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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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29th Apr 2019, 11:07 am | #1 |
Hexode
Join Date: Apr 2008
Location: Ramsgate, Kent, UK.
Posts: 252
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AVO VCM MK3 Replace/Good Scale
I am currently recording all my valve stock in a spreadsheet and would like to be able to make the spreadsheet 'highlight' any parts that would be considered as REPLACE according to the VCM's REPLACE/GOOD scale using the recorded value of gm (mA/V) compared to the average/bogey value quoted in the valve tester manual or manufacturer data sheet.
I remember seeing this subject discussed covered somewhere but cannot find it now. The question being, does anybody know the relationship between the gm value as set on the 'Set mA/V' control to the limits of the red/white/green sections of the REPLACE/GOOD scale on these VCMs? Adrian |
29th Apr 2019, 6:40 pm | #2 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,780
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Re: AVO VCM MK3 Replace/Good Scale
I think it's the same meter as the CT160, and the "replace" boundary seems to be set at half the nominal gm value on these instruments.
John |
29th Apr 2019, 6:52 pm | #3 |
Moderator
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,876
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Re: AVO VCM MK3 Replace/Good Scale
Unfortunately, the replace/good threshold is a bit arbitrary. Whether a valve needs replacing or will soldier on depends a lot on the application.
David
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29th Apr 2019, 7:03 pm | #4 |
Octode
Join Date: May 2009
Location: Linkoping, Sweden
Posts: 1,465
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Re: AVO VCM MK3 Replace/Good Scale
The red area goes from 0 up to 0.5mA/V, the white area from 0.5mA/V up to approximately 0.68mA/V and the green area from approximately 0.68mA/V up to approximately 1.35mA/V.
You can see the small lines a the top of the good/replace scale from 0 up to the 1mA/V line. But these are only measured values, if a valve works in an equipment is something else.
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30th Apr 2019, 8:32 am | #5 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,984
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Re: AVO VCM MK3 Replace/Good Scale
It classic era Tektronix valved oscilloscopes, they ran the cathode of each valve to -150V via a 33k resistor. That way the operating point and gain stabilises as the valve ages. Essentially the cathode resistor mimics a constant current cathode resistor, and in some applications a second valve was put there, where the anode resistance acts as the cathode resistor.
Tek scopes list the 545A, 547 etc all work perfectly cheerfully even when the valves were in very poor shape as measured on a valve tester. It is therefore bewildering that modern valved gear (like hifi amplifiers) have not cottoned on to this, and just use the normal grid biassing down to ground. So as the valve ages, the operating point changes and the anode current shifts - so the gain shifts. The point is that whether a valve needs to be replaced depends on what the circuit is in which it is used. Craig |
30th Apr 2019, 9:09 am | #6 |
Octode
Join Date: Dec 2008
Location: Wincanton, Somerset, UK.
Posts: 1,780
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Re: AVO VCM MK3 Replace/Good Scale
All very true, but the OP posed a very narrow question regarding the relationship between gm and the go/no-go areas on a Mk3 AVO.
John PS I have measured valves which come up with say 60% anode current (and gm) when biased according to the AVO data, but when the grid voltage is raised to produce the correct Ia, the gm comes good also (logical). However, some others are unable to climb to anywhere near the nominal Ia, presumably due to lack of emission, and these would presumably still give problems under Tek bias conditions. |
30th Apr 2019, 10:59 am | #7 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,853
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Re: AVO VCM MK3 Replace/Good Scale
As David said yesterday, AVO VCM meter readings can be a bit arbitrary. They are a bit more accurate than say - a Taylor or Mullard valve tester, but along with their other basic tests such as resistance & leakage, give the user a pretty good idea of the state of the valve in question.
There is a vast amount of Forum info available via "Search" on AVO's range of VCM's, so have a shufti to find out about meter accuracy & calibration. If folk have doubts about the accuracy of the mA/V(gm) of their AVO siblings - Mk3, Mk4,CT160 - - whilst reading Ia(providing its near enough the "Book" value) - (with all voltages set as per the AVO VDM)adjust the Vg by a small fixed amount & see how much Ia changes - this will give you mA/V. This should be nigh-on the same as the subsequent mA/V Dial "Test" reading. If it isn't - then calibration &/or fault finding is required. If you want to go the whole hog with a valued valve, then take a dozen or so readings of Ia & corresponding Vg & draw up a graph of the gm "Curve". Start with a low Ia, then gently raise it with changes of Vg. Take care not to exceed maximum Ia ! If you don't feel comfortable or confident in delving inside your AVO VCM - there is a small number of Forum/BVWS experts who offer repair/calibration services. Regards, David |