Fisher 800 push-pull output stage not symmetrical. Why?

pixpop · 21st January 2012, 05:28 PM

I'm messing with a Fisher Ta-800, and according to the schematic the output stage is not entirely symmetrical. Here's the schematic for one channel:

http://dl.dropbox.com/u/8094054/ta-800.jpg

Note that the output transformer primary is split 90/105, and R97 is different from R99.

Can someone splain this to me? I'm expecting the output primary to be split into 2 equal halves...

TheGimp · 21st January 2012, 05:42 PM

DC Resistance of most OPTs is not equal for both windings unless extreme care is made by the manufacturer.

This is because each successive layer in a transformer has a greater circumference, and hence greater resistance.

payitforwardeddie · 21st January 2012, 05:46 PM

If the primary is wound side by side on a split bobbin, then the the two halves will be identical in number of turns, inductance and resistance. If the two halves are wound one layer on top of another, then as the coil gets bigger, the resistance per turn goes up slightly. So, if you had taps every 25 turns, say, you would be able to see the rising resistance. Its not important. Only the turns and inductance matter. Hope this is Ok.

HollowState · 21st January 2012, 05:46 PM

The slight difference in winding resistance is normal. It happens because of the increased wire length due to coil build-up as the winding pack gets physically larger. As for R97 & R99, I suspect it may be a typo. The schematic looks like a Sams and mistakes are not uncommon for them. It may also be that the particular unit they reversed engineered (assuming that's what they did) just had an off-color resistor. Or maybe even an improper value. I'd say they should both be 1K ohm grid stoppers.

jjman · 21st January 2012, 05:47 PM

I think it's normal to have a difference in DC ohms there. Length of wire can differ (DC ohms) whereas the number of turns (AC performance) would be the same. R97 and R99 are grid stoppers so I wouldn't expect their difference to affect balance although I don't know why they would be different values. Maybe since there is a mystery cap under R97 (on the scheme) and it's removing some high frequencies and the higher R99 is acting as a low pass to equalize. Guessing.

jjman · 21st January 2012, 05:48 PM

4 simultaneous answers.

payitforwardeddie · 21st January 2012, 05:51 PM

So we must be right.

pixpop · 21st January 2012, 05:51 PM

Thank you, thank you, thank you, thank you.

Did I miss anyone?

pixpop · 21st January 2012, 06:27 PM

BTW, I just checked, and the two resistors are indeed different. 1k and 10k.

dcgillespie · 21st January 2012, 08:58 PM

It was very common for Fisher to use two different values for grid #1 stopper resistors, for the sole purpose of helping to control any potential parasitic oscillation tendencies in the output stage.

Dave

21st January 2012, 05:46 PM	#4
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	The slight difference in winding resistance is normal. It happens because of the increased wire length due to coil build-up as the winding pack gets physically larger. As for R97 & R99, I suspect it may be a typo. The schematic looks like a Sams and mistakes are not uncommon for them. It may also be that the particular unit they reversed engineered (assuming that's what they did) just had an off-color resistor. Or maybe even an improper value. I'd say they should both be 1K ohm grid stoppers. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

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21st January 2012, 05:28 PM	#1
pixpop diyAudio Member Join Date: Jan 2005 Location: Los Angeles	Fisher 800 push-pull output stage not symmetrical. Why? I'm messing with a Fisher Ta-800, and according to the schematic the output stage is not entirely symmetrical. Here's the schematic for one channel: http://dl.dropbox.com/u/8094054/ta-800.jpg Note that the output transformer primary is split 90/105, and R97 is different from R99. Can someone splain this to me? I'm expecting the output primary to be split into 2 equal halves...

21st January 2012, 05:42 PM	#2
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	DC Resistance of most OPTs is not equal for both windings unless extreme care is made by the manufacturer. This is because each successive layer in a transformer has a greater circumference, and hence greater resistance.

21st January 2012, 05:46 PM	#3
payitforwardeddie diyAudio Member Join Date: Feb 2011 Location: South Wales, UK	If the primary is wound side by side on a split bobbin, then the the two halves will be identical in number of turns, inductance and resistance. If the two halves are wound one layer on top of another, then as the coil gets bigger, the resistance per turn goes up slightly. So, if you had taps every 25 turns, say, you would be able to see the rising resistance. Its not important. Only the turns and inductance matter. Hope this is Ok.

21st January 2012, 05:47 PM	#5
jjman diyAudio Member Join Date: Jan 2009	I think it's normal to have a difference in DC ohms there. Length of wire can differ (DC ohms) whereas the number of turns (AC performance) would be the same. R97 and R99 are grid stoppers so I wouldn't expect their difference to affect balance although I don't know why they would be different values. Maybe since there is a mystery cap under R97 (on the scheme) and it's removing some high frequencies and the higher R99 is acting as a low pass to equalize. Guessing.

21st January 2012, 05:48 PM	#6
jjman diyAudio Member Join Date: Jan 2009	4 simultaneous answers.

21st January 2012, 05:51 PM	#7
payitforwardeddie diyAudio Member Join Date: Feb 2011 Location: South Wales, UK	So we must be right.

21st January 2012, 06:27 PM	#9
pixpop diyAudio Member Join Date: Jan 2005 Location: Los Angeles	BTW, I just checked, and the two resistors are indeed different. 1k and 10k.

21st January 2012, 08:58 PM	#10
dcgillespie diyAudio Member Join Date: May 2010 Location: Ball Ground, GA	It was very common for Fisher to use two different values for grid #1 stopper resistors, for the sole purpose of helping to control any potential parasitic oscillation tendencies in the output stage. Dave

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