General question about AVO valve testers

[David Pannell]

Continuing the seemingly ever popular discussions regarding the AVO VCMs, does anybody know the reasoning behind the Mk1 & 2 having one valve and two solid state diodes, to the Mk3 having two valves and one solid state diode, and then the Mk4 being all solid state?

Was having valves in the Mk3 seen as progress or expediency, or even an actual technical reason?

I understand that many Forum members owning these test instruments, tend to 'upgrade' from valves to solid state diodes - but how much better is this?

I recently carried out a temporary experiment on my own Mk2 by replacing 1 of the solid state diodes (neg grid supply) with a valve, just to see what if any difference there was. (The existing valve supplies the screen volts).

What I found was that the controls for backing off anode current, neg grid volts, and mA/V adjustment, seemed smoother and somewhat less erratic - almost more controllable.

Hence my original question - was AVO, in their wisdom, going forwards, then backwards, or backwards, then forwards?

I know that there are members of the Forum with much more experience of these instruments than I, who might be able to shed a little more light.

[Dekatron]

I haven't found any exact information printed in any AVO documents for the reason to choose valves or diodes in all of the AVO material that I have read. I have however discussed it with forum members and also other people who had an insight into the AVO manufacturing and repairs and the following reasons have been brought up which seems plausible to me:

*. AVO had access to valves via the contracts, military and civil, and also via other channels which made them decide to use them as they could get them cheap.

*. Another reason was that valves were readily available and made it easy to service the AVO VCM's, you just swapped them whereas a diode required soldering.

*. The metal rectifiers (1/6A) used in early AVO's where probably a way of getting rid of the valves, but they were large and also needed to be mounted much like a valve which used up the same space. You had to solder these too which took away some of the ease of servicing the tester.

*. Germanium and Silicon diodes where more expensive and not available in the voltages needed where the valves where used, at the time of design and manufacturing.

One thing that I like about using silicon diodes in the AVO Mk III and CT160/A is that you don't have the warm-up time of the internal valves any more. I have not seen any difference in how the controls work so I can't say that they will be affected apart from perhaps when the tester is warming up.

I can't judge if it is better or not to upgrade to silicon diodes but being free of the warm-up time is a big plus for me, and it was also the way that AVO went themselves.

One more reason to change to silicon diodes is that the valve used in the backing-off circuit in the CT160/A is carrying a higher current than the manufacturer datasheet specifies, it carries 12.5mA where the datasheet says 9mA maximum which means that this valve will deteriorate faster than the manufacturer data specifies under normal usage so you'll have to replace it more often.

[David Pannell]

Thanks once again, Martin, for your insight. I am most grateful.

Cheers,