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Old 1st August 2007, 03:40 AM   #11
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Haha, after I re-read things, I wasnt getting snippy about the rectifiers, I hope nobody took it like that. I think I was just excited.

I was referring to the phono stage in the Eico when I said I liked it, just noisy... not the Fisher, I was making a comparison in the heater supplies.

I shouldnt mix and match things so much I guess. I have not formed an opinion on the phono stage of the Fisher yet, although it is definately very quiet, but I have to get the amplifier sounding good before I can decide how it sounds.

Aaron
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Old 1st August 2007, 04:09 AM   #12
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Guys,

Check the data sheets. The 7189 is rated for the same plate dissipation as the 6BQ5/EL84, but the 7189 can take a higher anode voltage. The 7189A is rated for both higher plate voltage and higher plate dissipation than the 6BQ5.
6BQ5 Data Sheet
7189 Data Sheet
7189A Data Sheet

Tube World is asking (sic) $175 for Toshiba factory matched 7189A pairs. That's $350 for a new set of "finals". Even at the high price, the rare and out of production 7189As will go fast.

Jim McShane charges $21.50 for a matched pair of EL84Ms, which are 7189 equivalents. IMO, making the changes necessary to use 7189 equivalents is obvious.
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Old 1st August 2007, 01:27 PM   #13
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Some fine print on the data sheets... 7189 is rated for 400V and 12W "Design Center Values". 7189 is rated 10% higher for both, but it's "Design Maximum". These are just about equivalent ratings.

The 7189A DOES have connections defined for two pins that are "IC" - internal connection - on 6BQ5 and 7189. Since these pins are defined as "Do not use" on the 6BQ5, it should be OK to use 7189A for 7189 (and vice versa). I believe the Russian ones DON'T use these pins anyway - and not all manufacturers were consistent - tube testers may or may not show "short" on pins 1, 2, 6, 8, depending on brand.

I have a set of the Russian 7189s in a Lafayette receiver, and they seem to be happy at 420V or so...
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Old 1st August 2007, 09:23 PM   #14
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I was not disagreeing about the specs. I wonder if Fisher's rating of 20WPC was high? Should it actually be the 17.5 Watts?

As I said, everything is stamped 7189, including the chassis where the tubes go, rather than 7189A... and all of the literature, schematics, etc reflects this. However, it also all says 20 watts per channel.

I guess my question is this. I don't care if it'll only do 10 watts per channel. If its really only 17.5... then that's perfectly fine. What I am wondering is if there is an ERROR... and either it should be 7189As, and I DO need to modify things to use the tubes that are in it (which I kind of doubt)... OR- can I just leave the parts values alone and replace the caps and enjoy the stereo.

I never said it had 7189As to begin with. Someone based that on the specs of 20 WPC, which makes perfect sense to me if you look at the Data sheets. But I believe the amp was designed to use plain 7189s, because it has the original outputs, which are Fisher branded 7189s, the chassis SAYS 7189, as does all of the paperwork. So I'm confused as to why I'd need to modify anything (?)

Perhaps I'll scan the schematic and attach it at some point in the near future.
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Old 1st August 2007, 09:50 PM   #15
anatech is offline anatech  Canada
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Hi Aaron,
Quote:
I was not disagreeing about the specs. I wonder if Fisher's rating of 20WPC was high? Should it actually be the 17.5 Watts?
Probably. The rating in the tube manual is at a certain distortion with no feedback. Fisher is using feedback and may rate theirs at a different distortion rating. So it may be the same thing. Don't worry about it. Besides, you will not hear a difference between 20 watts and 17.5 watts.

Quote:
So I'm confused as to why I'd need to modify anything (?)
I think the previous post indicated that no mods were needed. Relax.

Quote:
Perhaps I'll scan the schematic and attach it at some point in the near future.
that would be great! Would you mind sending me a full resolution copy please? bhomester at gmail dot com.

-Chris
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Old 2nd August 2007, 12:38 AM   #16
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Aaron,

FTC regulations were not as tough in 1959 as they are currently, when claims about the performance of consumer electronics are made. It seems Fisher was "creative".


Quote:
I'm confused as to why I'd need to modify anything (?)
7189 plate dissipation should be limited to 12.5 W. per tube. It's a matter of reliability and service life. Changes in the operating conditions may be needed to stay inside that limit. If the operating conditions change, an external DC filament supply is needed for the phono section tubes.

Regulating 7189 screen grid B+ is something you can look into, as linearity will improve.
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Old 2nd August 2007, 02:10 AM   #17
anatech is offline anatech  Canada
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Hi Eli,
Does the current circuit operate the outputs outside this safety limit? I can't say as I have no information on this model.

-Chris
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Old 2nd August 2007, 03:06 AM   #18
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According to the spec sheet, the 7189 can run as high as a 400V plate voltage, this amp runs them at 380V. (According to the schematic... I still havent had a chance to take it apart and probe around in there.) This weekend I'll get to doing that, probably Sunday afternoon. I'll have to check and make sure it's not running too hot..

It could well be, mains voltage here seems to hover around 120 volts. If it was designed to operate at 115 or 117, chances are its running a bit on the hot side, and fast approaching the 400V.
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Old 2nd August 2007, 03:12 AM   #19
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Quote:
Originally posted by anatech
Hi Eli,
Does the current circuit operate the outputs outside this safety limit? I can't say as I have no information on this model.

-Chris

I don't know. I too am hoping to get my mitts on a schematic. Fisher was not stupid. It's quite possible that all is well, electrically.

The advertising copy has me wondering what is really going on. "Hype" aside, that IS a good looking unit. After overhaul, I expect it will also sound GOOD.
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Old 2nd August 2007, 03:14 AM   #20
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Fisher often operated the phono tubes below nominal heater voltage, for lower noise. If they ran 130mA , that would be about 13W per tube (guessing 422V on plate and 22V on cathode).

A matched set of output tubes is VERY important in this amp.

I'd also be interested in a scan if you get to it.
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