Modified AVO 2 Panel Valve Tester

[Draker4c]

Has anyone got an idea as to the additional panel added to this AVO 2 pane valve tester comprising 2 pots and 2 single throw switches with 2 jacks which allow for additional connection to valves under test? The kit was bought several years ago from a gent in Portsmouth and it appears to be an Admiralty pattern. I am loath to open the set up until I have some additional pointers! Does anyone have knowledge of such a mod? I looks well executed..
Many thanks
Hugh G1AUR

[Draker4c]

I notice there is a third smaller variable pot between the larger ones possibly some trimming function…
Hugh G1AUR

[Electronpusher0]

Could you show a photo of the back of the additional panel that includes the socket please.
Should be quite easy to trace the circuit of the panel.

The brown wire appears to go to the valve panel, these are quite easy to open up and far less risky to open than the meter panel, it would be interesting to see where the brown wire connects, that way we may be able to take a good guess at its function.

Peter

[Silicon]

It seems to have a facility to add a variable resistance by using three potentiometers wired as rheostats.
It also has bypass switches to remove the resistance from the circuit.

60K Ohm seems too high for heater circuit modification.

[Draker4c]

Thanks Peter and Silicon I should have time later today to open the valve base and get some more pictures. Would appreciate comments!
73 Hugh G1AUR

[Draker4c]

Well gained access to the underside of the valve board without too much difficulty. The wiring from the pot board disappeared down towards the end of the roller unit and… it appears one of the two wires is unterminated and the is soldered to ta terminal on the side of the terminal unit. A mod half done ? Will get the circuit out for clues! Photos follow.
73 Hugh G1AUR

[Draker4c]

Pictures lined to my last post..

[Draker4c]

On careful inspection of the circuit diagram I am none the wiser! The connected wire definitely connects to a terminal on the side of the roller unit the other is unconnected and sheathed in brown sleeving. There are two jacks on the added board with the potentiometers implying that additional test leads could be applied to the valve.
Hugh G1AUR

[Draker4c]

Hi Peter and Silicon any thoughts…?
73 Hugh G1AUR

[Electronpusher0]

I would like to see another picture of the underside of the additional panel, image 3 in the first post is missing the end of the panel with the socket that the brown wire is plugged into.

I have traced the schematic of what we can see so far but I need the missing bit.

Inside the valve base unit the brown wire seems to connect to one of the rods of the rotary selector switches. The rods are the inputs to the rotary switch, the outputs are the tags along the bottom of the assembly, one per valve base pin.

The rods are numbered clockwise, if you lay the panel face down and look at the end of the rods the rod at the top is number 3.

The function of each rod is :
1 Cathode
2 Heater +
3 Heater -
4 Grid
5 Screen grid
6 Anode
7 Anode 2
8 Diode 1
9 Diode 2
10 0V. This rod is missing on later AVO valve testers.

Could you try and see which rod the extra wire connects to please.
If it is No 4, grid, then I have a theory that the extra panel is to inject a signal or voltage to allow gm to be measured.

Peter

[Draker4c]

Peter thank you so much for getting back to me. There appears to be a shortage of circuit/ wiring diagrams of the AVO valve base units including the wiring for the roller selectors online although plenty for the meter unit. I will open the unit again later today and get the additional pictures. Could the circuit be an addition to allow gm measurements of newer high slope valves without the issues of parasitic oscillation?
Hugh

[peter_scott]

Please find schematic attached.

Peter

[Draker4c]

Thank you Peter Scott!
Hugh

[Electronpusher0]

Originally Posted by Draker4c

Could the circuit be an addition to allow gm measurements of newer high slope valves without the issues of parasitic oscillation?
Hugh

One of the shortcomings of the 2 panel tester is that the maximum gm it can read is 10.
The newer high slope valves often go above that, eg EL84.

I was simply thinking aloud that maybe the addition is to allow higher readings.

Parasitic oscillations are best dealt with by adding ferrite beads to the valve bases.

Peter

[Electronpusher0]

Originally Posted by peter_scott

Please find schematic attached.

Peter

The attached also shows the layout.

Peter

[Robert Gribnau]

A link to the information I gathered on this "Valve Tester with Universal Panel": https://drive.google.com/drive/folde...BvC54ja7EGnR-G

I own one and tested lots of valves with it.

[Draker4c]

Many thanks everyone I’ve had work commitments but plan to have more photos and check Peter’s advice. Incidentally Peter it was the as you said the possibility that the mod allowed higher readings that made me think that a provision was being made for higher slope valves.
Thanks again for all the advice and help.
73 Hugh G1AUR

[Draker4c]

Thank you Robert for all the information!
Hugh

[Stevie342000]

Very interesting it will be good to find out what the modification does in due course.

My thoughts agree with yours in that it enables it test higher slope valves.

My initial thoughts where that it did something to the Grid Bias...that may well still be the case, unless my thinking is out.....or it is an an anode load....

There are a few high slope valve such as EL84 or ECC88 that I will be using or may wish to test and if the thoughts on the mod turn out to be correct then it may be a modification that I may make to my AVO Two Panel.

[Draker4c]

Hi Stevie I’ll open the valve base unit tomorrow and investigate the wiring connections to the roller selector rods and from there to the valve bases. If there are subsequent connections to more recent valve type base(s) then it may also suggest that the mod is to cater for newer possibly high slope valves. On first inspection as stated earlier only one of the two cores from the connecting lead from the panel appears connected. The other is sheathed in AVO type brown cable wrap so the next puzzle is where this wire would be connected to complete the circuit through the pot panel.
Hugh