Jordan's Scott 296
Rebuild
First up is the replacement of the always
bad Selenium rectifier I have never seen one of these
that wasn't putting out at least 25% less then required
voltage to the 12AX7 heaters and more important the bias
for the output tubes.
Here is the pesky bugger before replacement
its the 1" x 1" device bolted to the side of
the chassis.
Before replacement
There are 4 connections to the original
and the new part they both have 2 connections marked ~
and one marked + and one - pretty simple replacement except
the new part has the markings on the rear so its easier
to solder the connections before you mount the new bridge
rectifier. In most Scott amps you also have to adjust
the value of the first dropping resistor on the filter
network because the new SS part is more efficient in this
amp its not required since Scott had the heaters supplied
with only -47 volts for 6 12AX7's in series that is less
then 8V for a 12V heater I now have 10.5V much better.
The other concern normally would be you would have to
much bias voltage but I am reworking the bias circuit
for full bias control so I will tackle that later.
Bridge Rectifier Installed
3 High Voltage Can Replacement
The cans to be replaced are the left one
in the bottom row and both cans in the top row. The other
can is for the bias & heater supply and out of hundreds
of amps I have only seen 2 of these bad this one proved
to be fine.
Before Replacement
With this being such a gorgeous amp Jordan
and I decided on using original type twist lock cans for
the power supply. Of coarse knowing me the way you all
do I found a way to do some upgrades while were at it.
The original first filter can was a 20/20/20/20uF @ 500V
I ordered a 80/40/30/20uF @ 525V for its replacement.
I kept the same 20uF on the 5AR4 rectifiers output and
installed 80uF where it would do the most good right at
the center tap of the output transformers. The remaining
2 sections of this first can are for filtering the dropping
voltages before it heads to the preamp section. Now the
2 top cans is where we have to get creative they are both
4 section cans 2 -20uF @ 450V and 2 -25uF @ 25V what we
do here is just replace the 2 450V sections with cans
and use small axial separates for the 25V section and
mount them else where in the amp. We used 2 40/20/20uF
@ 500V cans strapped the 2 20uF sectioned together giving
us 2 40uF section to replace the 20uF nice upgrade in
filtering again . You will notice that I also rerouted
the wiring to clear the area above the cans this is where
the bias controls will be added in the next step.
New Cans Installed
Here is how I keep track of the 4 sections
that I'm going to move to separates. This way I don't
have to get out that nasty magnifying glass to read them
schematics
Okay here is the next steps
Bias Controls and Cathode sensing resistors
Here is the 2 new bias voltage controls
installed in the spot I cleared in the previous step.
This is a pretty straight forward operation although it
require a good bunch of wiring changes. Scott's existing
setup was fix bias with balance controls per channel not
exactly precise to say the least this setup is Identical
to a 299C and 222C. What I did was tap into the bias voltage
supply for the balance controls and install a 25k potentiometer
to feed each balance control. You feed on leg the voltage
and hook the center wiper to the center wiper of the balance
pot. The other leg of the bias voltage control is run
to ground through the resistor this resistor value is
how you adjust the range of voltage you will be able to
apply to the grids of the output tubes. I also added another
filter stage on the wiper of the bias voltage pots. This
amp will now easily be able to run a wide range of output
tubes from EL34's to KT-66 and everything in-between with
its now wide range of bias voltage sorry KT-88 would be
a huge stretch for this amp.
Here they are on top of the chassis
Adding Bias Cathode Sensing resistors
This next step takes some serious rewiring
Scott has the Cathodes directly connected to ground and
also used these same pins to ground many other items.
So the first step is to disconnect everything from pin
1-8 of the output tubes and determine what has to be taken
back to ground. What were accomplishing in this step is
to lift the cathodes from ground with a 1% 10 Ohm sensing
resistor to allow us to actually measure the bias with
a ordinary multimeter. This takes some serious moving
and rewiring to accomplish. I install 2- 4- lug terminal
strips to make all the new connections. I strapped 3 of
the lugs together on each terminal strip then run a wire
from both to the main B+ filter cans ground connection
The output stage before changes
Cathode sensing resistors installed
Having full bias control is great but not
if you have to take the amp out of your system and get
under the chassis to measure it. So I install test points
on the rear apron of this amp. This is simple you drill
and install the tip jacks then run a wire to each 10 Ohm
resistor where it attaches to the tubes cathode. (Jordan
relax those tube are not yours I'm not firing this amp
up after all this work with $600 worth of tubes until
proper operation is confirmed)
Test points
First up is the Dynaural noise suppressor
this tube is a 6GH8 in the 296. All I do is replace the
caps and on electrolyte with exact same values. This feature
is basically worthless with today's modern turntable.
I call it the Dynaural bass suppressor
Before
After Major Surgery
First Gain Stage
These 2 12AX7's are additional gain stages that the 299
and 222 series do not have surprisingly I found that every
original resistors were dead on value wise so I left them
as is why mess with a good thing ! I replaced all capacitors
with Sonicaps and a few Russian Film and Foils here.
Before
After Surgery
Tone Controls and Gain Stages
These 12AX7's are basically Identical stages
to the 299 and 222 series of amps. I did my standard fair
replacing all Plate and Cathode resistors/bypass caps.
Also replaced all capacitors with Sonicaps. You will notice
the additional Sprague Atoms these were to replace the
25V section of the cans in the earlier can replacement
procedure.
Before
After Major Surgery
Phono Section
Pretty simple stuff here replace 2 caps
and unless the phono section is found to be operating
abnormally when I fire it up it should be good to go.
Before
After Minor Surgery
The final preamp Gain stage,phase splitters
and coupling caps to the output tubes.
The importance of this section are monumental
to the final sound of the amp. I replace all capacitors
and the plate resistors to start of and then may revisit
this area after the amp is tested on the scope. Usually
nothing additional is required. Again Sonicaps were used.
I upped the main coupling cap values slightly a interesting
side note this is the first Scott integrated that I have
ever seen with a reasonable value cap for the main couplers
with them being .25uF stock I used .33 @ 600V here.
Before
After Major Surgery
Well here is a complete shot of the amp
after the major work is complete ! Now its time to fire
it up ! It should work without issue but you never know
!!
