Mix Blog: Ampex 351 Resurrection, The Sequel — Part 2: Rectifying an Age-Old Problem

Don’t pass up Part 1!

After a few tweaks to bring the power connection in the Ampex 351 up to modern AC standards, my refurb of a second Ampex 351 electronics module continues…

An issue that plagues these aged beasties is that the power supply includes a selenium rectifier. As documented in Resurrecting an Audio Dinosaur, Part 2, selenium rectifiers—which were fairly common in electronics at that time—don’t age well. The voltage level of the DC supply output can deteriorate, they are prone to catching fire (!), and failure due to age is imminent.

[Side note: When the first Audio Dinosaur blogs were published, a Mix reader responded and insisted that this was a copper rectifier. A trip to the original Ampex manual (as well as quite a bit of discussion with folks having far more knowledge than I regarding vintage Ampex gear) confirmed that it is indeed a selenium ‘rect.]

Anyway, the solution for Audio Dinosaur One was removing that rectifier, which is mounted to the side panel, and replacing it with a tag strip that connects two 1N4007 diodes between the power transformer and power supply circuit board, and rectifies AC to DC

[Another note: there was a typo in the original blog stating that the diodes were 1N4004, but they were, in fact, 1N4007.]

The tag strip was a safe, reliable solution for the old rectifier, but it always bothered me that it looks kinda home-brewed. Enter our friends at M² Circuits, who make a small circuit board designed to host these diodes in a much more elegant fashion.

M² Circuits’ Sil-Rect PCB has pads for the diodes, as well as pads for voltage drop resistors. As noted in the first go-around, replacing the old rectifier with new diodes—which are more efficient—causes an increase in the DC voltage to the tubes. That voltage should be in the vicinity of 12.6 VDC but in the case of Dinosaur One, the voltage went up to 14.7 VDC. Way too high. The solution was to increase the value of the drop resistor on the power-supply circuit board, which reduced the voltage to around 12.2 VDC (close enough for rock and roll).

M² has recognized the fact that owners of Ampex 351s who want to replace the rectifier but don’t plan on doing a complete refurb may not want to mess with the original power-supply PCB to change the power resistor (it’s a huge pain in the arse to access), so they’ve added footprints on the Sil-Rect PCB for resistors that can be used to drop the voltage to the nominal level without disturbing the original circuitry.

The Sil-Rect shown in Figure 5 is being installed in Dinosaur One, which already had the 1N4007 diodes and the correct voltage going to the tubes, so you’ll notice that the resistor pads are jumpered.

Before mounting the PCB, I added a spacer, as well as a small circular insulator cut from a leftover drum head that sits between the circuit board and the spacer, just to be sure that there’s no chance of the PCB solder joints shorting against the side of the chassis.

The location of the solder pads for the AC inputs and DC output are such that I probably won’t need to extend the wires for Dinosaur Two as I did for Dinosaur One. The result is a much neater and more reliable solution!

To be continued…