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Gates SA-70 Preamp Builds

3–4 minutes

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Hello everyone! I wanted to share some insight on the Gates SA-70 preamp, which I build somewhat regularly. I have four of these units in our studio and have sold well over ten in the past few years. It’s a fairly basic tube design and was one of the first tube preamps I felt comfortable building after the RCA BA-2 style.

A quick note on working with tube equipment: Tube gear can be scary and dangerous to work on due to the extremely high voltages. It took me many years to become comfortable and confident in building tube equipment. Patience, double-checking, and being mindful of your hands can go a long way in ensuring safety when working with high voltages.

All that to say—don’t be scared! Be smart! I’m always happy to share my experiences and lessons to help others feel more comfortable.

Coil Audio’s CA-70 is the most prominent current iteration of the Gates SA-70. Their products are awesome and inspired many of the additional features I add to my builds.

Let’s first talk about tubes. The original SA-70 schematic calls for a 6J7 and 6C5 tube. Depending on your power supply (solid state or tube rectified), you may require an additional tube. I like to leverage a tube rectified supply with a 6X5GT. Sourcing good quality, tested, new old stock tubes can sometimes be difficult. If I’m looking for a deal, I start with eBay or Reverb and look for sellers with good reviews. If I want ease and quality, I’ll go directly to a tube dealer, typically Viva Tubes or Tubes and More.

For the 6J7, there a few alternative tubes that can also be used in this position: 6SJ7 & the mil-spect 5693. One benefit of these alternatives is not needing to run a grid cap cable to the top of the 6J7.

Now lets take a look at the schematic. Below you’ll find the schematic I use for our builds. This schematic has some tweaks and modifications including the addition of a negative feedback control, switchable bass control (adding capacitors to the feedback loop) and tube power supply.

Here is an example schematic showing the 6SJ7 or 5693 (they have the same pinout) in the 6J7 position.

You’ll notice, compared to most solid state microphone preamps there isn’t a ton of parts. Speaking of parts, here are some of the components I typically use in our builds:

  • Transformers
    • Input transformer should be in the region 600 ohms to 50k ohms
    • Output transformers should be in the region of 15k ohms to 600 ohms
      • Same as above there are many vintage and current options
        • My favorite current production and least expensive is the Edcor WSM series, these sound great.
    • Power Transformer (specifically for the tube rectified supply)
  • General Parts
  • Enclosure
  • Bass Control
    • This 5 position switch only adds subtle changes to the bass response of the preamp. I leverage the follow capacitor values:
      • .001 uf
      • .0022 uf
      • .0056 uf
      • .008 uf
      • .01 uf

I wire everything point-to-point (which I can get better at) and this alleviates the need for a PCB or circuit board. I will use tag board to help layout different parts of the circuit if it is easier.

Here are some pictures of a recent builds to get a feel for the layout and design:

This has been quite a bit of information, so I’ll pause here for today.

Tomorrow I’ll come back with more details on the features, adding a DI input, adding a VU and some good troubleshooting steps.

Good additional Resources:

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