This is my trusty Precision 10-12 tube tester.
I have owned it and maintained it for many
years,
and it has proven to be dependable and
reliable.
I’ve never had any issues with it at all.
After I acquired it, I opened it up and
replaced the
old paper capacitors, and installed a fresh
5Y3GT
rectifier tube.
I also lubricated the various switches and
levers.
Build-quality is excellent, and the many
switches
seem to be very good quality.
It has a late and updated roll chart MRC (F2B)
A.
Not shown, but also included is a 9-pin
Neonoval
adapter for Compactron tubes.
The documentation the manufacturer provided for
the instrument is excellent, and describes how
the
tester works.
It also includes the schematic
diagram. Once I began to understand the how and
why, I was able to test for example Russian
military tubes even though there is no factory
setting guidelines for these tubes (of course).
I have labeled the key setting switches to
remind
myself at a glance what they do…
The A switch selects the anode voltages and
load
options for the tube under test. For example, the
12AX7 is tested under higher voltage and lower
loads than a high-perveance, low-mu power tube
like the 6CA7.
The B switch selects the heater return pin.
The C switch sets how much signal voltage is
applied to the tube in order to sweep the tube
through an appropriate plate-current load line.
The D switch adjusts the meter sensitivity for
an
appropriate reading relative to the settings
used
on the A and C switches.
The E switch sets the heater voltage.
There is a line adjustment potentiometer but
this
tester was designed back when the standard line
voltage was closer to 110 volts than today’s
more
common 120 volts, so I always applied some
“English” to this setting by adjusting the
meter
needle to hit 47 or 48 instead of 50.
If you want to be very precise, you can… Just
above the A switch there are two sockets and
the
one on the right is a 12-contact utility test
socket.
You can insert a Digital Multimeter probe into
any
of the 12 contacts and read the voltage
present.
With this, you can adjust the line voltage so
that
the heater voltage is precise… say 6.3 volts,
and
this way you know the line-voltage setting is
exactly where it should be.
Below the A-E switch bank, there is a
12-position
lever-switch bank, and these 12 positions
correspond to the 12 positions available on the
above-mentioned 12-pin socket, or any of the
pin-
positions on any of the other tube
sockets. Each
of the 12 positions can (and must be) selected
to
one of four options, W,X,Y,or Z. Z is the control
grid (grid no.1), Y is the plate, X is the
screen-grid
(grid no.2) (used for pentodes for example),
and W
is 0-volts (Ground). Any electrode not connected
via the selector switches is connected to
0-volts by
default.
So for example, a half of a 12AX7 tube has pin
no.1 connected to the plate, and pin no.2
connected to the control grid (grid no.1) so to
test
this triode, you set Lever 1 (pin 1) to Y (the
plate
connection), and lever 2 (pin 2) to Z (the grid
connection).
I put together my own little manual with the
settings for common audio tube types… twin
triodes, pentodes, power tubes, etc, so I could
just
read the settings out of the manual instead of
having to scroll through the tube chart every
time
I switched to a different tube type. You will receive
this manual.
The US military was impressed enough with
Precision tube testers that they approved the
Precision model 920P for use by the war
department during WWII. Three pages from the
manual for the 920P are shown for information
only, but thanks to the schematic I can tell
that the
military version was set up in generally the
same
way as the civilian 10-12. The documentation
supplied with this tester also includes the
schematic diagram.
If you read the instruction manual carefully,
you
can select an appropriate load and voltage
setting
via the A-switch by seeing how the engineers
set
the A-switch up for various tube types (small
signal voltage amplifiers versus large signal
power
amplifiers for example), and you can use the
12-
pin utility socket to read the voltage applied
to the
control grid.
By comparing these setups to known
tube types and applications (small signal
versus
large-signal for example, or low-mu versus
high-
my), you can use similar logic to arrive at
appropriate settings for uncharted tube types,
such as the Soviet military tubes.
….
I first got interested in vacuum tube audio
back in
1977, and began collecting tubes and studying
tube audio design.
I have included pictures of several of my
projects,
a preamp and two pairs of monoblock tube
amps.
One of the pairs of tube amps uses triode
connected 6FW5 deflection tubes and features a
fully regulated power supply. The second pair
uses ultralinear EL34 / 6CA7 tubes and the iron
and chassis from a pair of Dynaco MKIII
amps.
These are shown for credibility only, not
included.
If you poke way back in my ebay feedback
listings
you will find a lot of positive feedback for
vacuum
tubes and transformer items.
Check my other listings for maybe more vacuum
tubes including 12AX7A, 5751, 12AT7, 6201,
12AY7A, 6072, 12AU7, 6189, 5814, 6DJ8, 6922,
5AR4 / GZ34, 5V4, and etc.