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Needs instruction to adjust bias for Fisher 800C

206.8.118.2

Posted on March 26, 2001 at 22:57:44
aol908


 
Please help. Thanks.

aol908

 

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Re: Needs instruction to adjust bias for Fisher 800C, posted on March 31, 2001 at 09:17:38
You can't, Fisher receivers use fixed bias. If your bias checks low you need to replace the bias supply bridge rectifier and possably bias filter caps. Also it would be a good idea to replace the coupling caps to your output tubes (7591,s in the 800-C). If you are handy with a soldering iron you can get rebuild kits for Fisher receivers fron Vacuum Tube Valley (www.vacuumtube.com).

 

that's great to know, posted on April 2, 2001 at 09:07:30
aol908


 
I am expecting to receive on shortly. Since that's the case, I don't have to worry about ajusting it. Just "plug and play" - Thanks.

 

Whoa, daddy-o!, posted on April 8, 2001 at 19:14:31
funkwrench


 
Just because the bias is fixed on an 800 does NOT mean that you cannot adjust it. It just takes a bit more effort. If you can track him down, you should contact the Fisher Doc and score one of his re-build plans. The plans discuss the rectifier and shunt resistor for your particular iteration of the 800. VTV rebuild plans probably do the same.

The Fisher doc recommends that the bias on these things be increased from about -18 v to -21 to -25 volts (this is from memory, I may be a few volts off). 7591s run fairly cool at that bias, hence the tubes are saved. But since we now have reasonably priced replacement tubes from EH, that may no longer be an issue. I say run em hot.

The point of all of this is that you need to check your bias and replace the bias rectifier if you are under-biased.

Good luck!

joe

 

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