Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Ed_Chambley on April 17, 2012, 03:26:23 pm
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I found one of these in really nice condition and seems like a wonderful PA amplifier. Thing is clean as a pin and powdered up right away. I attached a preamp and speaker and it plays great. It does need a little cleaning of pots and some caps, maybe all need replacing. Anyone ever messed around with one of these. I am trying to figure out what to do with it. It is actually a 4 channel amp with 2 rectifier tubes. Anyway, I have heard these old PA amps can be made to have a guitar input fairly easily. Open to suggestions.
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I'm a a Stromberg-Carlson fan. I've got 3X Stromberg-Carlson APH-1100's mono blocs powering the front speakers in my home theater system; and rebuilt an Stromberg-Carlson ASR-444 for my son. Vintage PA amps, like your AU-57, with the thin-leaf transformers are usually great for hi-fi use.
Checkout this site: http://www.audiophool.cjb.net/MadeInRoch.html (http://www.audiophool.cjb.net/MadeInRoch.html) Great site to roam around in. Enjoy your amp!
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where i'd start... i attached the original and a marked up schematic.
B+ values are a best guess; moving the pre-amp B+ chain to the 425V rail will add ~100V across the board, but will have a greater load, so may be less than 100V, but then again i'm recommending that we lose two bottles, so may be more.
screen voltage (335V source) will rise some with less load - no worries.
disconnect/remove V1 and V4 and associated ckts/pots/jacks -
change mix resistors from 100K to 470K - try it with 100K first, move to 470K for a bit more gain.
removed R23 & R30 and changed R21 & R28 from 47K to 100K. experiment with R22 & R29 values - you may like them as is, change both, or just change one for some differentiation of character between the channels. you may wish to bypass R26 and/or R33 for more gain - experiment.
you may need to diddle with R57 (15K) to bring PI back into balance.
changed R65 to 10K; it was 39K.
i have not run a sim on the PS; again, please don't be surprised that B+ values are not what i estimate.
--DL
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I'm a a Stromberg-Carlson fan. I've got 3X Stromberg-Carlson APH-1100's mono blocs powering the front speakers in my home theater system; and rebuilt an Stromberg-Carlson ASR-444 for my son. Vintage PA amps, like your AU-57, with the thin-leaf transformers are usually great for hi-fi use.
Checkout this site: http://www.audiophool.cjb.net/MadeInRoch.html (http://www.audiophool.cjb.net/MadeInRoch.html) Great site to roam around in. Enjoy your amp!
Very cool site. Thanks.