Fisher Console Amp Mod?
- Thread starter crswanson
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cademan
Lunatic Member
YES to all of the above! 
The tuner/preamp runs off of the amplifiers main power transformer so you would need the amplifier to use the tuner/preamp.
You can use the amplifier by itself, but you will need to know how to turn on the amplifier without using the tuner/preamps on/off switch.
Post a picture of the underside of the amplifier and I or anyone here can show you how its done.
The tuner/preamp runs off of the amplifiers main power transformer so you would need the amplifier to use the tuner/preamp.
You can use the amplifier by itself, but you will need to know how to turn on the amplifier without using the tuner/preamps on/off switch.
Post a picture of the underside of the amplifier and I or anyone here can show you how its done.
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I don't know for sure about the 460, but the 680 has three series 12AX7 heaters in the preamp serving as the cathode resistor for the output tubes. For stand-alone operatio, you need to add a cathode resistor - 36V/150 mA = 250 Ohms @ 6W - better make that a 10W or preferably more.
Sorry for jumping in here but,,, Tom I've been looking for this info for months. Does this also apply to the 481A? If so would the resistor simply replace the three heaters? Looking at the schematic (place the resistor between the lines feeding pins 2-3 of the mpx connector? Of course that may prove to be a challenge since Ive already removed the original connector from the amp chassis.
Thanks
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I’ll take the chance of creating more confusion by sharing some pictures of my modified 440a amplifier and an untouched 481a amplifier. The amps look very similar except the 440 uses 7247 preamp tubes and the 481 uses 12AX7.
The first picture shows my 440 amp. The yellow wire coming from the tuner \ preamp plug is wired with the EL84 cathode pin 3. You can see in the picture the white 250 ohm 10 watt resistor that I added. At the very bottom of the same connector you might be able to see where I jumped two black wires. This bypasses the preamp \ tuner switch and the amp now powers up when plugged in. I added the 250 ohm resistor based on advice in a different thread that I can't find now. Eventually I’ll add a power switch.
The second picture is the unmodified 481 amp. The amps look very similar but the connectors are a little different. When I get around to it, I plan on adding the same 250 ohm resistor to this amp. I think it should fit nicely on the terminal strip lug where the yellow wire goes now. I would just ground the resistor to the closest power supply cap.
Good luck. The 440 amp sounds nice.
I also have a 660a amplifier that uses 7591 power tubes. I'm using this amp right now and also added the same 250 ohm resistor. In addition to the resistor I added 10 ohm 1/4 watt resistors in series with supply going to the cathode. This allows me to measure voltage across the resistor and adjust the bias appropriately.
For the 440 amp I'll add these resistors by moving the yellow wires going to pin 3 of the EL84 to the unused pin 1. I'll then add the 10 ohm resistor between pin 1 and pin 3. I've read that the resistors also add some protection to the output transformers. They can also be used to match tubes. Below is a link that describes the tube matching process.
http://enginova.com/Tube_matching.htm
The first picture shows my 440 amp. The yellow wire coming from the tuner \ preamp plug is wired with the EL84 cathode pin 3. You can see in the picture the white 250 ohm 10 watt resistor that I added. At the very bottom of the same connector you might be able to see where I jumped two black wires. This bypasses the preamp \ tuner switch and the amp now powers up when plugged in. I added the 250 ohm resistor based on advice in a different thread that I can't find now. Eventually I’ll add a power switch.
The second picture is the unmodified 481 amp. The amps look very similar but the connectors are a little different. When I get around to it, I plan on adding the same 250 ohm resistor to this amp. I think it should fit nicely on the terminal strip lug where the yellow wire goes now. I would just ground the resistor to the closest power supply cap.
Good luck. The 440 amp sounds nice.
I also have a 660a amplifier that uses 7591 power tubes. I'm using this amp right now and also added the same 250 ohm resistor. In addition to the resistor I added 10 ohm 1/4 watt resistors in series with supply going to the cathode. This allows me to measure voltage across the resistor and adjust the bias appropriately.
For the 440 amp I'll add these resistors by moving the yellow wires going to pin 3 of the EL84 to the unused pin 1. I'll then add the 10 ohm resistor between pin 1 and pin 3. I've read that the resistors also add some protection to the output transformers. They can also be used to match tubes. Below is a link that describes the tube matching process.
http://enginova.com/Tube_matching.htm
Attachments
That was it!!!! All is working as it should and I'm loving it
. I ended up connecting qty 2 100ohm and 1 50 ohm in series. Project 481A = doneAttachments
Well, there's trouble in paradise. After a few minutes of listening,there will be some static and one of the 7189's will start to glow very bright. I realize something is breaking down but have no idea where to start looking. Its only the one tube position and I did try swapping out the tube.
jaymanaa
RIP 1961-2018
Possibly, have you cleaned the tube pins and sockets? I use electrical spray (the kind that leaves no residue) and a small metal bristle brush (brass is nice). For the sockets, I just use an old tube and the spray. Spray some in the holes, and wiggle the tube in and out. Hmm, that didn't sound exactly right.:scratch2:
That was it!!!! All is working as it should and I'm loving it
Don,
Glad to see you got her up and running!
It's possible. I have an amp that I had to re-design the bias system to get a tube to stop redplating. Keep an eye on it for a while.
I'm sure these amps sound great as stand-alones, but I'm not sure I could bring myself to gut a Fisher console unless it was trashed.
Thanks for all the help with my 481A guys. It has been playing for a good 4 hours or more now and sounds great. As a matter of fact I never thought very highly of the EV 8's I'm playing the fisher through until now. They didn't sound very good with any of my SS recievers but they sure sound good with the Fisher.
Thanks again
Thanks again
