Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: offabit2001 on December 01, 2015, 12:25:01 am
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Hi guys. Was wondering if some of you guys who have way more knowledge then I do, could help me out. I have an old 2204(jcm 800) based amp I threw together a few years ago using chinese caps and junk parts. For the most part it sounds good for what it is. I've since bought a few better amps and this thing is itching to be turned into a power amp. It runs 6550 tubes and has great bass response. So here where I need the help. What to I need to add in front of the 12ax7 phase inverter as an input tube. Since I already have the socket and the 2 extra 12ax7's, that would be the tube I'd like to use. I want to drop the master volume and input in from there. Here is basically the layout from what I remember, give or take. Would you guys please help me with the values. I have 2 rack mounted preamps and would like to run them into it. You can see what i added value wise, but i'm sure they are off. Any help would be greatly appreciated.Thanks. So to some it ---
-Make power amp from old 2204-
-Drop master volume-
-Add input stage using an extra 12ax7-
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this would work. gain of 12AU7 biased as shown is about 7 un-bypassed and has an input threshold of about 20V. it's fairly symmetrical (low distortion) with the B+ indicated. with the .1uF coupling cap, roll-off is around 12Hz.
--pete
EDIT: assume the wiring is as looking at the pot - not from rear as shown. sorry! :icon_biggrin:
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Thank you sir. You guys are great.
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There needs to be a cap between the wiper of that pot and 1M resistor on the board. Otherwise, the pot will mess up the bias on that LTP PI tube.
But you really don't need to add anything except an input jack. Just snip the wire between the Treble pot and the Master Volume. Connect an input jack between the Treble pot and the Master Volume pot. (See attached) This allows you to use the 2204 as before. And when you plug in an external preamp, the 2204 preamp is disconnected.
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fixed. i knew that...sorry brain fart.
--pete