Nothing exciting here, just a few corrections to the available schematic for the S5500-II. Mine is what I presume to be a late unit, serial number K39256 with date codes on the transformers from week 6, 1963.
Two main differences between my unit and the published schematic are the bias supply and the circuitry around the phase inverter.
Firstly the bias supply. On this one, it uses what amounts to a full wave center tap design, but the center lead is the voltage source and the ends of the transformer ground via the diodes and a drop resistor. This one in stock form also had a bias control pot that is not on the schematic. This can be seen here:
In stock form, there is also no means of monitoring the current through the output tubes. The S5000 manual says to basically adjust it for peak power output and minimum distortion, which is not something Joe Homeowner can do. This is silly and needs to go away. To fix that, I modified one of Dave's designs for a Fisher and installed a bias/balance board with a set of test points on it. Schematic for the board is here:
Some pics of the board in place. This isn't really mounted, its essentially sitting there on a blob of hot snot currently. I need to make some spacers to stand it off the chassis and mount it permanently, but basically its sitting where the old bias pot was and where it will live once its screwed down. The wire terminals are the test points, and are arranged in the order the tubes physically sit in the amplifier. The center one is ground.
The test points are wired to the cathodes of the output tubes. The cathodes ground through the typical 10 ohm resistor method. A handy-dandy 10 conductor cable gets connection to and from the output tubes. Wiring this in does require breaking the common bias feed connection as well. To make this land all neatly, the stock 2 post terminal strips (ground + isolated terminal) need to be changed to a 3 post. Mine are in the mail, so no pics until I get them in. Included is a pic of the stock setup with the cathode resistors in place. You can see the stock terminal strip for the bias feed where the resistors are soldered.
Unfortunately the stock bias supply rig doesn't provide enough voltage to make the bias/balance rig work properly. On to re-working the supply a bit to get some more juice. Essentially all I did here was remove the 22 ohm resistor from the positive side of the supply and move it to the negative side. This gave me another 6.5 volts negative on the supply, which was plenty enough to get the tubes to bias properly. In the process I realized that a 12ax7 heater wired in parallel is a 21 ohm resistor. In stock form, the phase inverter for the right channel is DC heated, and the inverter on the left side is AC heated. A small wiring change gets both of these on DC heat, and several watts of heat out from under the chassis. It also slightly unloads the power transformer. Nothing earth shattering here, but its basically free.
I'll post other stuff as I get there. This is a work in progress, and interest in working on it is variable with temperature. The colder it gets, the less inclined I am to work in the shop.
Cheers
Two main differences between my unit and the published schematic are the bias supply and the circuitry around the phase inverter.
Firstly the bias supply. On this one, it uses what amounts to a full wave center tap design, but the center lead is the voltage source and the ends of the transformer ground via the diodes and a drop resistor. This one in stock form also had a bias control pot that is not on the schematic. This can be seen here:
In stock form, there is also no means of monitoring the current through the output tubes. The S5000 manual says to basically adjust it for peak power output and minimum distortion, which is not something Joe Homeowner can do. This is silly and needs to go away. To fix that, I modified one of Dave's designs for a Fisher and installed a bias/balance board with a set of test points on it. Schematic for the board is here:
Some pics of the board in place. This isn't really mounted, its essentially sitting there on a blob of hot snot currently. I need to make some spacers to stand it off the chassis and mount it permanently, but basically its sitting where the old bias pot was and where it will live once its screwed down. The wire terminals are the test points, and are arranged in the order the tubes physically sit in the amplifier. The center one is ground.
The test points are wired to the cathodes of the output tubes. The cathodes ground through the typical 10 ohm resistor method. A handy-dandy 10 conductor cable gets connection to and from the output tubes. Wiring this in does require breaking the common bias feed connection as well. To make this land all neatly, the stock 2 post terminal strips (ground + isolated terminal) need to be changed to a 3 post. Mine are in the mail, so no pics until I get them in. Included is a pic of the stock setup with the cathode resistors in place. You can see the stock terminal strip for the bias feed where the resistors are soldered.
Unfortunately the stock bias supply rig doesn't provide enough voltage to make the bias/balance rig work properly. On to re-working the supply a bit to get some more juice. Essentially all I did here was remove the 22 ohm resistor from the positive side of the supply and move it to the negative side. This gave me another 6.5 volts negative on the supply, which was plenty enough to get the tubes to bias properly. In the process I realized that a 12ax7 heater wired in parallel is a 21 ohm resistor. In stock form, the phase inverter for the right channel is DC heated, and the inverter on the left side is AC heated. A small wiring change gets both of these on DC heat, and several watts of heat out from under the chassis. It also slightly unloads the power transformer. Nothing earth shattering here, but its basically free.
I'll post other stuff as I get there. This is a work in progress, and interest in working on it is variable with temperature. The colder it gets, the less inclined I am to work in the shop.
Cheers
