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The Fisher Custom Electra IV 440A

Joe Willis

Joe Willis

Active Member
Hi, just picked up a fisher 440A console, el84 Push pull. she's a beauty. I'm going to restore as a console on this one.

here is a link to the service manual:
http://fisherconsoles.com/service%20manuals/Fisher%20custom%20electra%20iv%20e44%20sm.pdf


Before I get going, couple of questions, first any suggestions to changes in circuitry that I should consider.

Should I keep coupling caps the same size? (.047)

C106 AC Line suppression cap. No value is given in service manual(that I can see) I'd like to replace with new x1/y2 safety cap. Anyone know what size that is?

What diodes would be best to replace the voltage doubler circuit?

I'd like to increase capacitance in the power supply. Is there a max capacitance after the diodes like there would be with a tube rectifier? Suggested values and locations to increase power supply capacitance?


that should be a good start.
Thanks as always.
pics to follow.
Joe
 
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An 0.01uf safety cap would do fine. Not sure what the original value is, but thats a typical value on a lot of gear.

UF4007 or 1N4007 would be fine for the voltage doubler. UF4007 should be slightly cleaner, but if you use a 0.01uf 630v film cap across the 1N4007's it works out about the same. The stock ones are probably still OK though if you don't really feel like messing with them.
 
The original coupling cap size of .047uF into 330k ohms provides a -3dB point of 10Hz. That seems appropriate for the amp.

I would limit the first PS cap to 200uF. You could add extra filtering by adding an RC filter to the PS input. You could try 47uF -> 100 ohms -> 200uF and then take the B+ for the output tubes from that cap.
 
I did a 440A as a stand-alone and tried to keep it as close to stock as possible (big-ass power resistor for the bias circuit excepted). The OPTs are pretty small so going with bigger coupling caps is not going to net you much, as is stiffening the power supply with bigger caps. Quality parts (the resistors are really good quality so keep 'em if they are within spec) are a better use of your time and money. Sprague 716p film caps work really well in this application, IMHO.

-D
 
enough said! Thanks, I placed my parts order. Just sticking with the basic spec. time to start cleaning her up while I wait for the parts.
 
Ok, Power amp is working, not sure about the pre side of things yet no sound when source connected through pre but it works when I plugg into the power amp directly.
So moving on, what would the proper procedure be to bias the el84's via the bias adjustment pot. I think it should run at about 150ma for the quad. I haven't installed any cathode resistors and am wondering if I can just take a voltage drop across R125. or is there some built in test point I'm missing?
Joe
 
Pin 3 should be right around 44V as there are 4 tubes on the preamp that are fed from here. Follow the schematic to Pin 6 (for the POS + side) and to Pin 9 (gnd or NEG - side) of the circular 9 pin connector. There should be 44V there. Adjust to get 44 volts across the heaters. Pin 2 should then fall into line at about 33V.



My 481A in my CE VII has 12ax7's so the voltages are different @ 21V Pin 2 and 33 for pin 3.

Larry
 
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