Archive for Ampex

Vintage Tube Mixer Direct Ouputs (RCA, Ampex, Altec, Etc.)

Posted in Modded with tags , , , , , , , on July 30, 2013 by AnalogAudioRepair

Not a lot of great pictures this time, but something I do get requests for.  There are quite a few nice tube mixers around that sum two or more inputs into a single output.  Often folks would like to use these as individual mic preamps with their own outputs.  Your only choice is to use them as a single channel pre, or add some sort of direct out.

The issue is that these preamps use a single tube (or half of a dual triode) to amplify each input, then one or more tubes for EQ and output stages.  Of course it would be costly to add identical output stages to each channel, even if the power supply could handle it.  So the quick and dirty method is to take an unbalanced signal right off the coupling capacitor from the first stage.  The downside is that these outputs can’t drive much of a load, and have a reduced level compared to the main outs.  The upside is that they generally work well into the high impedance inputs of most converters and other modern gear.  Also, by using a switching jack, the signal can be connected to/disconnected from the mix buss automatically.

I have done this recently with a vintage RCA mixer (MI-C38690) and an Ampex MX-35 and both clients were quite pleased with the results.  The RCA also got input pads and a nice panel for all the connectors as well as some power supply cleanup.  The Ampex was additionally modded with line inputs on the first two channels.

RCA tube mixer with input pads and direct outs

RCA tube mixer with input pads and direct outs

RCA Tube Mixer Wiring

RCA Tube Mixer Wiring

RCA Power Supply Mess - Before Servicing

RCA Power Supply Mess – Before Servicing

Ampex MX-35 Line Inputs

Ampex MX-35 Line Inputs

Ampex MX-35 Direct Outs

Ampex MX-35 Direct Outs

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Ampex 601 Tube Preamp & DI Mod

Posted in Modded, Restored with tags , , , , , , on March 22, 2013 by AnalogAudioRepair

This was a fun project that involved removing some tape deck electronics and modding the remaining circuit into a tube mic preamp with DI.  As you can see, several of the tubes are not needed when using this as a preamp.  A whole bunch of other parts came out too which cleaned up the crowded underside of the chassis a good bit.  The headphone jack is now the DI input and the Input/Tape switch selects either the mic or DI.  The Line Rec Level pot is now the DI level control and an XLR output jack has been added on the side.

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