Avo Valve Tester

[pwdrive]

When I was a lad I went the Radio College in Manchester at Brooks Bar (Mr Woods) to study for a ticket for radio officer but I got sidelined by a young female...

Lawrence.

[Sparks]

I trained at Wray Castle, 1959/61.   The college was started by Mr. Tomlinson who owned Brooks Bar.   He had just retired and wanted to form a small radio college to keep himself amused.   Eventually, it gained international fame and went on for many years.  He here is the promotional Utube video:   https://youtu.be/91xL2WJeN1o
It was not like that when I was there, it was all very rough and ready.
Bob

[pwdrive]

Thanks for the link, the college at Brooks Bar was an old converted Victorian house, all a bit sparse as I remember it.

Lawrence.

[pwdrive]

However, I did end up on a ship once as a passenger (actually twice on the same ship) The TSS Stefan Batory, Gdynia outbound to Montreal in the 1970's, I boarded at Tilbury Docks, I remember a storm 10 about 500 miles west of Ireland, did the same trip back a few years later, was scrapped later:

https://www.hollandamericablog.com/2010/...d-maasdam/

Lawrence.

[Sparks]

I am now quite baffled by this tester.     The test results  do not see to be either consistent, or accurate.     I decided to investigate.

With the test switch in the normal central position,  the manual say that a standing  0.5 Volt ac negative charge is applied to the grid, and when the switch is pushed over to Mutual Conductance, a  0.5 Volt positive charge is applied to the grid.      What on earth is a 0.5 Volt positive ac supply? considering the well-known fact that transformers produce ac only!
A close-up of the relevant part of the circuit is shown.   LS3 is the 1 Volt secondary of the main transformer (centre-tapped) that supplies these two mysterious 0.5 Volt positive or negative charges.    It even has - and + clearly shown on the ends!!!!

To further complicate matters, I put the meter between grid and cathode and found 1 Volt ac.    This did not change when I pushed the switch across to MC.

Bob

[Terry]

What your meter can't tell you is the phase difference between the two one volt signals.

If half wave rectified AC is applied to the anode, the valve will only conduct on the positive half cycles.

Depending on the phase of the signal applied to the grid during the anode's positive half cycle, the grid will either see a negative half cycle or a positive half cycle as the switch is moved from one position to the other and conduct accordingly.

   

[pwdrive]

Lets take the anode voltage first, suppose the anode voltage is selected to be 250 volts, it's clearly 250 volts AC from one of the secondary windings of the transformer, so between anode and cathode we have 250 volts AC because the other end of that winding is connected to the cathode, grid bias aside, the valve can only conduct when the anode is +ve with respect to the cathode ie: the valve only conducts during the +ve half of the AC cycle.

The set zero/mA/V switch is arranged to supply the grid voltages, when the anode voltage is supplied with the +ve half of the AC cycle one end of the grid winding will be -ve with respect to the center tap and the other end will be +ve with respect to the center tap, the center tap is also connected to the cathode, therefore a change of grid voltage of 1 volt can be obtained by switching the grid between one end of the winding and the other.

So far as I can make out...

EDIT: Was writing this while Terry posted.

Lawrence.

[Sparks]

It isn't rectified at all! It is coming straight from the transformer as shown in the diagram. So why do they describe an ac supply as being either positive or negative? I took the meter movement out again so I could get at the switch. I gave the contacts a real good clean with the finest grade wet & dry paper, and also contact cleaner. Although it seems to be working, it is showing most of my valves as very low. An uused EF80 just got into the green, but not by very much. Despite this, all the valves work in a one valve receiver - all with good volume and low HT (45V).
I am beginning to have doubts about valve testing in general. I have just received my vintage radio components catalogue, and he is offereing ECH35 valves with very low in emmission at 25p each, but saying they work fine in radios! I can beleive it - that has been my experience as well!
Bob

[Mike Watterson]

Re-read the VCM163 article I linked.

Almost all valve testers are essentially halfwave. Neither Analogue, ordinary DMM or even true RMS meters can give the same results as the built in meter calibration. That's why when I was replacing Valve tester meters:
1) Got a meter with same FSD current sensitivity. More important than terminal voltage or movement resistance, though it can help if close.
2) Scanned current scale.
3) Resized it to not just size, but arc of replacement meter, printed and fitted replacement scale
4) Did suitable calibration if possible.

You can usually calculate on external regular meter readings to find the actual readings if you know how the valve tester works and what your DMM or analogue test meter does.

[pwdrive]

AC is both +ve and -ve.

Lawrence.