HomeA Rebuilt and Modified Altec 1567A Mic Preamp-Mixer Head

Here is a modified Altec-Lansing 1567A microphone preamp-mixer head that includes a channel breakout modification. Besides a near-total rebuild of the vintage unit, highlights of the modifications include:
A photo of the modified unit's front panels is shown below. If you are considering a standard breakout modification, do not let this photo scare you. In this particular case, my client had some specialized requirements. In addition to getting five independent tube preamps, he wanted the ability to cascade channels in various ways to experiment with distortion effects, including driving input transformers into saturation. All input and output jacks are thus accessible from the front, on an "auxilliary panel" which is permanently attached to (below) the vintage unit. This panel also hosts toggle switches that control relays near the input transformers in the vintage unit, and the "pre-triode attenuator" knobs and pads (in CH1 and CH2, a lot of attenuation is needed when experimenting with transformer distortion). It's possible to get a standard channel breakout mod of an Alltec 1567A without using up additional rack space.

Photo of front panels of modified Altec 1567A
For a complete description of this equipment, including theory of operation and its performance, you can read the following liberally-illustrated reports. They are Adobe Acrobat (PDF) documents. The "Quick Start Guide" (1.09 MB), available when I shipped the gear, includes unpacking and power-up instructions, a summary of the modifications, schematics of recommended patch cables, and instructions for rack-mounting. The "Technical Report" (5.72 MB) followed a few months later, and has complete schematics, circuit descriptions, and performance test results. The block diagrams crowded into your screen below are presented at greater resolution in each of the report PDFs. Compare these block diagrams to the generic one shown on my breakout modification hub page. Notice that channel mute switches are not needed for this Altec 1567A modification because the mix bus is removed (it can be reconstituted externally if desired).

Block diagrams of Altec 1567A before and after modification

If the idea of a channel breakout modification resonates with you, bear in mind that it need not be this complex. Feel free to email your questions and comments to me at the address shown on my home page (clarkhuckaby.com). Thank you for your interest.

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