Avo Valve Tester

[pwdrive]

Thinking about it further, I am wondering how it can successfully test a valve when there is no provision to set or change the grid volts?
Bob

As suggested earlier, read the circuit description, in a nutshell when testing for gm (mutual conductance) the control grid potential is changed by 1 volt when the mA/V position is engaged, when it's not engaged the control grid is supplied with 0.5 volts -ve with respect to cathode, the meter is then adjusted to set zero, when the mA/V position is engaged the grid is supplied with 0.5 volts +ve with respect to cathode, that gives a change of grid voltage of 1 volt, the meter is calibrated in DC current terms therefore the scale can be read off in milliamps per volt (mA/V)

Lawrence.

[Sparks]

I will read it, but afraid I never was much good at theory, and it is unlikely that I will understand it, on account of the fact that there is so much to read, but I hope to figure it out by use, and comparing known good valves to known poor ones!
Bob

[pwdrive]

I will read it, but afraid I never was much good at theory, and it is unlikely that I will understand it, on account of the fact that there is so much to read, but I hope to figure it out by use, and comparing known good valves to known poor ones!
Bob

There's not a lot of theory in it, some folks can make the mistake of trying to hard and missing the point in the process, sometimes me included.

When trying to figure out a particular part of a circuit in a schematic it can help to print a copy and high light with a marker pen or pencil the relevant bit's when the relevant switches are in their selected position.

Basically mutual conductance (mA/V) is given by a change of anode current for a change of grid voltage whilst the anode voltage is held at a fixed voltage, the anode is fed from a low impedance source compared to the anodes resistance so in the grand scheme of things the anode voltage will be relatively constant for a change in anode current, all that's required now is to change the the grid voltage by 1 volt and note the change in anode current, that's indicated by the meter, if the anode current changes say by 5 mA for a change of grid voltage of 1 volt then the mutual conductance would be 5mA per volt (5mA/V)

With your tester when you set the meter to zero, anode current still flows (standing current) but that current isn't now indicated on the meter as it's been balanced out "backed off" so far as the meter is concerned, that's what the zero control, rectifier and it's feed winding etc do, when the mA/V switch is engaged the grid goes more +ve therefore more anode current will flow, the current through the meter is now not in balance with the increased anode current and thus will give a deflection that indicates the change of anode current, it might sound complicated but it's all Ohms law kind of stuff

Lawrence.

[Sparks]

Thanks,
I did push the mA/V switch to one side during test, and the needle did go up.   I can see what you mean now it has been explained in simpler language, and I can look into it further.     When I first got a computer, I was completely baffled as to how it should be shut down.     When I was told I had to hit the "start" key, and shut it down from there, they looked at me (in the shop where I purchased it - I had shut it down by pulling the plug )  like I was simple and declared "everyone know that!"     Sadly, for folk like me (with a mind unclouded by fact) a lot of things are beyond my reasoning powers!
Since yesterday, I have learned a lot about valve testers (fancy me not knowing they used AC instead of DC )    
Next week, I have chance of getting another one, so I hope it has more modern valveholders in it, plus a manual.    The chap who owns it has been told by his wife that he has to get rid of all his radio "junk."        So I am hoping to provide a good home for his other tester.
Bob

[pwdrive]

Not all valve testers use the AC method, I used to have a 1-177-B valve tester (basically a Hickok) that used the DC method but to keep the anodes supply impedance low a mercury vapour rectifier was used.

Lawrence.

[Sparks]

          I made a test panel some time ago that is used in conjunction with my "Poor man's Valve Tester," designed and produced by myself.    The other half of it is a box with a one-valve receiver in it, an EF91 is on top.   It is tuned in to a station with the EF91 in position.    There are flexible leads with plugs on them for Heaters, Cathode, Grid 1, Grid 2, and Anode.
          To test a valve, the EF91 is removed, and the valve on test is put in the appropriate socket in the panel shown.    The electrode plugs from the main unit are plugged into the appropriate red sockets along the top of the valve-holder panel.   Any links required such as G3 to cathode etc, are made with double-spade jump leads to the bottom terminals.  There are flexible leads with plugs on them for Heaters, Cathode, Grid 1, Grid 2, and Anode.  There are also two  sockets on the main unit,  with a shorting switch, inserted in the HT line for checking the anode current.   The meter is plugged into them and the shorting switch turned to off.      HT and LT supplies come from an external source, a variable DC power supply, also designed by myself, that will supply from 0V to about 300 DC.     Again, nothing mind-bending, just a variac with isolating transformer feeding into a conventional full-wave silicon rectifier, and smoothing circuit.      The LT comes from either 2, 6 or 12 Volt rechargeable lead acid batteries or another variac for higher heater voltages.   I am often told it is not up to much for testing valves, but it has served me very well for some time now.    
         I will be making up a lead and plug to connect my home-made valve-holder panel to the Avo Valve Tester!
Bob

[pwdrive]

Yes, connecting your home brew panel to the main panel of your Avo valve tester would be the way to go, less faffing around and you could test some of the more modern valves with a bit of luck.

Lawrence.

[Sparks]

Today, I replaced the mains lead and grommet.    Noticing that the window was loose, and rather dirty, I removed the entire meter movement, made and fitted a brand new window made out of clear acrylic, which improved it a lot.   I tried to clean up the meter face, but didn't push my luck as I didn't want to rub everything off, so it is still a bit grubby, but better than it was.     Then I cleaned and polished the front panel and it now looks almost brand new.
Bob

[pwdrive]

It's scrubbed up well, looks as though the meter pointer has whacked the stops at sometime or the scale has shifted.

Lawrence

[Refugee]

I love the home brew valve tester panel.
I have still got my home brew transistor tester and it still works.
It tests the base current at a fixed collector emitter voltage with 5 collector current settings like a poor mans curve tracer.
It has an original metal canned 741 op-amp and a couple of transistors and LEDs in order to set the collector emitter voltage.
I built it in the 1970s when LEDs were newfangled things.
A pencil and paper was the order of the day until I got my first electronic calculator for HFE tests.