Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: serbay662 on November 10, 2020, 07:49:14 am
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Hello to everyone
I'm doing the restoration of the amplifier.
There is missing information in the diagrams I marked. Can you help with this?
I am also open to suggestions for updates for this amp.
Thanks in advance.
http://b.baldach.free.fr/farfisaFR40/FarfisaFR40_Schematic.jpg (http://b.baldach.free.fr/farfisaFR40/FarfisaFR40_Schematic.jpg)
(https://i.ibb.co/rG8GsPb/Inked-Whats-App-Image-2020-11-03-at-20-11dsd-06-LI.jpg)
(https://i.ibb.co/10tt7CH/Inked-Whats-App-Image-2020-11-10-at-11-00-40-LI.jpg)
(https://i.ibb.co/sjyLh8x/Inked-Whats-App-Image-2020-11-10-at-11-00-47-LI.jpg)
(https://i.ibb.co/6Wx2wqy/Inked-Whats-App-Image-2020-11-10-at-11-01-04-LI.jpg)
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Welcome.
Tone networks are a lot about ratios. We see this one has some parts about 20 times lower impedance than others. The pocked cap is likely about 20 times the one above. Not fussy, use nearest 20% value, or whatever in in your drawers and close. (Yes, the "250p" is wrong, the answer is right.)
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The G2 node resistor is 470r-1K on nearly all 2*EL34 amps, so why not here?
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The missing mix resistor can probably be same-as the other mix resistor.
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The cathodyne looks screwy but seems to be correct As-Drawn. The gain-stage V5a is bootstrapped from the V5b cathodyne cathode, boosting gain. Merlin may have a clearer drawing on his site.
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Frikkin' Adobe Flash Update/UnInstall......
Not relevant here except I needed a very different program, which has gone bad, so needed a reboot to flush Window's tiny pipes, and Flash came up telling me to kill it. Gladly!! But will it stay dead? Or keep on being the #1 back-door on the internet?
Anyhow I got my PDF 'printer' back to 2007 spec (working) and here is the 'fisa plan mildly cleaned.
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The G2 node resistor is 470r-1K on nearly all 2*EL34 amps, so why not here?
i see K5 so maybe was 1.5K ?
--pete
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Maybe. Won't be big difference?
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The G2 node resistor is 470r-1K on nearly all 2*EL34 amps, so why not here?
Thanks for your answer, but I'm new to tube amplifiers. :dontknow:
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The G2 node resistor is 470r-1K on nearly all 2*EL34 amps, so why not here?
i see K5 so maybe was 1.5K ?
--pete
I alway see European writing are 1K5 for 1.5 k
So K 5 = 500 ohms
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...European writing are 1K5 for 1.5 k
So K 5 = 500 ohms
500 would usually be "500"
I agree Pete has a point: "K5" is likely 1k5 with the "1" lost to time and old paper.
And it is not critical. Less than 500 might buzz a bit. Over 3k will give a "saggy" quality which may be appropriate in a Farfisa. Much over 3K (like 10k) will significantly reduce maximum power.
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Maybe. Won't be big difference?
a few volts more drop, as you state.
my thoughts when i saw partial G2 value in the schema were farfisa took published conditions to the literal. please see attached.
--pete
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...European writing are 1K5 for 1.5 k
So K 5 = 500 ohms
500 would usually be "500"
I agree Pete has a point: "K5" is likely 1k5 with the "1" lost to time and old paper.
And it is not critical. Less than 500 might buzz a bit. Over 3k will give a "saggy" quality which may be appropriate in a Farfisa. Much over 3K (like 10k) will significantly reduce maximum power.
You could be right