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.