07-13-2015, 11:12 AM
One of the interesting parts of the restoration was duplicating the two 200uFd @350V electrolytics in the power supply. I wound up using two can capacitors that are 50uFd X 4 @350V and paralleled them. The one that is above ground potential on the outside metal was about the same size as the original one but I had no insulating sleeve, so I made one using some card stock in a trapezoid shape to wrap the can twice. Once for the inner shorter portion of the top and another to wrap all the way down to the bottom of the outside metal. It was slightly taller than the can height so I could roll the top edge over a circular disc piece of the same card stock. Then I printed a label stating it was 50uFd X 4 configured to make a 200uFd@350VDC. The electrolytics came from Antique Electronic Supply and were ideal for the job. I had one of the bakelite insulation wafers to mount it on the chassis, so I did not have to research that. The end result looks like many OEM insulated caps for voltage doubler power supplies.
One thing I learned about Fisher 500K ohm balance controls is that they are linear taper, not audio taper. Of course the volume control is a 500K ohm audio taper (log taper) control with the loudness tap. I was lucky to find one on ebay, although it did not have an on/off switch. I may eventually try to find a snap switch (also called a micro-switch) with a long straight lever and cut a small slot in the chassis metal, then put custom made brass tab on the control shaft held by a press-on nut that will grab the control shaft and use that to turn the amplifier on. For now a slide switch is handling that task.
The TL082 Op Amp that handles the preamp output is mounted on the back of the balance control. It allows a modification to the driver stage input to prevent Miller effect at the triode grid. It is supplied + and - 11VDC from the LV EFB power supply that also handles DC feeds to the preamp tube heaters plus providing about -50VDC that is used to handle the negative bias adjustment circuit.
Joe
One thing I learned about Fisher 500K ohm balance controls is that they are linear taper, not audio taper. Of course the volume control is a 500K ohm audio taper (log taper) control with the loudness tap. I was lucky to find one on ebay, although it did not have an on/off switch. I may eventually try to find a snap switch (also called a micro-switch) with a long straight lever and cut a small slot in the chassis metal, then put custom made brass tab on the control shaft held by a press-on nut that will grab the control shaft and use that to turn the amplifier on. For now a slide switch is handling that task.
The TL082 Op Amp that handles the preamp output is mounted on the back of the balance control. It allows a modification to the driver stage input to prevent Miller effect at the triode grid. It is supplied + and - 11VDC from the LV EFB power supply that also handles DC feeds to the preamp tube heaters plus providing about -50VDC that is used to handle the negative bias adjustment circuit.
Joe
