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Fisher 400 - Output tubes: resistors - supposed to be there?
I have a Fisher 400 Receiver recently purchased off Ebay. I'm in the
process of adding cathode resistors to the 7868 sockets - pin3 to chassis. However, I've found 2 resistors that don't appear in the schematic for my serial number range (36290T). There is a 220 ohm, carbon resistor /10% from pin4 to chassis for output tube V15 and the same value resistor from pin5 to pin3 for tube V17. My questions a 1. Are they supposed to be there? 2. What is their purpose? 3. Should they remain - if I'm adding the cathode resistors? Pete Snyder |
#2
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Pete, Those resistors are on the schematic (although not adjacent to the output tubes) and thus do "belong" in the circuit. You can find them in series on the schematic to the left of the 6V string of heaters in parallel. They form a voltage divider that's center-tapped to ground and I suspect they reduce hum. It's a good idea to install the 10 ohm 1/2 watt cathode resistors like you planned. Don't forget to install a pot in place of or in addition to R34 to adjust the bias voltage. Hope that helps. Jon From: (Pete Snyder) Organization: http://groups.google.com Newsgroups: rec.audio.tubes Date: 4 Nov 2003 19:14:05 -0800 Subject: Fisher 400 - Output tubes: resistors - supposed to be there? I have a Fisher 400 Receiver recently purchased off Ebay. I'm in the process of adding cathode resistors to the 7868 sockets - pin3 to chassis. However, I've found 2 resistors that don't appear in the schematic for my serial number range (36290T). There is a 220 ohm, carbon resistor /10% from pin4 to chassis for output tube V15 and the same value resistor from pin5 to pin3 for tube V17. My questions a 1. Are they supposed to be there? 2. What is their purpose? 3. Should they remain - if I'm adding the cathode resistors? Pete Snyder |
#3
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Also, ( and you may have already realised this, but just in case ... )
after you install the 10 ohm cathode resistors, move the 220 ohm resitor lead that was on pin 3 of V17 and connect it to ground. Jon Yaeger wrote in message ... Pete, Those resistors are on the schematic (although not adjacent to the output tubes) and thus do "belong" in the circuit. You can find them in series on the schematic to the left of the 6V string of heaters in parallel. They form a voltage divider that's center-tapped to ground and I suspect they reduce hum. It's a good idea to install the 10 ohm 1/2 watt cathode resistors like you planned. Don't forget to install a pot in place of or in addition to R34 to adjust the bias voltage. Hope that helps. Jon From: (Pete Snyder) Organization: http://groups.google.com Newsgroups: rec.audio.tubes Date: 4 Nov 2003 19:14:05 -0800 Subject: Fisher 400 - Output tubes: resistors - supposed to be there? I have a Fisher 400 Receiver recently purchased off Ebay. I'm in the process of adding cathode resistors to the 7868 sockets - pin3 to chassis. However, I've found 2 resistors that don't appear in the schematic for my serial number range (36290T). There is a 220 ohm, carbon resistor /10% from pin4 to chassis for output tube V15 and the same value resistor from pin5 to pin3 for tube V17. My questions a 1. Are they supposed to be there? 2. What is their purpose? 3. Should they remain - if I'm adding the cathode resistors? Pete Snyder |
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