1 ohm cathode
standoff resistors . Mount the
47K(/39K/33K) resistor in
place of the 56K on the board,
cut appropriate foil traces to
insert pot via jumper leads,
and cut cathode pin trace to
each output tube (if the tube
sockets are wired with flying
leads just interrupt & re-route
these) and wire a 1 ohm
resistor in series with each to
complete the circuit. 

What you'll end up with, if
everything goes well, is a way
to set the bias current using a
millivolt meter (1 mv=1ma
when measured across a 1 ohm
resistor, read from each test
point to ground) from outside
the chassis with the amp still
assembled and speaker load
connested. 

This is kind of a quick
explanation. So if I've left
anything out, especially
regarding this particular model
amp which I'm not familiar
with, someone else out there
might have some corrections
or tips pertinent to your amp. 

Good luck, 

Doc