Audio and Video Talk > Valves / Vacuum Tubes

AVO VAlve Testers

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Just a heads up for those that have a AVO valve tester that needs repair and or calibration, Charles introduced me to an expert based in Pretoria, Hans Kapetjin, who is an AVO expert.

He is an ex-CSIR engineer.

Anyway he serviced and calibrated my MK1 and MK2 machines for a very reasonable fee.

Contact me if would would like his contact details.

The "super" Brimar 33A/101K (12AT7) double triode and the AVO valve tester

Found some Brimar 33A/101K NOS valves.

Took me some time to find out what this valve really is.

The box:
The box is much bigger than the ordinary box for these type of valves.  The valve is secured in
an innovative method in the box. One one side is a printed indicator, when replacing the valve
in equipment. 

The valve:
Beneath its number is a neat acid marking.

This number appears in the AVO valve data manual and its settings is the same as for an ECC 83 valve.

This is not correct:

The correct setting is as for an ECC 81 (12AT7)

i came onto this very late; not sure why I did not interest myself earlier, as I have probably tested my fair share of valves in my professional life (does a quantity of many hundreds qualify me?  :) This ever since I joined the CSIR in February 1960 . . . (We have used several generations of AVO valve testers there.)

One matter that might be of interest (I did not notice it coming up earlier). That is that the AVO's I know of use rectified a.c. (the raw product) for testing. This saves on components (it is one matter to keep such a supply within value over a wide spread of anode current drawn; it is another to do so with a rectified filtered d.c. supply as in normal use.)

The one matter that then needs to be kept in mind, is that the values of current read is r.m.s based and might (probably will!) deviate from what will be obtained under fixed d.c. conditions as listed in valve manuals; this is particularly so with high-Gm power valves.

One therefore has to be careful equate the AVO readings with in-circuit values in the normal steady-state d.c. conditions of valve operation. The main problem is that an AVO-reading will involve an 'averaged' gm-value based on non-linear behaviour as a result of the anode current going from zero to peak value and back 50 times a second.. This will give a different value than when measured under steady-state d.c. operating conditions.

An AVO test is therefore valid with the figures and results published by them, but does not necessarily represent the d.c. powered behaviour involving a fixed Gm value. (If a valve was a perfectly linear device a simple conversion factor would have been possible.)

I am not aware that any later AVO testers used pure d.c.; that would naturally have changed matters. (After the advent of semiconductors such operation would have become possible.)


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