Here's an empty moving coil step up transformer box for use with the Ampex B-17331-1 octal base mic input transformers that were used in the old 600 series and 351 tape recorders. Other transformers that are pin compatible and will also work in this box include Triad HS-273P, Microtran M8030 and M8031 and possibly others. No transformers are included. This is a custom engraved box using high quality Belton sockets and gold plated Switchcraft jacks.
You can see this has two gain switches as well as a ground select switch. The gain switches operate by putting the transformer primaries in parallel for high gain, or series for low. The actual amounts of gain and load impedances depend on the transformer installed.
The ground switch lets you choose how to provide shielding for the enclosure. It doesn't significantly affect the left and right channel audio grounds which are individually hardwired through from in to out. In the switch's center position, all three grounds (left, right and chassis) are all kept separate, meaning that you'll want to run a separate ground wire from the SUT's ground post to the ground post on your preamp. Keeping all three grounds separate until they join at your preamp is usually going to be the technically best way to do it, but generally the box can be attached to a single audio ground without causing problems, and might be more convenient. You can ground the box to the left audio ground using the "L" position of the ground switch. The "L+R" position joins all three, left, right and chassis grounds together at the box, which does occasionally end up producing the lowest noise, particularly when you're using audio cables that are wired without both shields attached on both sides in order to prevent ground loops. Usually, you won't need to worry too much about what the ground switch is doing, just wire the box however is convenient, and then put the ground switch in whatever position provides the lowest noise. Proper grounding of the enclosure can't always fully prevent the transformers themselves from picking up magnetic hum, and for this reason you may still need to position or rotate the SUT box so that the transformers pick up the lowest noise from power transformers and AC turntable motors. Generally speaking, these old microphone transformers are pretty well shielded against magnetic noise, and if you hear any hum at all, you can usually get rid of it with just a little tweaking.