Sansui 1000A Valve Receiver
Here we've got a Sansui 1000A Valve Receiver. Their are a few variants of the 1000A/T topographies plus different bias configurations to note.
This particular unit came in for extreme hum and red plating issues. A few modifications can/should be integrated as well. For a more consistent reliable cathode draw or bias current a 10 ohm 1/4-1/2W MOX should be used at the 7591 power valve cathode pins to ground, remove the original jumper and replace. One can also tie the dependent bias potentiometers to a single control if desired.
Another issue brought to my attention was the 117VAC original rating. If we look at the driver section schematic we see posted Vdc of 470Vdc+ for a plate voltage. Most modern homes here stateside operate in 120-125VAC range. Looking at initial Vdc rating we see peaks of 482/480Vdc. so increasing come capacitor voltages is a good here.
7591 Pin-Out Configuration
For the red plating, after ruling out mismatched 7591 valves we look at the bias circuit. The grid resistors 1K and bias 200K resistors are typically out of spec on these units, another common fail point is the oil-type .3uf coupling capacitors. It is highly recommended to replace these with a 500Vdc+ electrolytic or polypropylene in the given voltage. I also recommend replacing electrolytic axials here as the axials in the time period in my opinion are dodgy at best. Monitor again for about 30mA draw at the cathode resistor and that solves that. A thing to keep in mind, because of the auto-biasing 7591 topography, matched quads are of some benefit here. The 1st stage ax7's were fine but the consistent left channel hum was the result of a bad 6AN8 valve.
New coupling capacitors (.33 630Vdc PP, new .47 & .22uf PP capacitors, new 30uf radial panasonic FC high temp long life electrolytic)
Keep an eye out for our Sansui 1000A Restoration posts coming soon!