Wednesday, January 9, 2013

Ampex 601 2 Ch Brute Force Preamp Conversion w/ Altec Iron

 Tablebeast TB-A601 Ampex 601 2 channel with Altec 4722 and 15095:

This is my new king of the hill. What an awesome, AWESOME preamp this turned out to be. I started with an Ampex 601 reel to reel tube electronics module. I then gutted and dismantled the whole thing. I separated the parts from stuff I planned to reuse and stuff I planned to discard. Of the discarded some parts, some are kept for other projects and spares while others are just thrown away. Sprague bumblebee caps? Yeah, I keep those when the values are not right for the conversion, you bet! Old cap cans and ALL the wiring? In the garbage it goes!
The next step is to prepare the chassis for my intended design. I rearrange the tube sockets in my needed layout and add a few sockets as needed. Then I install the new Weber cap cans with mounting brackets. The back panel gets all the Switchcraft chassis-isolated jacks, XLR input and pad/phase switches for channel 1 and pad phase switches are also added to the channel 2 input on the front.
When considering iron for this unit, I chose some of my favorite affordable and readily available vintage transformers. The Altec 4722 and 15095 were what I went with. I wired the inputs so that they will take the 4722 of course, but it will ALSO accept a 15095 or 15335 as an input depending n application. The outputs get 15095 plugs, but can also accept 15335 units as well. This makes the octal plug-in format function as Altec originally intended, where you can swap out the transformers easily to modify gain staging and impedance matching depending on what you stuff in there. Oh yeah, they also sound freaking INCREDIBLE!
The tubes in here are the 5879 Pentode for preamp duties, 12AU7 for line amp/output duty and the classic 5Y3 providing voltage rectification. They are set up as a pair of my Brute Force preamps which have insane amounts of gain. They are perfect for ribbons with low noise and the high gain, but really I have yet to find a mic that I don't like with it. Of course you can just turn the gain down when using a hot mic and if that doesn't get you in the sweet spot, engaging the pad and turning back up will get you there.
The inputs and outputs give you a LOT of different options. Of course it has balanced XLR inputs for mics and 1/4" DI inputs for instruments or other hi-z sources, but this unit also has dual outputs per channel. One output is tapped before the output transformer and is connected to an impedance balanced TRS 1/4" output jack. It can be used simultaneously with the transformer balanced outputs that are also on TRS 1/4" jacks. Now the impedance balanced outputs are HOT and I LOVE hitting an actual tape machine with this output as it will roast that tape if desired. The transformer on the output has a stepdown that knocks the signal down while balancing and adjusting impedance as is my preferred output for getting maximum coloration as it allows yo to crank the circuit a bit more.

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