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Old 7th December 2005, 06:06 AM   #1
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Default Fisher 500B OPT/Feedback Questions

A while back , I bought the iron from a Fisher 500B on Ebay (power and output XFMRs). I plan to build a new 30-35W stereo power amp using the iron and a nice quad of EH 7591s. I'm not sure about the primary impedance of the OPTs. I wanted to know this so I could plan out the gain of the amplifier, as I'm going open loop for my first try. Does anyone know the primary impedance off the top of their heads, or will I have to torture the transformers with a sig gen to make them reveal their secrets?

I was looking at the schematic of the 500B for inspiration, and I noticed that the designer returned the cathode side of the phase splitter to the summing node along with the classic feedback network. Can anyone wih more vacuum stte experience than me explain the rationale behind this? Are we talking nested feedback loops here?

What I'm thinking of trying first is a simple open loop setup using a 6CM7 as preamp and splitter, with the high gain triode as a common cathode amp with solid state current source loading, followed by a classic "concertina" style phase splitter using the low gain triode. Next would be a feedback approach using a folded cascode tube/SS hybrid as the first gain stage, and the same splitter.
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Old 7th December 2005, 03:10 PM   #2
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You are going to have both frequency response and damping factor problems with open loop running in pentode mode. Consider triode mode for your 1st breadboard attempt. The 7591 is as easy to drive as a 6V6. So, extrapolating Poindexter's "Musical Machine" is something to consider.

Poindexter's Site

BTW, take a look a the concurrent PP 7591 thread on this site.

PP 7591 Thread
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Old 7th December 2005, 04:53 PM   #3
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I've been following the other 7591 threads, and have gleaned some ineresting stuff from them. Still looking for answers to my other questions, though. If I'm forced to scope out the primary impedance of my output XFMRs, though, I'll post the information here.
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Old 7th December 2005, 04:57 PM   #4
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You can measure the ratios with an ordinary voltmeter, albeit not without some danger and thrills. Measure your line voltage. Connect a line cord to the primary, then connect a voltmeter to the secondary. CAREFULLY plug it in and note the voltage reading. Unplug, then break out your calculator.
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Old 7th December 2005, 05:24 PM   #5
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See this page for more info...


http://www.one-electron.com/trans_p1.pdf
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Old 7th December 2005, 06:41 PM   #6
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I'm not crazy enough to apply line voltage to my precious audio iron, even through a variac. A sig gen at a few kHz and a scope will work just fine, and won't zorch anything. At a few kHz, the loading effect of the transformer magnetizing current shouldn't be an issue. If I was paranoid about this I could also gin up a buffer to go between the sig gen and the transformer, but I'll try the simple approach first.
BTW - thanks for the information - that was pretty much what I seeking. Now about the 500B phase splitter feedback to the summing node...
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Old 7th December 2005, 07:42 PM   #7
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I don't have a print handy, and I also have intermittent Rumzheimers, but it seems that Fisher would often stand a concertina splitter on an asymmetrically large cathode resistor, put a higher positive voltage on the grid with a voltage divider from B+ to grid to cathode resistor and balance out the AC signal with a variable pot/resistor combo at the plate.

So they could operate the first stage at a higher plate voltage and more linear operating point and still direct couple it to the concertina.

I think I remember that..... don't I?
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Old 7th December 2005, 07:54 PM   #8
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Wait, my brain is still on phone modem....loading...

Maybe the concertina had a self bias resistor and the voltage divider went to the junction of that and a larger value resistor from there to ground.

Whatever they did, it was to operate the first stage at a higher than usual plate voltage and use direct coupling to the concertina.

Maybe stage one would be a good place for mu-follower with that trick.
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Old 7th December 2005, 07:57 PM   #9
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Wrenchone,

Are you planning on using a preamp with gain with the new power amp?
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Old 8th December 2005, 12:14 AM   #10
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Maybe we're talking about something else. I've got a 400 and I've had Fishers, here and there, in for repair, including 500s. And I don't remember anything that brought global feedback back to the splitter. That would probably have caught my attention. Or, maybe not.

I think the coolest thing that could happen here, is a schematic graphics program, so people could draw it and put it in with the post. Instead of draw, scan, file and send to image host, etc.

7591 is actually even easier to drive than 6V6. An EL-84 front end with some headroom would do it.
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