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Fisher 500C biasing

CSkoo

CSkoo

New Member
Hi,

I got a nice serviced and recapped Fisher 500C from ebay recently as entry to tube world. It comes with a quad of EH 7591 output tubes. If I chang the tubes next time, let say Tung sol or JJ tube, do I need to adjust the bias? As the seller told me that no biasing pot is available and the Fisher 500C is fixed bias. Any tubes that I change need proper bias?

I am a newcomer to tube world. As I read from the Audiokarma, many people say that Fisher 500C is a very good receiver, that why I choose this model.
 
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short and sweet of it, yes most likely new production tubes need adjusting, and no in stock form there is no way of doing that. You can however add the adjustments and a means of checking things, which I would personally say is a good thing to have.
 
Means that I need to look for a tech to add the bias adjust pot and to adjust the bias regularly?:rolleyes:

Other tube such as 12AX7 replacement need to bias after the tube replacement?

If I change back to EH tube as before, do I need to do the bias?

I am not familiar with those tube electronic nor the electrical thing. Without knowledge in this area it is very dangerous to do it:eek:!

My Fisher 500C has been properly bias by the seller and the selenium rectifier also has been replaced. It can be enjoyed for a period.

How we know that when the tube need to be changed? And which tubes need to be changed? The manual of Fisher 500C does not mention much about the tube.
 
I added the 10k ohm bias pot and it seems to work great. I do have a question about how I installed it though.. I was confused with the THREE legs, was hoping for a simple TWO legs. I used the singled out one and then one of that appears to be in a pair. I cut the 3rd one off. Is this correct? or have I messed up again?
 
Best to connect 1st and center leg, looking at it from left to right with the middle leg on the upper side, the left and center leg together, leaving the 3rd leg single. Then connect the input side to the doubled leg, and the single is output. This is considered now a variable resistor. Pin 2 is the wiper.

10k pot.JPG
 
CSKOO; I would recommend either the EH's (good tube with a large envelope) or the Tungsols (more expensive but still good). JJ's are junk AFAIC. Too many Infant mortality failures in the past. Some guys use them but I don't recommend them in any fisher product unless you have an IBAM board and bias them cold and bring them up to 70%of Max dissipation over a matter of weeks. Still there will be failures IMO. Read this examination of the NEW STOCK Tubes vs. Old Stock 7591's. EH's and Tungsols come upwards of 90-99% but the JJ's fall fairly short. http://www.tronola.com/html/7591a_tubes.html
 
larryderouin said:
Best to connect 1st and center leg, looking at it from left to right with the middle leg on the upper side, the left and center leg together, leaving the 3rd leg single. Then connect the input side to the doubled leg, and the single is output. This is considered now a variable resistor. Pin 2 is the wiper.

View attachment 1222375
Click to expand...
larry - need a little more clarity. attached is the pot I am using with legs numbered. You saying connect 1 and 2 together? surely not 2 and 3 together... ?potentiometer.jpg
 
Either Way is fine. All it does is reverse the way you turn it for biasing up or down. 1&2 will give you MAX Resistance counterclockwise. 2&3 Clockwise. Input on the jumpered legs.
 
larryderouin said:
CSKOO; I would recommend either the EH's (good tube with a large envelope) or the Tungsols (more expensive but still good). JJ's are junk AFAIC. Too many Infant mortality failures in the past. Some guys use them but I don't recommend them in any fisher product unless you have an IBAM board and bias them cold and bring them up to 70%of Max dissipation over a matter of weeks. Still there will be failures IMO. Read this examination of the NEW STOCK Tubes vs. Old Stock 7591's. EH's and Tungsols come upwards of 90-99% but the JJ's fall fairly short. http://www.tronola.com/html/7591a_tubes.html
Click to expand...
Thank you Larry!
 
ducati2 said:
larry - need a little more clarity. attached is the pot I am using with legs numbered. You saying connect 1 and 2 together? surely not 2 and 3 together... ?View attachment 1222383
Click to expand...
Are you sure that pot is adequate for the intended use? If the contacts in the pot burn out there will be no regulation on the7591 tube and it will go Chernobyl on you.
 
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