AVO VCM Type 3: Calibrating for Rectifiers & Diodes

[radiozero]

Hi. I have a MK 1, type 3 VCM. Probably just known as MK 1.

I need to ensure that when I test my 5R4GY the reading is sufficiently accurate.

The way that rectifiers and diodes are tested is that the device is put under a load, such that a good valve would pass a pre-determined current, or range of current.

The VCM has several load positions for diodes/rectifiers: 1mA, 5mA, 15mA, 30mA, 60mA, and 120mA.

In the case of my 5R4GY, a load is chosen such that 60mA of current should pass if the valve is good.

The load AVO chose, is effectively 1800R shunted by a 8uF capacitor.

In order to test this arrangement, I created a circuit in Duncan Amps PSU Designer II.

The results were:

I(D1) = 59.825 (Mean) -- 119.71mA (RMS)

I(R1) = 57.499 mA (Mean) -- 60.331 mA (RMS).


Looks like things are okay, because the simulation gives a mean current of 59.825 mA, and the VCM meter I believe shows mean currents.

Okay, now according to an article I have, the way to test the VCM calibration on the 60 mA range is to place a milliammeter and 560R resistor in series with a 1N4006 diode or equivalent to represent a "good" valve rectifier.

The VCM meter itself should read what the outboard millameter is showing (IL) x 1000 divided by 781. If the outboard milliameter did show 60 Ma - the VCM meter would read 76.82 on the 0-100 scale.

All of the tests from 1 mA - 120 mA should produce a VCM meter reading of between 76.82 and 77.46. Plus or minus 5%.

PS. I wonder why a value of 560R was chosen for the series resistor. And why: (IL) x 1000 divided by 781.

[radiozero]

The reasoning of AVO as to the scale must be as follows:

When 60mA flows, the VCM meter is going to read about 80 on the 0-100 scale. Say 60mA current made the VCM meter go to 80.

That means the meter is showing the actual current x 0.75. Looking at the "Good" (green) scale, the limit on the left is 50 on the 0-100 scale. That means the actual current is 50 x 0.75 = 37.5 mA. So, AVO decided that a recifier is deemed "good" when the current is 37.5 mA or higher on the 60 mA test.

[Dekatron]

On later AVO VCMs there is an extra line drawn at the center of the GOOD scale and that line lines up with 74 on the mA scale, so it is 74% of full FSD.

You can read about the scales and the design AVO used here: https://www.vintage-radio.net/forum/...ad.php?t=86262 in a large document that I wrote on this and other aspects of the design.

The early AVO VCMs use the same scale but don't have the line drawn, but the middle of the GOOD scale is still at 74%.

The design of the measurement is the same for all of the AVO VCM Mk I, Mk II, Mk III, Mk IV, CT160 and CT160A - what differs are voltages and therefore shunt and series resistors. There is a small difference on the outer scale for measuring resistance as that depends on the voltage/current used for the calibration circuit but it works the same.

I've included an AVO Mk I/II scale and an AVO Mk III scale for comparison.

/Martin

[radiozero]

If, when you set up a test circuit, to ensure you get accurate results testing rectifiers, you substitute a semiconductor for a valve, the article I'm reading says place a 560R in series with IN4006 (current 60 mA range). Then the VCM meter should read about 76.82.

Ignoring the fact that perhaps the meter should read 74, I note that in Duncan Amps PSU Designer II, this diode resistor combination results in current through the diode of 47.566 mA. But, we should be getting 60 mA surely. That means I'd need to connect a 270R in series with 1N4006, because that gives current through the diode of 60.467 mA.

So, the 560R in series is wrong? In order to simulate a vacuum tube rectifier, the value should be 270R. It seems.

[Dekatron]

Did you include the internal resistance of the series resistors in the tester and the internal resistance of the "rectifier" tube in the calculation for the PSU designer-II? Those are the only resistance that I can think of that can affect the current in the circuit apart from the voltage being half wave rectified.

What voltage do you see across the 560 Ohm resistor if you measure it with a modern DMM?

/Martin

[radiozero]

In Duncan Amps PSU Designer II, you can select a 5R4GY rectifier. That is included in the library of components. I don't know what value of internal resistance might have been associated with that rectifier.

The series resistance in the VCM, associated with the shunting of the meter is 1.584256R (1.58R) assuming R34 is 1.5K and the meter resistance is 120R. That will have I think little effect on the current thru the rectifier. So I did not include it.

When I ran the simulation with the load of 1800R, which is the load resistance connected on the 60 mA range of the VCM, I was pleasantly surprised to see the mean current thru the 5R4GY given as 59.825 mA. Because as understand it, a good valve loaded with 1800R, should result in a current of 60 mA.

Then, what I've gone and done is done a simulation with the test set up, given as 560R in series with 1N4006. In this test, to test VCM rectifier calibration, I'm assuming that when the meter shows 76.82% (perhaps it ought to be 74%) the current is 60 mA. One would think that is the point of this test set up, to produce a current of 60 mA.

Perhaps it does, but not in the Duncan Amps simulation.

I wonder if I have the capacitor in the right spot.

[Radio Wrangler]

With the 5R4 working into a resistive load with 8uF across it, there is some similarity to a 5R4 in its intended sort of application, except it will be being tested as a half-wave rectifier when the device will most usually be run in a full-wave circuit.

Because of that 8uF capacitor holding charge, the current conducted by the half-5R4 will be appreciably less than half a cycle long, and the current won't be a half-sine either.

This raises the question of just what number you would attach to such a current when it's really a complex waveform and simplified values like mean, RMS, peak etc are all waveform dependant.

It also raises the question of just what sort of current reading AVO had in mind when they chose the shunting and scaling of the meter.

And then there is the issue of what a DVM reads when carrying an interesting current waveform. On a DC range, it'll probably be the mean, but I've come across some units which tend to read peak, and I've got one which reads true RMS even on a DC range. Of course all these values converge when measuring pure DC, but the matter a lot measuring rectified DC. Measuring the current in the load resistor gets the advantage of filtering from the capacitor, and it gives a fair indication of whether a rectifier is doing its job.

Calibrating these valve testers against any sort of conventional meter is a very involved process, I'm afraid.

David

[Dekatron]

Measuring the voltage across the 560 Ohm resistor and then dividing it with 560 (or whatever the resistance measures) will give you the current flowing in the circuit, but you will have to use a modern DMM on the DC range to get correct measurements. Older DMMs are not good enough when they encounter half wave rectified voltages, or other waveforms but modern ones give a good enough figure for these measurements I have seen. I use Fluke 87 III and 87 V. Earlier I had a Beckman RMS 3030 but that was more than 12% off while measuring this voltage in an AVO Mk IV, but my Flukes were spot on.

AVO recommends the U52 for rectifier tests on the AVO Mk III and beyond.

Since the mains voltage is quite skewed these days due to all of our electrical appliances and also due to a lot of communicating protocols using the mains wires you will need to use either a motor-generator or a pure sine-wave inverter to make a correct calibration (as good as it can be done anyway) as you will only have a pure sine-wave to work with then. The transformers themselves affect the sine-wave too but not as much as what is present on the mains voltage from other stuff. I made these checks myself and found that I got a difference of some 3-5% between using the mains voltage unchanged compared to the pure sine-wave inverter - it also looked a lot better on an oscilloscope. However since you are probably going to use the tester on the mains voltage without a pure sine-wave inverter you will have to check how it behaves then and maybe calibrate it for that use.

In the document I wrote I have some figures of the grid and anode voltage where you can see that the tip of the sinewave is skewed, it is not a nice sinewave signal as it should be, but with a pure sine-wave inverter it looks like it should.

/Martin

[radiozero]

My VCM is MKI.

Aaah, so AVO themselves recommended a test circuit - which is plugging in a U52.

What is nice about that, to some degree, is that the data sheet actually shows Effective Valve Resistance.

Anyway, if you sought to replace a U52, with a 1N4006 plus series resistor, what you would do is select a resistor value, by trial and error (probably), until the current was the same as when the U52 is plugged in.

Or seek to use the value given for Effective Valve Resistance.

[Dekatron]

Just remember that AVO did not intend the rectifier tests to test a valve continuosly as it will damage the load resistors if you run them over their ratings!

The service manual for the AVO VCM163 mention these tests and other tests.

[radiozero]

On page 10 of VCM163 Service Manual, we see that in fact AVO indicates as a standard rectifier a DD058 semiconductor in series with 150R. Then to plug in a U52 and the reading should lie between 70 and 80 on 0-100 scale.

As I understand it, rectifiers with low effective supply impedance/internal resistance would test good at near 100, whilst high impedance perhaps 60.

To get more accurate results, you would make an adjustment to your readings. For instance, if you got a reading of 80 using your standard rectifier, and you knew it's resistance to current flow, and if you knew the internal resistance of the rectifier under test, you could make an adjustment to the reading. It might then be, that a perfect rectifier, of the type you are testing, should show 70 on *your* meter.

I say your meter because according to AVO for the VCM163 a standard rectifier could produce a meter reading anywhere from 75 to 90 for the 60 mA position.

[radiozero]

Here is the table of expected meter readings for VCM163, when using the standard rectifier (DD058 in series with 150R):

D/R Range Reading Mean
(mA)
180 67-82 74.5
120 70-85 77.5
60 75-90 82.5
30 80-95 87.5
15 85-95 90
5 87-97 92
1 95-103 99

These figures reflect the use a a single standard rectifier. Ideally, you would use a standard rectifier for every range.

Then AVO say insert a U52 and the meter should read 70-80. The mean is 75, prety close to 74.5, which is the mean for the 120 mA range (only).

[Dekatron]

AVO were only interested in showing that the meter was showing the correct measurement on one setting/scale.

And by making sure that the reading is correct on one range and also checking that the voltages and resistors are correct for the other ranges it will give that the other ranges are correct too, so you only need one valve or diode/resistor combination to check all ranges if you measure the other parameters.

I am not sure that I wrote that the 74% value for the 1mA/V line position comes from the fact that AVO says that the meter should show 22.2uA at this line, that divided by the FSD current of 30uA results in 0.74 = 74%. A little bit off-topic on this is that on a lot of AVO meters that I have checked, that has the correct 30uA FSD the line is at 22.5uA which corresponds to 75%, those meters had the more modern moving coil meter that is used in the VCM163 although they were placed in AVI Mk IVs and CT160s having 30uA FSD and 3250 Ohms internal resistance at 20 degrees Celsius - they didn't contain the old swamp resistor which was wound on a bobbin but a similar swamp resistor to what is found inside the Ia meter on the VCM163, one wound on a piece of SBRP and dipped in glue.

Also, the tolerances for the tester is some +/-5% and many valves show a spread of +/-10% or even more but are nevertheless deemed as "new" so you will have quite a large spread for a good valve.

I always compare any AVO that I repair to readings from my Roetest valve tester and I am not satisfied with the calibration until I get comparable readings.

/Martin

[Dekatron]

You can see exactly how the circuit for D/R measurements in the CT160 and its calculations for the scale were done in this chapter in the document that I wrote, Pages 39-40 chapter 2.6 shows this.

You can substitute values in the equations for the other testers and you should get the same results.

/Martin