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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: ampex preamp posted by thermionic emitter on April 15, 2007 at 19:01:21:
T.E.This sounds like an Ampex 402, and I have one. It dates from the early 60's, and sold for some $180 in my 1962 Allied catalog (only the Marantz preamp was more expensive and it came with an electronic crossover). Quite a sum back then. Definitely get the Sams schematic Waldenpuddle linked to. There are two versions of the phono section in the schematic, neither of which is exactly like mine, so there may be more variants too. If the ganged volume control works well you're in luck. I have'nt been able to source a drop in replacement (dual ganged 500K with a 40% tap). The schematic shows a rather complex volume compensation circuit which works with this pot (and which is not switchable in and out) and my unit is different here from the schematic too. Plan on swaping the selenium rectifier for a modern diode bridge. I recently measured the response on mine, and found that the bass rolls off around 50 Hz, and the centers of the tone controls are not accurate. The tone control pots can be rotated to center them better, the bass on mine needs some boosting while the treble needs some cutting. My ears had been telling me this all along. Make sure you have the tubes located in the correct locations, as swapping a 12AX7 with the one 12AU7 will give poor sound. This is easy to do, mine has no markings on the chassis, just a sticker on the cover. Notice that the 12AU7 output section is the controversial cathode follower, which might benefit from a solid state current source as per Allan Wright's Tube Preamp Cookbook, but I have'nt gotten this far with mine.
Good luck!
Paul
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Follow Ups
- Re: ampex preamp - Paul Eizik 11:20:33 04/16/07 (2)
- Re: ampex preamp - waldenpuddle 11:12:54 04/17/07 (1)
- Re: ampex preamp - Paul Eizik 20:42:48 04/17/07 (0)
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