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 Post subject: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 5:11 pm 
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The Fisher 600T has two different circuits for the amplifier section, one with an adjustable bias one without

The adjustable bias was removed in the last production runs

Which circuit is best suited for the amplifier section?


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amp_schem.png
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amp_600T.jpg
amp_600T.jpg [ 211.5 KiB | Viewed 442 times ]


Last edited by Omer on Oct Fri 13, 2017 5:30 pm, edited 2 times in total.
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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 5:16 pm 
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Please post a link to the relevant schematics----or just snips from the relevant sections.

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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 6:11 pm 
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Can a NTE5801 or 1N5801 relace the 2N2326 diodes in the biasing circuit?


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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 7:17 pm 
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2N2326 is not a diode....

Going back now to look at the schematics....

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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 7:29 pm 
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I doubt if there will be any noticeable difference in performance. In principle, you can get the bias more accurate with the earlier circuit, but the diode approach gives some temperature compensation.
There's a good chance that the change was made simply to save money.

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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 9:54 pm 
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Looking at the two schematics, I see that the older one had the adjustable bias and the newer one has a zener diode as part of the replacement circuit. This looks like an improvement made by a technological innovation.
The improvement mainly helps in manufacturing. No one need adjust the bias any more, so changing from the manual adjustment to the zener diode doesn't make sense unless the adjustment is off and you don't know how to get it back.

Best regards,

Bill


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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Fri 13, 2017 11:35 pm 
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**zener** diode? Where?

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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Oct Sun 15, 2017 1:04 pm 
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It's a 1N2326 germanium temperature compensating diode...not 2N2326 and not a zener.

http://www.datasheetlib.com/datasheet/8 ... state.html

Dave


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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Dec Sat 02, 2017 1:47 am 
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After restoring my 600T recently, the voltages for the amp ( with thermal diodes, 1N2326) section gave different readings than are in the schematics, when I was listening to 8 ohm speakers

The thermal germanium diodes 1N2326 gave Vf readings on my Fluke 115 diode tester section between 0.126 to 0.131 Since my experience with silicon diodes have always given me a consistent forward bias readings, I was considering putting a 1K trimpot, with the wiper connected to the anode, one of the sides of the trimpot connected to the cathode, and the other end connected to where the anode was connected I am thinking that this would make the forward bias more accurate per output transistor

I am also wondering if the 10 ohm resister in the circuit, should I return the 10 ohm trimpots?

Also, how would one adjust the bias, and what voltages should I be looking for?

Any thoughts would be immensely appreciated


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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Dec Sat 02, 2017 1:51 pm 
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I'm not sure what you mean by a "thermal diode". What you have is a regular Ge diode which is used for temperature compensation. Are you tuned in to how that works?

Theory aside, you've presented two different circuits that were used in this model. I'd be inclined to stick with on or the other. That said, there is certainly no harm in replacing a fixed resistor with a pot....assuming you know how to set the pot.......which brings us to an actual answer to one of your questions..:)

The bias for an output stage like this affects the crossover distortion. It can also affect the quiescent power dissapation, which in turn affects the operating temperature. I think manufacturers typically specify where to set the bias in terms of idling (quiescent) collector current, based on lab tests to determine the best balance between distortion and safe thermal design.

If you don't have the specs on setting the bias, then you need to set up some kind of distortion measurement...using a scope or a distortion analyzer. When you find the basic relationship between bias and distortion, then you need to do the thermal stability analysis to determine the best compromise.

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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Dec Sat 02, 2017 7:48 pm 
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Thank you for explaining the function of the temperature compensating diode

However, there are many people out there , that think that they can shove a Schottky diode to replace it , or return the trimpot and go back to the first generation circuitry

I have now decided to replace the diode, with an AC125 germanium transistor and a 1K pot


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 Post subject: Re: Biasing Circuit in the Amplifier in a Fisher 600T
PostPosted: Dec Sat 02, 2017 11:26 pm 
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Whatever floats your boat........:)
certainly beyond my competence level.

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