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Fisher EFB thread

RS Steve

RS Steve

Tube Junkie
Subscriber
Let's create this thread for the purpose of having one source for proper information concerning the installation of Dave Gillespie's EFB modification for Fisher tube equipment. I suggest we keep the unneeded chatter down and just post verified as "correct" schematics, pictures, and advice for the installation in different models. If this works out, maybe Dave can chime in with his experience and helpful suggestions. We can always weed out posts that are just for establishing the threads function and understanding as we go forward. But what we need are complete posts containing all the information for said model to limit confusion. We can always confer via PM to get things correct and verified before posting here, that way the thread doesn't go on forever. We want the information easy to find and not taking hours to read through.
 
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This looks like one of the final versions of the EFB installation into a 500-C. This schematic had other modifications also, so some of the shaded areas do not pertain to the EFB.

500-C modded schematic EFB only.jpg
 
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Here is the modification needed to convert a Fisher 500-C EFB board so it works in the Fisher 400, two resistors circled.

Fisher 400 EFB changes from 500c version.png
 
Hi Steve,
The circled in red are the two components I changed for my 400. Having started with one of your 500c boards, I'm not quite certain the values you used for the BIAS pot and 39k resistor. It adjusted just fine to a good voltage going into my IBAM board so it worked just fine. Just thought it would be good to document the values you used on your pc board.
Thanks so much, my 400 sounds great.
John

edit: I just saw your 500c EFB schematic in another thread which shows just a 100k pot in the bias adjust portion of the circuit.
 
I've been trying to understand better Dave's EFB and what component values need to be changed to modify the board for each particular Fisher receiver.
I came up with this schematic which contains the modifications Dave has listed in the forums as I understand them. I've listed component values for early 400, late 400, 500b/800b, and 500c/800c.

Any comments or corrections would be appreciated.

Schematic Fisher Universal EFB.jpg

Schematic revised twice to reflect corrections listed below.
 
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Already found one error: "GND: to output tube screen grid only" should read "GND: to ground"

Corrected in schematic above.
 
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Thanks Steve and gadget. Schematic above has been corrected.

Why are the dumbest mistakes the hardest to find?
 
I have been using a 100K pot in place of P and R2 on my board, allows a very good range of adjustment for bias needs in different models and tube brands..
 
Thanks Steve. The 100K pot looks like it would simplify things for all the models I've listed.

I'll add it as an option on the next schematic revision.
 
Here's the amended schematics, that I redrew, showing the entire DCG mod (input simplification, revised phase compensation, cathode feedback, power supply upgrade, and EFB) for the Fisher SA-300:

sa-300_audio_circuit_gillespie_mod_900.jpg


sa-300_power_supply_gillespie_mod.jpg

Verified working.

sa300_mod_underneath_10-20-2023.jpg

ALL of these mods should be executed SIMULTANEOUSLY- this upgrade should be considered a single package. Each part relies on the others, to work correctly.

EDIT: Just learned from Dave Gillespie- that the STF12NK60Z MOSFET can replace the now-obsolete STF10NK50Z, in any EFB screen regulator. Drop in replacement with even higher voltage and current capacity.

Regards,
Gordon.
 
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GordonW said:
Here's the amended schematics, that I redrew, showing the entire DCG mod (input simplification, revised phase compensation, cathode feedback, power supply upgrade, and EFB) for the Fisher SA-300:

View attachment 3016992


View attachment 3016981

Verified working.

View attachment 3016982

ALL of these mods should be executed SIMULTANEOUSLY- this upgrade should be considered a single package. Each part relies on the others, to work correctly.

Regards,
Gordon.
Click to expand...
Nice Gordon, many thanks!
 
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