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Old 20th May 2022, 1:05 pm   #1
J0nt383
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Default AVO VCM MK IV question/help.

Hi!

As for what i can search myself to in the forum i have not found any pointers to this particular problem.

To give a little story about this meter (for fun), it has been an inventory of the swedish marine, with big probability the hms halland (j18) vessle. rescued by previous owner in 1984 (?) and then just been sitting in a cabinet.

Head on to the problem, i noticed some deviating voltages trough the neg grid control and found that it had been repaired with some carbon resistors where one where off by 10kohm, changed to specs from service manual and everything is spot on! trough out the SM, looks good on scope etc etc.

But.... when testing a tube like EL34 where there is a little current so to say (ECC81-83 are just fine) it reads a good value steady and fine with meter switch at 100ma. the issues start when "backing off", the needle just climbs, and climbs, and climbs regardless how many times i back it off to zero it keeps climbing. any pointers? i had a tought of changing everthing in it to nice new parts, but it it really nessesary?

//Jonas
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Old 20th May 2022, 5:17 pm   #2
Craig Sawyers
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Default Re: AVO VCM MK IV question/help.

How long are you leaving the EL34 to warm up? Power valves can take several minutes and more for the anode current to stabilize.

Craig
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Old 20th May 2022, 5:31 pm   #3
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Default Re: AVO VCM MK IV question/help.

I have in the past tested EL34 that were reading well over 100 Ia and the only solution to getting the meter off its stops was to add more neg grid volts ( 13.5 from memory is the book setting ), so tuning the reading, but if it won't stabilize at say 15 or 16 then there may be a fault elsewhere.
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Old 20th May 2022, 6:22 pm   #4
J0nt383
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Default Re: AVO VCM MK IV question/help.

Can it be that simple? me beeing to fast?

left it at test/100Ia for 10minutes this time and it is stable when backing off.

resluting in: 61mA and gm of 12, seems fair, right?

//Jonas
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Old 20th May 2022, 7:22 pm   #5
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Default Re: AVO VCM MK IV question/help.

That sounds good, glad it was sorted for you, these VCM's are never usually an easy fix.
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Old 20th May 2022, 7:55 pm   #6
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Default Re: AVO VCM MK IV question/help.

Nice! thanks. is the mu readable in this meter somehow or does it require some calculation? perhaps already covered somewhere in the forum, will check.
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Old 21st May 2022, 6:19 am   #7
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Default Re: AVO VCM MK IV question/help.

Sorted it out, thanks again!
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Old 21st May 2022, 7:10 am   #8
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Default Re: AVO VCM MK IV question/help.

mu is not readable on an AVO valve tester.
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Old 21st May 2022, 8:11 am   #9
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Default Re: AVO VCM MK IV question/help.

Quote:
Originally Posted by Craig Sawyers View Post
mu is not readable on an AVO valve tester.
Yes, i sorted that out after a bit of reading. but, since mu is the ratio of change between Va/Vg at a given current, wouldnt there be a way of calculating this? or finding out rp? from what i can findout by reading mu = gm x rp? this just became a jungle
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Old 21st May 2022, 1:29 pm   #10
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Default Re: AVO VCM MK IV question/help.

While the AVO VCMs use what amounts to halfwave-rectified sineusoid waves for their electrode voltages, the meter scalings are set to compensate for this and produce readings comparable to what a true DC test would give.

So while Va and Ia are bouncing around at mains frequency, the meter pointers stay stable. There is no indication of the slope of Ia versus Va. So there is no way to see the anode curve slope impedance. The Gm measurement is fine, and gives the other half of the information you need to calculate mu. But a crucial factor is missing.

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Old 23rd May 2022, 5:10 pm   #11
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Default Re: AVO VCM MK IV question/help.

QUOTE=J0nt383;1471951]
Quote:
Originally Posted by Craig Sawyers View Post
mu is not readable on an AVO valve tester.
Yes, i sorted that out after a bit of reading. but, since mu is the ratio of change between Va/Vg at a given current, wouldnt there be a way of calculating this? or finding out rp? from what i can findout by reading mu = gm x rp? this just became a jungle [/QUOTE]

Looking at the valve data here:

https://frank.pocnet.net/sheets/129/e/EL34_Mullard.pdf

It looks as though there's enough slope on the Ia-Va curve to enable a calculation to be made by doing a couple of measurements with the valve tester that should give a good approximation of ra, with some additional measurements you could also do the calculation by drawing a tangent on a plotted curve in order to calculate ra, the tangent should be drawn so that it intersects the Ia axis, ra being Va divided by the difference between the operating current and the current indicated at the intersect.

Lawrence.
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Old 23rd May 2022, 6:30 pm   #12
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Default Re: AVO VCM MK IV question/help.

mu is really determined by the physical structure of the valve. Which is why it is a fairly flat characteristic. And why if it is quoted, it is often a single value.

Although the curves seem to vary by a lot for a 12AU7, for a typical Ia of 10mA mu only changes from 17 to 20 for Va from 300V to 100V https://frank.pocnet.net/sheets/093/1/12AU7A.pdf

Craig
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Old 23rd May 2022, 7:05 pm   #13
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Default Re: AVO VCM MK IV question/help.

Quote:
Originally Posted by Craig Sawyers View Post
And why if it is quoted, it is often a single value.
Probably because it's referenced to a single value of operating parameters.

It's often rounded up or rounded down from the calculated.

The published valve data isn't set in stone anyways due to valve tolerances etc.

I did a ra calc. using the published Ia-Va curves for the EL34 and it wasn't that far off from the publish figure.

Lawrence.

Last edited by ms660; 23rd May 2022 at 7:21 pm. Reason: rewrite
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