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


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.


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.


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.


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.


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 !!


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