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

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

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