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How to figure out the bias point of fisher X-100A

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7n7is

7n7is

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Joined 2006
sciliterature.50webs.com/RelativityDebates.htm
I made a stereo preamp based on the circuit in the picture which is based on a Mullard preamp. I only used the phono and tape head EQ networks and left out the bass and treble controls. I'm using regulated DC on the 6267 filaments which was the only way I could get rid of the hum. I tried a hum balance control, but it would only cancel hum in one channel depending on how it was adjusted. One possible way is to use separate filament transformers for each preamp channel and use separate hum balance controls for each channel.
 

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K

kenpeter

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Joined 2007
I too own a fisher X-100-3 with 7189 sockets.
Currently stuffed with JJ EL84, and working fine.

I don't know if its an X100A or not? No letters
to indicate, and this thread is the first I've ever
seen a proper schematic diagram that matches
the 7189's that I appear to have.

Other X100 had different output tubes, those
schematics I already had at least two different
versions.
 
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7n7is

7n7is

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pchw

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Joined 2006
Mine is definitely different from 7n7's, not just the look. It uses a voltage doubler, not GZ34. The manual says X-100-3 and X-100-A. I suppose they are the same. Attached is the complete service manual that I have. I hope someone will find it useful.
 

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john29302

john29302

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Joined 2009
I got that same amp......just fixed it...the cap and resistor feeding the cathode/heater voltage divider thingee....i at first relaced the cap and a nice plume of smoke and loud whizzing sound was really something to behold. later i replaced both the resistor and the cap....evidently when the resistor opens the voltage is sucked down the cap. it wasnt as exciting as the fireworks though. anyways it sounds great...all made in holland fisher tubes. note the balance control doesnt work like other amps. no channel is muted out L or R. and hey....v1 and v6 are the phono premp/eq sand are the series strung cathode bias circuit.......so how you remove them and play phono?? and you thought they were bass/treble...no...how would removing them allow you to still have bass/treble?? what thats been smoking
 
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kenpeter

Member
Joined 2007
Before finding this thread, the only x100 schematic I had, showed 7591's.
Which is completely bizarre, cause octal, not even the same size socket.
I'll dig and see if I can find that drawing...

I've got 9pin 7189 sockets with JJEL84's in actual X100 amplifier I've got.
Evidence to believe 7591 variants with big octal sockets must exist too?
If so, doubtful they could bias the same...

But I don't think its the "wrong amp".
 
john29302

john29302

Member
Joined 2009
right...fisher fooled many on these x101 and x100. console units vs. stand alone amps ...not sure what to think myself. 7n7is shows a picture of what i got...and sams photphact has a picture of it on the x101 photofact folder. i have the x101b folder as well, and an amp. x101b is 7591, which is not gold on front and whose phono preamp is not sitting sideways (laying horizontally) and he probably converted to the 7591xyz when these nos 7591 became scarce. heck i gave away 2 recievers thinking tubes were never coming back. who new they are cheaper now than then? anyway both are great. my x101b needs a power tranny....anybody know which way to turn...actually hammond makes on .75 an inch taller and to get rewound is like 225 plus shipping. yikes
 
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