AVO VALVE TESTER
large 2-piece valve tester. The module with the valve bases was available
seperately at the price of £3 3s, allowing the tester to be upgraded
as new valve types came along. Indeed this tester must have been made for
many years as some of the bases on this unit are for post-war valves ; it
is certainly a later model as the heater voltage has an additional divide-by-7
switch for testing 1.4v filament valves.
The connections to the valve electrodes are made via a selection of thumbwheel switches.
The picture opposite is an advert that appeared in the April 1939 edition of the "Service Mans Manual". Click here for a larger view [153K].
Visitor Frank Adams recals using one of these instruments in the late 40's :-
"This model was the first tester to be produced using unrectified A.C. to simulate D.C. testing. The Patents justify the theory of this (not to my entire satisfaction) but set out the basis on which all future ACWECO and AVO valve testers were designed. [Some later models did substitute raw rectified A.C. but the principle was the same.]"
"The problem with this 1st. generation tester is that it was designed for use with the early types of valves which had low mutual conductance and a relatively high effective anode impedance. The upshot of this is that the design envisaged testing in the region of zero grid bias. From memory the testing procedure involved changing the grid bias about -1 volt to zero. Unfortunately whilst this was O.K. with early types of valves which operated with little or no grid bias later types took much too much anode current even with the anode voltage setting as low as possible".
In addition it seems that the instrument can suffer from parasitic oscillations with more modern valves.
|Thanks to Chris Colebrook for a copy of the instructions. A later valve chart was found on the Valve Data CD.|
|Seems to be all there and undamaged (if you ignore one of
the rubber feet missing) and in physically sound condition, though as seems
to be the way with valve testers the lead for connecting to a valve's top
cap is missing.
I've seen couple of these testers with duff meter movements. If you have one of these then it is worth noting that the movement is essentially the same as used in the old Avo 7 multimeter.
|Rescued from the Radiophile's auction at their October 2001 Shifnal bash for £70.|
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THE VALVE TESTER GALLERY
13th January 2002