Testing CV5112 on AVO Mk3

[Archie74]

Hello everyone. Testing a CV5112 on a calibrated AVO Mk3. Check (C) everything fine. Check (H) the valve heats up and the meter needle is rising and falling, very lively. Any advice on why this is happening would be greatly appreciated or is the tube faulty. Many Thanks

[Dekatron]

Do you have another CV5112 to compare measurements in the AVO against, to see if it behaves similarly?

Could the CV5112 be gassy or could it be due to flash overs?

Maybe bad contact in the socket or in the roller selectors/switches? Try to turn all of them back and forth and see if the problem disappears or not (only turn them when the tester is powered down!).

[Archie74]

Hello Martin, sorry but i am new to this. Is the valve no good if it is gassy and what is a flash over? I do have another and this behaves similarly. Thank you

[David Simpson]

Archie, I see from your past posts that you also have a Taylor 45 Valve Tester. What readings do you get on that for the CV5112?

It's always good to welcome folk to the valve testing fraternity, and you've mentioned that you are "new to this" - so, I beg you - please don't get drawn into a long-winded thread by asking folk where to poke and prod inside the MK3 to carry out repairs. There are one or two highly skilled & recommendable AVO VCM experts down in England who (for a fee) repair and calibrate MK3's, MK4's, CT160's, etc.

Just wondering - you didn't buy the MK3 from Cornwall by any chance?

Old Taylor 45's are probably only worth £200 tops and not that difficult to repair. Whereas AVO MK3's are worth £400 - £500 plus in good nick and proper AVO spares are like rocking-horse manure. A replacement AVO 30uA meter is worth the same as a complete Taylor4 5 for example, as is the Thumbwheel Switch assembly!

Regards, David.

[Archie74]

Hello David.

I don't have the Taylor anymore. The AVO was calibrated and serviced by CVA Classic Valve Amps in Nottingham so I definitely will not be opening her up! I was just wondering why the CV5112 was performing like it is?

Thank you.

[chriswood1900]

I wonder could it be oscillating it has a high-ish GM at 10.

[David Simpson]

Thing is Chris, the valve wouldn't exhibit PO problems until Archie switched to the "Test" function - when Va & Vg is applied. On "Check (H)" the MK3 is still in Mohm function & hopefully resolves on infinity or high Mohms. The "very lively" rising & falling of the meter's needle is something more than just an inter-electrode insulation problem.
Have you a Megger Insulation Tester Archie ? A wollop of 500 or 1000V between electrodes would aid diagnosis.
Had a look at the CVA website - the guy seems highly qualified & knowledgeable. Looks like a MK2 & a MK4 on the shelf, plus two CT160's on the bench, in his picture.
Should the couple of revered VRR&R/BVWS valve tester guys we all ken be too busy, perhaps V/T owners should give him a try. Nottingham is pretty central. Anyone else had dealings with him, or know him ?
Perhaps there is a clued-up V/T owning Forum guy living near Archie who could help him ?

Regards, David

[Archie74]

Hello David,
Sorry but i don't own a insulation tester. Thanks to you all for trying to help me.

[barretter]

Originally Posted by chriswood1900

I wonder could it be oscillating it has a high-ish GM at 10.

The gm of the CV5112, which is the military equivalent of the 3A/167M is 47ma/V. It could quite easily be oscillating.

[Archie74]

What does this mean if it's oscillating?

[Cobaltblue]

This is what Wikipedia has to say about it.

In this case its unintentional positive feedback.

https://en.wikipedia.org/wiki/Electronic_oscillation

Cheers

Mike T

[David Simpson]

Mike T & Barretter, as I said to Chris is post7 - how on earth do you expect the valve to oscillate RF in "Check(H)" mode. To quote the MK3's Manual :- "Heater hot, & Leakage tested between Cathode & Heater, and all other electrodes strapped together". I.e. NO Va, NO 45mA Ia, and certainly NO Gm function, because the heffing Anode & Grid are "Strapped" together ! Meter Readings are taken off the zero to 25Meg Ohms range.
If Archie wants to now tell us that the meter was "rising & falling, very lively" when "TEST" was selected - - not "CHECK (H)" - - then that's a totally different scenario.
For testing any high slope valves on any AVO VCM - - I use a Ferrite Beaded extension box.
Has anyone looked at the eBay prices for CV5112's ? £200 - £600, that's the cost range of most AVO VCM's !

Regards, David

[Cobaltblue]

Originally Posted by David Simpson

Mike T & Barretter, as I said to Chris is post7 - how on earth do you expect the valve to oscillate RF in "Check(H)" mode. Regards, David

I was merely answering the query what oscillating was in this context.

without further information I don't think we can be sure what's going on.

I did notice that this uses a B8B base not the most reliable but as the pins seem to be gold plated you would expect a good connection to a newly inserted valve.

Cheers

Mike T

[Dekatron]

What setting do you have on the top roller selectors as well as on all front rotary switches?

[Archie74]

The setting i have on top roller is 200 610 430 with Heater 6.3, Neg Grid 3.5, Anode 200 and mA/V 30

[Dekatron]

That seems ok.

Can you send your tubes to someone else for checking?

[David Simpson]

The situation we have here is an AVO VCM - maybe worth £500, and a hellish expensive valve - possibly worth £500 - in the hands of someone( lets kindly say - not seemingly experienced in valve theory & testing) who doesn't seem sure of what he's doing. Possibly a recipe for disaster !
Is there no one near Leicester, experienced in valve testing, who can help(hands-on) Archie PDQ. Even if it results in a "here's the good news, here's the bad" assessment.

Regards, David