Avo Valve Tester

[Sparks]

Well it isn't! The readings are only correct with G3 left disconnected! In this 1937 tester, there is no provision for connecting G3 up anyway. The pin connections to the valve-holder panel are F, F, G1, G2, A, plus D1, D2 and A for rectifiers! My own valve-holder panel has a provision where I can add links, so I naturally linked G3 to the cathode when first testing the EF39. When the result was way out, I took the link off, and the readings became normal!

This doesn't give me any cause for concern, I long ago discovered that theory does not always match up with fact when it comes to radio, and I don't even bother questioning it these days! It was merely an observation!
Bob

[Mike Watterson]

Well it isn't!      The readings are only correct with G3 left disconnected!

This doesn't give me any cause for concern, I long ago discovered that theory does not always match up with fact when it comes to radio,

There is always an explanation for something so well understood as radio.
Real radio theory always matches reality when the measurements are done properly and the correct theory properly applied.

It gives me great concern as is means something is wrong. An electrode should always be connected to somewhere, never floating. I'd not want visitors to the site to get the idea that valve testing can be done with unconnected electrodes.

It absolutely connects to cathode in Pentode mode (g2 and anode are isolated supplies) and on an EF39 it *might* connect to anode if g2 and anode are using the same supply (triode mode testing). I'd have to check what the Philips data or construction says.

If there is no separate g2 HT, then it's only testing pentodes as triodes (g2 connected to anode).

[Sparks]

"If there is no separate g2 HT, then it's only testing pentodes as triodes (g2 connected to anode)."

Your statement above, indicates you are getting confused, not me!     I never said there was no G2 (Screen grid) supply, because there is, and I set it correctly!      There is no G3 (Suppressor grid) provision in the tester.     The valve-holder connection pins are:
Filament, Filament, Control grid (G1), Screen grid (G2) and Anode (A).

As I said, I am as aware as anyone that G3 should be connected to the cathode, and that is what I did initially, but the resultant readings were way out.   As soon as I removed the link, they showed correct.    Not just on one valve, but a number of them!

For me, the 1937 valve tester is an interesting piece of very vintage gear that is giving me no-end of amusement!      If you can explain it, very good, but I am not in the same league as you, as far as radio theory is concerned, and never have been!     But I will be very interested to see what light you can throw on this!

Bob

[Mike Watterson]

"If there is no separate g2 HT, then it's only testing pentodes as triodes (g2 connected to anode)."

Your statement above, indicates you are getting confused, not me!     I never said there was no G2 (Screen grid) supply, because there is, and I set it correctly!      There is no G3 (Suppressor grid) provision in the tester.     The valve-holder connection pins are:
Filament, Filament, Control grid (G1), Screen grid (G2) and Anode (A).

As I said, I am as aware as anyone that G3 should be connected to the cathode, and that is what I did initially, but the resultant readings were way out.   As soon as I removed the link, they showed correct.    Not just on one valve, but a number of them!

No, I just haven't looked at the manual. There are many old testers, even from 1950s, that only triodise the valve, no separate g2 HT.

I'm not confused at all.
I've not ever seen G3 supply on a tester, because it's ALWAYS connected to Cathode, Anode, G2 or Filament. It's never floating and on types not intended for RF it's usually internally connected to the cathode. Always the cathode or if a direct type, the filament, if it's a Pentode mode test. The entire point of a Pentode vs a Tetrode is that is has a third suppressor electrode (need not be a grid, can be a pair of pins or plates) that must be below anode lowest voltage or you get secondary emission, a current flow from anode to Screen (g2) which causes the  Tetrode Kink. Beam Tetrodes or Kinkless Tetrodes are really Pentodes too (named differently for patent reasons) when they realised an electrostatic lenses does same thing as the third very open screen.
   

G3 (suppressor) has to be connected, not floating, wrong readings means connections are wrong, or something else is wrong.

   

Shown innards are from an EF89. You can see that the electrons are really two opposing beams from centre and thus the outer most open "grid" can be replaced by two pairs of plates, one pair between the each of the two anodes and the g2 (sceeen) grid.

So if you are only getting what seems like reasonable readings on EF39 with the cathode floating, then something is very very wrong. Don't shrug and ignore it. Double check connections etc.

[Sparks]

OK, I have checked the wiring as you suggested, and it is all correct! Maybe the valve data book is wrong? After all I did find they had got the GM and RA colums wrongly labelled!

My valve data for an EF39 shows:
1 = Metalising, 2 = Heater, 3 = Anode, 4 = G2, 5 = G3, 6 = NC, 7 = Heater, 8 = Cathode. Top cap = Grid 1.

It might be because linking pins 8 and 5 on my valve-holder unit is not a short, direct link, but is a fairly long chain linking all the pin 5s on all eight valve-holders and there may be some inter-action across the tangled mass of wiring under the unit!
Bob

[pwdrive]

I assume the voltage settings etc are from the Avo valve data and not any other source?

Lawrence.

[Sparks]

Yes they are, but I believe I have solved it. The problem was the valves, and not me! I have been well-aware for the past 55 years or so that G3 should be connected to the cathode. For that reason, I incorporated in my valve-holder test panel, a row of pin-linking terminals. When I tested the first few EF39s (three or four of them), I had the cathode/G3 link in. The valves (except one) all showed rather low in both gm and emission. I noticed that when I removed the link, the readings became more what I expected. Today, I tried with some hardly-ever-used EF36 and EF37 valves that have the same bases as EF39s and similar characteristics. These all gave excellent and accurate readings on both gm and emission. When I removed the link, the readings went haywire. I tried again with the EF39s and found that when I took the links off, the readings changed when the link was removed, so conluded that there was something wrong with the first few valves that I tested! I dug out some more EF39s that gave a good selection of results from a few poor, some average and several excellent conditions. When I removed the links on the excellent ones, the readings did go down a bit, but not much. I have quite store of boxed valves that have never even been unpacked yet, and it will be interesting to see how they show up on it. I suppose I shouldn't "experiment" by messing about with known facts, but experiments on things that shouldn't have worked have seen me out of a few tight spots at sea when no spares were available, and shore servicing had given up because the correct valve or component was not available!
Bob

[Mike Watterson]

It takes a while to get confidence in a "new" piece of test gear, especially when it's ancient!

I'm glad everything makes sense now!

[Sparks]

For me, nothing in radio and electronics makes a great deal of sense, and I often find that things that shouldn't be are, and things that should be are not! No doubt lack of knowledge on my part, but I wouldn't have it any other way!
Last night, on Ebay, I bid for a Taylor 45a valve tester that had only gone up to £50 a few minutes before the end. In the last few seconds, bids came flying in, and in the last second, my high bid of £88 was topped by £90!
I will wait and see how the feedback goes, because I am curious as to whether it worked, or not!
I don't really feel too bad, because my Avo Mk1 only cost £65 on the car boot sale a few weeks ago!
Bob

[Sparks]

I have now almost completed the panel interface between the valve-holders and the Avo Mk1 valve tester.   (Fixed the two units on a baseboard.)   The tester is connected to the appropriate valve pins by short jump leads between the top black panel and the upper row of red pin sockets, 1 to 9.       Any electrode links that may be required such as Anode to G2, or Cathode to G3 are completed by putting short jump leads with spade terminals on each end, across the appropriate black screw terminals along the bottom of the lower panel.     In order to forestall anyone saying how horribly dangerous it is, because of the exposed brass bolt ends on the lower screw terminals, the final task is to make nine insulating blanking buttons to glue in the tops of the terminals.    Nine steel washers will be placed below the terminals to lower the height of the bolt heads in the tops of the terminals.    The cable, top right, is plugged into the side connection of the valve tester.
When testing more modern pentodes, such as the EF95, there is some instability in the form of hand capacity.    After setting the zero, the needle goes well up into the scale when I remove my hand from the control.   The only answer so far seems to be, keep hold of the control when at zero, and push the joystick over with the other hand, then it is OK.
I see I have missed off the "d" from "required" in the central info strip!
Bob