Mix Blog: Ampex Resurrection, The Sequel

A few years ago here, I chronicled my restoration of an Ampex 351 electronics module that was originally part of a tape machine, but with a bit of modification could be used as a microphone preamp. The donor of that unit—my friend and audio mentor Tony Ungaro—was kind enough to give me a second Ampex 351, which had been sitting idle for a few years.

The first unit was refurbished using replacement PCBs (printed circuit boards) made by a company called Recording With GAS, which I believe is now defunct. Until recently, there was no other option for replacing the stock PCBs, and refurbishing the old ones is (in my view) a nonstarter due to the inherent unreliability that comes with age. In fact, some of those PCBs were made of phenolic, which doesn’t age well and can crumble.

Not quite ready to give up, I kept the project on the back burner but would occasionally resume my online search. Lo and behold, while surfing last year to see if anyone else in the universe was fixin’ to refurb these beasts, I came across a company called M² Circuits. It turns out that M² not only restores vintage Ampex tape machines, but now also sell a variety of replacement circuit boards that can be dropped into the original 350/351. I was ecstatic!

I’ve learned quite a bit since my first rebuild, and part of that was something that I did very close to the end of the project, but it probably should have been one of the first tasks. In the case of Dinosaur Number Two, I decided to address this issue ASAP, because it involves a safety issue: the “death cap.”

First, a bit of background. During the time when Ampex 350 tape machines were manufactured (ca., early 1950s), electrical outlets in the United States were non-polarized, and there was no dedicated connection from the AC service ground to chassis ground. The two-prong AC plugs used on audio gear (and many appliances) could be plugged into an AC outlet right-side-up or upside-down, meaning that the hot side of the AC supply could be connected to the neutral line in the device, and vice versa.

To create a chassis ground and reduce RF interference, many manufacturers of audio gear added a capacitor between the neutral of the power cord and the device chassis. This situation will be familiar to techs who work on vintage guitar amplifiers.

There are two problems with this. The first is that, if the power plug is inserted into the AC outlet “upside down,” the hot line of the AC service is now connected to the chassis via a capacitor. While the current flowing through this capacitor is very small, it’s enough to wake you up if you should, for example, be holding on to your electric guitar and a microphone at the same time. The other problem is that if the capacitor fails short (as opposed to fails and opens), the chassis of the device will now pass full AC voltage. Yikes.

The Ampex 351 has two of these “death caps.”

One of the caps is soldered between the AC hot lead (black wire) and chassis ground lug, the other between the neutral lead (white, but discolored to a yellow-ish color in this example) and the same chassis ground lug. The smart thing to do is to remove them and replace the old Hubbell connector with a proper IEC connector, which will provide a three-pin AC plug with proper ground.

In a small miracle of modern technology colliding with old technology, it so happens that a modern IEC receptacle fits just about perfectly into the hold vacated by the old Hubbell receptacle, which saved me from the need to hack away at the rear panel of the 351 with a chainsaw (or maybe a Dremel tool).

I did have to drill two holes for the screws that hold the IEC receptacle in place. Here’s a view from the rear panel:

The tricky part about wiring the IEC receptacle is ensuring the correct connections. A trip to the original Ampex schematics confirmed that the hot wire for the 351’s power transformer is black, and the neutral wire is white. The chassis ground lug gets connected to the ground terminal on the IEC (Figure 3).

And now we have a vintage Ampex 351 with proper connection to modern AC service, thus keeping everyone nearby safe from getting zapped! Having made the modification, I was then able to briefly test the unit to verify that it works and is ready for the full treatment. Stay tuned!