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Improving Fisher's Heater String Cathode Bias System

Hello to all and a belated thanks for the help with my KX-100. I finally got around to doing the Zener modificaiton. I was hoping someone here could take a look at my numbers and see if they're reasonable:

C1 466v (440 spec)
C2B 400v (365 spec)
C3A 389v (350 spec)
C3B 377v (340 spec)
C3C 209v (185 spec)

Output tube current

V7 32mA
V8 31mA
V9 60mA
V10 51mA

V7/V8 Screens/Plates 400v/462
V9/V10 Screens/Plates 399/460

Thanks
 
Output tubes V9 and V10 are too high. You need a better matched pair to the other pair or install an IBAM/IBBA.
 
Thanks so much. I had a feeling this was a problem. If I install IBAM/IBBA, what's a good target for output tube current?
 
junkjer said:
Thanks so much. I had a feeling this was a problem. If I install IBAM/IBBA, what's a good target for output tube current?
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Same as your v7 & v8, they look pretty ideal from here, from my limited knowledge. Others may concur.
 
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I'm at
junkjer said:
Thanks to you both. Given current tube prices I'm inclined to go with IBAM/IBBA so now I must choose between those two mods.
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I'm in same boat. Haven't tackled any of my 4 projects yet (2 ea. 500c , a 400 & an x-101-d) . I need to make up a master list of all parts needed for necessary mods to all & make a mass order. I have a 1 lb. coffee can full of domestic-production 7591s, plus what's in the unit's on hand & I don't want to toast any due to shrinking supplies/ escalating replacement costs. Projects delayed due to multiple hobbies & reasons, I guess.
 
The 7868 and 7591 are literally the same tube with different pinouts. What works for one usually works for the other. I've put both IBAM and IBBA in 400's, and 800c's with no change in the material list on Dave's diagrams. I also have a Sansui 1000A (7591) with an IBBA. No change to materials either and works great.
 
Thanks for clearing that up for me. A couple more questions as I prepare to install this. In Dave's original schematic for the IBBA mod, the electrolytic caps are installed with + to ground. I assume this is because the machine he was working on used negative filament supply voltage. This is not the case in the KX-100, so am I correct in thinking the IBBA caps' negative leads should go to ground?

Also, am I correct that in the KX-100, IBBA replaces R70 and R69, so those two resistors can be removed. But does C4 (100uF) remain?

And, is there any issue with having already installed the Zener mod?

Thanks again!
 
Thanks for a great improvement again Dave. I’ve done this twice and it’s always gratifying when it plays after you’re thru. I consider myself a parts replacer not a diagnostician. Thanks for all you guys help.
 

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Cat. Change out R74 (450Ω) and replace with zener's. Otherwise should be same.

From a "D" schematic. The manual is on Fisherconsoles. Arrow is direction of Cathodes.
ScreenHunter 704.jpg
 
larryderouin said:
Cat. Change out R74 (450Ω) and replace with zener's. Otherwise should be same.

From a "D" schematic. The manual is on Fisherconsoles. Arrow is direction of Cathodes.
View attachment 2775016
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Outstanding! If not for being against post office rules, I'd mail you a beer, Larry. I'll research which ones to use, I might even have some in the reclaim or n.o.s. stash.
 
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