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Introduction to the Ampex 350-2, Revision 1.1
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Unlike later stereo Ampex recorders (such as the 351-2 and later machines), the 350-2 stereo machines
used full-track erase heads and a unique bias-coupling arrangement between master and slave electronics.
The master electronics was essentially unchanged from 1953 to 1957 (the years when the mono 350 and
the stereo 350-2 were produced) but there were various issues of the 350-2 slave electronics (including
early ones with a bias buffer amplifier and later ones without this buffer amp).
Keep in mind that no factory-issued Ampex 350-2 recorders had independent bias oscillator linked
together (as did the later 351-2 and AG-350-2 recorders). But many technicians in the 1950s, 1960s, and
1970s made their own 350-2 recorders by taking two 350 master electronics and coupling the two bias
oscillators together (normally such modified machines will have a stereo erase head). This works well if
done correctly. One method was to tie the two bias-oscillator grids together; this was often done with a
two-conductor (plus shield) cable going from one master electronics to the other.
Note also that the early multichannel mastering recorders (such as the 300-3 and 300-4) used the 300
transport but with 350 mono slave units and a special master bias oscillator (built in on a chassis the same
size as a 350 power supply) that was not used in 350-2 stereo recorders.
Unless you understand the unique bias-coupling arrangement of the 350-2, setup will be very confusing
because there is no erase current meter indication on the master unit (because the
erase head is plugged
into the slave, not the master
).
Identifying master and slave units
The master unit has the ac mains input connector, two fuse holders, and normally a female 4-pin Cinch-
Jones transport power connector. Typical catalog numbers are 5701 or 5701-1.
The slave unit has a blank plate instead of the mains input connector. It has one fuse holder and normally
a
male
4-pin Cinch-Jones connector. Catalog numbers for slave electronics varied and are often different
than those shown in documentation. Early 350 slaves have a single-ended bias buffer amplifier using a
6F6 tube (in place of the 6SN7 used in the master); later slaves do not have a bias buffer amp and will
have a blank plate at this chassis location.
Sometimes you will find a slave unit with a push-pull bias-buffer amplifier (used to reduce bias
distortion). These units use a 6SN7 bias buffer tube and can be identified by the presence of two toroidal
transformers under the chassis. This 350 slave was generally only used with 300-3 or 300-4 recorders—
and only then until these recorders were supplied with special versions of the Ampex 351 slave
electronics (typically catalog 30960-11) around 1958.
There is a special power interconnect cable (used only with 350-2 setups) with two female Cinch-Jones
connectors (one for the 350 transport and the other for the slave electronics) and a male Cinch-Jones
connector for the master electronics.
Note that very late model 350-2 electronics used a 6-pin transport interconnect cable but these are rather
uncommon.
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Prepared by David Dintenfass, Full-Track Productions, Seattle, Washington © 2020 by David Dintenfass
