AVO VCM MK3 Replace/Good Scale - UK Vintage Radio Repair and Restoration Discussion Forum

RF Burn · 29th Apr 2019, 11:07 am

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

John_BS · 29th Apr 2019, 6:40 pm

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

Radio Wrangler · 29th Apr 2019, 6:52 pm

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

Dekatron · 29th Apr 2019, 7:03 pm

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.

Craig Sawyers · 30th Apr 2019, 8:32 am

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

John_BS · 30th Apr 2019, 9:09 am

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.

David Simpson · 30th Apr 2019, 10:59 am

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

29th Apr 2019, 11:07 am	#1
RF Burn Hexode Join Date: Apr 2008 Location: Ramsgate, Kent, UK. Posts: 252	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
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,779	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
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,857	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 __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

29th Apr 2019, 7:03 pm	#4
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	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. __________________ Martin, Sweden

30th Apr 2019, 8:32 am	#5
Craig Sawyers Dekatron Join Date: Nov 2010 Location: Oxford, UK. Posts: 4,976	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
John_BS Octode Join Date: Dec 2008 Location: Wincanton, Somerset, UK. Posts: 1,779	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
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,851	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