Which AVO VCM is best? - UK Vintage Radio Repair and Restoration Discussion Forum

David Pannell · 4th Oct 2010, 3:51 pm

Hello everyone,

I have just joined the Forum, and am anxious to have the above question answered.

I have been an electrical/electronics engineer for over 40 years, and am the proud owner of a Mk 2 AVO Valve Tester which I have had longer than I can remember.

I will, in due course, add to my profile, which I hope will provide common ground for active discussion and participation in the future.

However, in the meantime, I am expanding my activities into the repair of valve amplifiers, and would like to hear other members' opinions regarding the VCMs.

Finally I look forward to a long and harmonious association with the Forum.

With kindest regards to all,

David.

Dekatron · 4th Oct 2010, 6:39 pm

Hi David,

And welcome!

My view on which AVO VCM that is best is in the list below, remember that this is my personal view which comes from my needs:

1. Any of them as long as it is calibrated and it can test the tubes I need!
2. The AVO VCM163
3. The AVO Mk IV
4. The AVO CT160 and the AVO Mk III
5. Any of the older AVOs like the AVO Mk II, AVO Mk I and the AVO Two Panel Valve Tester.

The most important thing is that the AVO Valve Tester (or any other Valve Tester for that matter) is calibrated no matter which of them you use, and that you are sure that it is really calibrated. The best thing is to have a few calibration tubes that you use regularely to check the calibration with and also have a few calibration tubes that you use very seldom to compare the results from with your "daily" calibration tubes. Checking the calibration regularely is important as small changes in measurements can occur if any of the components are degrading or if for instance the meter is loosing its magnetic flux and not giving the correct measurements over time. When you have learned to check the calibration of your AVO Valve Tester it is quite easy to not just use a calibration tube but to actually measure the voltages that should be present inside a calibrated AVO Valve Tester.

I hope this helped some, but please remember that it is my personal view and that might differ from your needs.

Lucien Nunes · 4th Oct 2010, 6:50 pm

Hello David and welcome.

Luckily that isn't quite as difficult a question to answer as 'which valve tester is best?' The conventional answer would be the VCM163 because that is the latest and most highly specified unit that allows both Gm and Ia to be read simultaneously.

In truth they are all perfectly respectable instruments although there are significant circuit differences between say your Mk.2, the popular Mk.4 and the close cousin the CT160. The latter uses a bridge circuit which is nice in theory, although IIRC it does not have so high a range of -Vg available which might be needed for certain audio output valves. The Mk.2 is best in this regard.

The general caveat with all AVOs is that they subject the valve to non-DC conditions, which a purist will tell you invalidates the readings to a degree. However AVO were not foolish and made instruments that give repeatable, predictable readings that end users of valves accept as definitive. AVOs also require careful handling as the meters are more or less irreplaceable; there have been many threads on that subject here.

The original 'two-panel tester' doesn't qualify as a VCM and is unsuitable for some later valves, although it has an elegant simplicity as an instrument.

FWIW I am a Mk.4 user although I have a Mullard high-speed just for fun and used to use a Funke that belonged to someone else. This was an accurate tester but the AVO is more pleasant to use.

Lucien

David Pannell · 5th Oct 2010, 9:33 am

Hi Martin and Lucien,

Thank you for your welcome, and for your interesting comments.

Calibration is not a problem, as I have close contacts with a local calibration house, and have also been calibrating the VCM myself for many years along the lines mentioned.

I wonder if anyone knows what the significant differences are between the various Marks of this instrument. It would be interesting to know, particularly if they all have the miniature rectifying valve VR92, or whether the later versions were fitted with a silicon diode.

Also I am intrigued to know if anyone has substituted this valve for a more conventional rectifying valve, rather than swapping it for a silicon jobbie.

I like to keep things as original as possible and I do have some spare VR92s, but in the event, I wonder if fitting a more conventional valve is possible. I want to avoid eventually having to use solid state if possible.

Cheers.

David.

Dekatron · 5th Oct 2010, 10:31 am

Hi David,

If you read what I have written on the AVO Mk IV and also the AVO CT160 here on the forum for the last months you will see a lot of interresting information on the AVO Valve Testers. Search for threads I have either started or threads I have commented on.

I have not worked with the AVO Mk I nor the Mk II myself but in later models like the AVO Mk III and the AVO CT160 you can replace the valve rectifiers (6AL5W and equivalents) with Silicon diodes but you will not only have to recalibrate the tester then but in some cases you will have to change resistors to compensate for the lower voltage drop across a Silicon diode.

You can read what Martin Scobie did to refurbish his AVO Mk II here: http://www.marie34.freeserve.co.uk/page9.htm - but he did not change the VR92 and unfortunately he did have problems fitting Silicon diodes across the meter to protect it, however my friend Euan MacKenzie in Australia has fitted modern Silicon diodes across the meter without any adverse effects on measuring.

If I ever come across an AVO Mk I or Mk II I will write an article on them too. My focus is nopw on my refurbishing of my VCM163 and a few shipping damaged Taylor 45Ds.

David Pannell · 6th Oct 2010, 12:36 pm

I believe that some of the later VCM instruments used the 6AL5W military grade rectifying valve, presumably for deriving the negative grid volts supply.

As the VR92 is almost impossible to obtain, I was wondering if I might try out a 6AL5W, with possible adjustment, to see if it works on the Mk2. Heater current is 300mA as opposed to 150mA, but the transformer windings look more than man enough for the job. It might even be feasible to parallel up the anodes and cathodes.

Any thoughts, or am I

?

David.

4th Oct 2010, 3:51 pm	#1
David Pannell Retired Dormant Member Join Date: Oct 2010 Location: Horsham, West Sussex, UK Posts: 77	Which AVO VCM is best? Hello everyone, I have just joined the Forum, and am anxious to have the above question answered. I have been an electrical/electronics engineer for over 40 years, and am the proud owner of a Mk 2 AVO Valve Tester which I have had longer than I can remember. I will, in due course, add to my profile, which I hope will provide common ground for active discussion and participation in the future. However, in the meantime, I am expanding my activities into the repair of valve amplifiers, and would like to hear other members' opinions regarding the VCMs. Finally I look forward to a long and harmonious association with the Forum. With kindest regards to all, David.

4th Oct 2010, 6:39 pm	#2
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: Which AVO VCM is best? Hi David, And welcome! My view on which AVO VCM that is best is in the list below, remember that this is my personal view which comes from my needs: 1. Any of them as long as it is calibrated and it can test the tubes I need! 2. The AVO VCM163 3. The AVO Mk IV 4. The AVO CT160 and the AVO Mk III 5. Any of the older AVOs like the AVO Mk II, AVO Mk I and the AVO Two Panel Valve Tester. The most important thing is that the AVO Valve Tester (or any other Valve Tester for that matter) is calibrated no matter which of them you use, and that you are sure that it is really calibrated. The best thing is to have a few calibration tubes that you use regularely to check the calibration with and also have a few calibration tubes that you use very seldom to compare the results from with your "daily" calibration tubes. Checking the calibration regularely is important as small changes in measurements can occur if any of the components are degrading or if for instance the meter is loosing its magnetic flux and not giving the correct measurements over time. When you have learned to check the calibration of your AVO Valve Tester it is quite easy to not just use a calibration tube but to actually measure the voltages that should be present inside a calibrated AVO Valve Tester. I hope this helped some, but please remember that it is my personal view and that might differ from your needs. __________________ Martin, Sweden

4th Oct 2010, 6:50 pm	#3
Lucien Nunes Rest in Peace Join Date: Apr 2008 Location: London, UK. Posts: 2,508	Re: Which AVO VCM is best? Hello David and welcome. Luckily that isn't quite as difficult a question to answer as 'which valve tester is best?' The conventional answer would be the VCM163 because that is the latest and most highly specified unit that allows both Gm and Ia to be read simultaneously. In truth they are all perfectly respectable instruments although there are significant circuit differences between say your Mk.2, the popular Mk.4 and the close cousin the CT160. The latter uses a bridge circuit which is nice in theory, although IIRC it does not have so high a range of -Vg available which might be needed for certain audio output valves. The Mk.2 is best in this regard. The general caveat with all AVOs is that they subject the valve to non-DC conditions, which a purist will tell you invalidates the readings to a degree. However AVO were not foolish and made instruments that give repeatable, predictable readings that end users of valves accept as definitive. AVOs also require careful handling as the meters are more or less irreplaceable; there have been many threads on that subject here. The original 'two-panel tester' doesn't qualify as a VCM and is unsuitable for some later valves, although it has an elegant simplicity as an instrument. FWIW I am a Mk.4 user although I have a Mullard high-speed just for fun and used to use a Funke that belonged to someone else. This was an accurate tester but the AVO is more pleasant to use. Lucien

5th Oct 2010, 9:33 am	#4
David Pannell Retired Dormant Member Join Date: Oct 2010 Location: Horsham, West Sussex, UK Posts: 77	Re: Which AVO VCM is best? Hi Martin and Lucien, Thank you for your welcome, and for your interesting comments. Calibration is not a problem, as I have close contacts with a local calibration house, and have also been calibrating the VCM myself for many years along the lines mentioned. I wonder if anyone knows what the significant differences are between the various Marks of this instrument. It would be interesting to know, particularly if they all have the miniature rectifying valve VR92, or whether the later versions were fitted with a silicon diode. Also I am intrigued to know if anyone has substituted this valve for a more conventional rectifying valve, rather than swapping it for a silicon jobbie. I like to keep things as original as possible and I do have some spare VR92s, but in the event, I wonder if fitting a more conventional valve is possible. I want to avoid eventually having to use solid state if possible. Cheers. David.

5th Oct 2010, 10:31 am	#5
Dekatron Octode Join Date: May 2009 Location: Linkoping, Sweden Posts: 1,465	Re: Which AVO VCM is best? Hi David, If you read what I have written on the AVO Mk IV and also the AVO CT160 here on the forum for the last months you will see a lot of interresting information on the AVO Valve Testers. Search for threads I have either started or threads I have commented on. I have not worked with the AVO Mk I nor the Mk II myself but in later models like the AVO Mk III and the AVO CT160 you can replace the valve rectifiers (6AL5W and equivalents) with Silicon diodes but you will not only have to recalibrate the tester then but in some cases you will have to change resistors to compensate for the lower voltage drop across a Silicon diode. You can read what Martin Scobie did to refurbish his AVO Mk II here: http://www.marie34.freeserve.co.uk/page9.htm - but he did not change the VR92 and unfortunately he did have problems fitting Silicon diodes across the meter to protect it, however my friend Euan MacKenzie in Australia has fitted modern Silicon diodes across the meter without any adverse effects on measuring. If I ever come across an AVO Mk I or Mk II I will write an article on them too. My focus is nopw on my refurbishing of my VCM163 and a few shipping damaged Taylor 45Ds. __________________ Martin, Sweden

6th Oct 2010, 12:36 pm	#6
David Pannell Retired Dormant Member Join Date: Oct 2010 Location: Horsham, West Sussex, UK Posts: 77	Re: Which AVO VCM is best? I believe that some of the later VCM instruments used the 6AL5W military grade rectifying valve, presumably for deriving the negative grid volts supply. As the VR92 is almost impossible to obtain, I was wondering if I might try out a 6AL5W, with possible adjustment, to see if it works on the Mk2. Heater current is 300mA as opposed to 150mA, but the transformer windings look more than man enough for the job. It might even be feasible to parallel up the anodes and cathodes. Any thoughts, or am I ? David.