Avo Valve Tester

[Sparks]

Well, I don't know how the valve tester works (apart from a vague idea) although I know what the meter does. I tried measuring with a DMM and an AVO 8, and none of it made sense. But nothing so far has explained why they describe an ac voltage as being positive or negative. There is no half-wave about it, it is coming straight out of the transformer!
Bob

[Sparks]

I do know that ac is positive and negative, but they have clearly put + and - on each end of the winding on the diagram. As is is flipping from one side to the other, I don't see how they can say one end is positive and the other end is negative!
Bob

[pwdrive]

I do know that ac is positive and negative, but they have clearly put + and - on each end of the winding on the diagram.    As is is flipping from one side to the other, I don't see how they can say one end is positive and the other end is negative!
Bob

It might be for illustrative purposes in conjunction with the detailed circuit description.

Are there any markings on the transformer for that winding?

Lawrence.

[Terry]

Bob,

if you have two transformer windings with voltages A and B and connect them together in series, you can get two results.

If the voltages are in phase, the two voltages will add together so the result is A + B.

On the other hand, if the windings are out of phase, they will subtract, giving A - B.

That is what is happening when you operate that switch: in the centre position the two AC voltages appear out of phase, as far as the valve is concerned, so the 1V winding is subtracted from the anode supply i.e.: the sum goes negative - in other words, down. When the switch is operated the 1V signal phase is reversed so that they add - the sum of the voltages now goes positive - in other words, up.

I've simplified this to hopefully make it easier to understand - it is really the anode current that we are interested in and the meter needle should show an increase when the switch is operated - in other words the current (which is now a pulsating DC that does not go below zero) increases or moves in a positive direction, as indicated by the meter.

[Sparks]

Well, that makes it much easier to understand, and it does make sense to me now. Whoever wrote the manual should not have put positive, and negative on the ends of a transformer winding! It appears that "everyone" knows what it means except me! It was years before I discovered you should not pull a memery stick out of a computer without ejecting it correctly! When I found out, I was again told "everyone knows that!"
There are no markings on the transformer, and because there are so many tappings, I have not even been able to locate the one volt winding.
Bob

[pwdrive]

The + - designations for the grid supply transformer winding aren't shown on the EMER schematic but they are on the patent specification schematic.

Lawrence.

[Mike Watterson]

The valve (even if Pentode or Triode) makes it halfwave. The + and - on AC windings simply indicates phase as Terry explained. So if you put 15V + 15V AC from a separate transformer in series in correct phase then you have 30V, or nothing if reversed. Add that 30 one way round to a 220V primary in a German Radio and you now can feed 250V, or if you live in 220V land, and have a 250V primary, you can reverse the 30V and it adds 30V to to 220V to feed the 250V transformer.

Matching phase on modern circuit diagrams (or transformers) is shown as a dot or circle, or if not shown, then OV is the opposite to a dot and the winding end with voltage is "dot". In older times they marked relative phase with + and -, to indicate 0 degrees and 180 degrees, or relative phase the same (zero degrees).

[Sparks]

That sort of thing is what made me leave the electronics  "rat race" in 1992.     The above simpler explanations have all helped to add to my store of "useful knowledge."     But I still don't understand why a valve showing "clapped out" can work as well as one showing very good!      In many way, having a valve tester has now muddied the water even more for me.    My old "homebrew" valve tester gave me a much more realistic pitcure of the condition of  valve, and I may well develop the circuit further!
Bob

[ppppenguin]

In many cases a valve tester won't tell you how a valve will perform in a real circuit. 2 main areas:

1: High power, especially pulse power as found in TV line scan. A tester simply can't simulate these conditions.

2: High frequency. A valve tester is working at or near DC which tells you nothing about how a valve will work at RF.

Many radios etc are very tolerant of variations in valve performance so even a valve that tests badly may work OK.

[Mike Watterson]

Unless you are using it to derive curves, a valve tester really only verifies if the valve is similar to a new one or not.
You'll find that resistors in the old radios can be off by over +/- 30% or worse and the radio is fine, they were often only +/- 20%, whereas now if I buy 5% resistors they are often +/- 1%.
Similarly capacitors other than for tuning (which tend to be stable mica) can be -50% and +300% or more and the radio will work.
So similarly the radio may seem fine even if the valve has fallen to 30% emission.

The VCM163 tester I borrowed was very useful as it tested inter-electrode leakage. It's worth adding a 300V to 600V DC power source* limited by two 1 M Ohms in series and using a uA meter to measure leakage when heater is running. Leakage from G2 to G1 is a particularly bad issue in tetrodes, pentodes, Hexodes, etc and seems common on some rimlock power out types.

(Either a voltage doubler using 2 x 1N4007 or the electronics of a flash gun of disposable camera with 300uF replaced by 1uF plastic foil)