Fisher 480A question

[burmashave]

I just bought a Fisher 480A modified from console to stand alone use. I can't get a lot of info online re it's schematic and service issues. Is this amp self biasing? If not, can somebody guide me to the directions for biasing it myself with the understanding that it's still basically stock other than stand alone use? Also, I see on an older thread that somebody on a different but same model amp increased the value of a "green" resistor that was too small considering there was no preamp or tuner hooked up anymore. Can somebody advise re that also please?

Thanks in advance.

 

[RUmad]

Sounds like a familiar story. Sounds like one of the models referred to in a YouTube video by AK member @Blueglow. If he can’t lend an ear personally, I remember the video as being extremely informative. Also, you may want to ask a mod to move this to the Fisher forum, you’re likely to get tons of help there. Good luck!

 

[burmashave]

Sounds like a familiar story. Sounds like one of the models referred to in a YouTube video by AK member @Blueglow. If he can’t lend an ear personally, I remember the video as being extremely informative. Also, you may want to ask a mod to move this to the Fisher forum, you’re likely to get tons of help there. Good luck!

sweet! didn't realize there was one. Thank you! On my way.

 

[FlaCharlie]

Not hard to find. Google brought this up right away . . .

https://sites.google.com/site/fisher480a/Home/fisher-e49-schematic

In the left margin there is a link to restoration notes.

The Google search results: fisher 480a schematic

There's a pot labelled bias adjust so it's obviously fixed bias (which actually means adjustable) as opposed to cathode bias.

 

[burmashave]

There are no pics in that link anymore so it’s tough following the narrative. I’ll keep looking. Thanks

I found biasing instructions posted in the Fisher forum so I’m set with that

 

[larryderouin]

FlaCharlie. The 480A/481A/490A are all variants of the same amp (different plugs to accomodate different tuner/preamps for the console). They are an adjustable cathode bias, where adjusting the pot will change the pin 2 voltage.

Burmashave. Use the 1964 Custom Electra Service manual. www.fisherconsoles.com Set the pin 2 voltage to NO MORE THAN 21V. 19 to 20 is a bit better as you won't be running the tubes at the ragged edge.

Stand-Alone mods from my archives for the 480, 481, & 490

Stand-Alone only without IBAM Mod.


Location of OPTIONAL IBAM Board. IBAM board will give you ability to bias each tube individually. Note change in cathode resistor circuit. Tie in IBAM to cathode circuit. E,F,G,&H are location of connection of 10 ohm cathode sampling resistor on IBAM BOARD. Test points connect between resistor and pin 3 on each tube. You'll have 4 test points. Pin2 to 21V and pin 3 to 33v. The adjustments are interactive so it may take some finagling to get them all set equally. Once that's done, you shouldn't need to touch it again unless you change a tube.


Dave's IBAM board for 6bq5 FISHER Amps.

And the text file instructions below

 

[Blueglow]

I made a youtube video on this exact topic a few weeks ago. Its worth the watch, I think...
Mark

 

[FlaCharlie]

FlaCharlie. The 480A/481A/490A are all variants of the same amp (different plugs to accomodate different tuner/preamps for the console). They are an adjustable cathode bias, where adjusting the pot will change the pin 2 voltage

Thanks, great info on these. I didn't look closely at the schematic, I just saw the adjustment pot in one of the pics. I have a 440-A and an SA-16.