Testing 5R4GY rectifier on AVO MKII

[radiozero]

Hi.

Ia on the AVO tables, says 60 mA.

On the METER SELECTOR inner scale, I should set that to 60.

I think so.

I got confused a bit. I was setting it to 15 (because there isn't a 20) going from the 20mA/V figure given in the table. I think I've corrected myself. I was reading wrong column.

[radiozero]

My valves are coming out just on the lower end of "GOOD". I think I will try to calibrate the AVO to make sure of correct readings. Shown in diagram is basics of test circuit for rectifiers.

[radiozero]

This is what AVO manual says:

The Basic Method of Testing for Diodes and Rectifiers


Any simple emission test at a low voltage must necessarily give rise to a purely empirical figure for the valve in question which cannot necessarily be co-related with any one of the maker's characteristics and which, owing to the fact that it relates to a lower bend portion of the rectifier characteristic may vary widely for any given type of valve.

The important function of a rectifying valve is that it will, under suitable reservoir load conditions, produce sufficient current to operate the apparatus which it intended to supply.

The fundamental requirement, therefore, is the basis of rectifier testing in the Valve Characteristic Meter. A sufficiently high AC voltage is applied to operate the valve above the bend in its characteristic, and to ensure that its internal voltage drop is negligible.

With a suitable reservoir condenser in circuit, the DC load is adjusted to corre4spond to a number of DC current conditions, i.e. 5mA, 15mA, 30mA, 60mA and 120mA. The actual current flowing in the load is then indicated on a meter shunted to correspond with the DC load required. The meter reading will then indicate as a percentage, the comparative efficiency of the valve on the basis of this required DC load.

Each half of a full wave rectifying valve is tested separately thus enabling matching of two halves to be checked and any tendency to produce hum by partial half waving to be indicated.

The pre-deternined load figures are chosen so that they not only give a sufficiently wide range of currents to cater for the normal requirements of electronic apparatus, but also correspond to the DC maximum emission figures usually quoted by manufacturers in their rectifying data.

Signal diode valves are similarly tested, but a lower AC voltage is applied and comparison is made with a single DC load figure of 1mA, this figure being normally more than sufficient to cover the rectified signal current that would be obtained. The basic operating circuit of the diode and rectifier system is shown in Figure 2.

[radiozero]

MAKING SURE THE VCM IS CALIBRATED OK: As far as I understand, when a valve in inserted and the valve is tested for anode current, anode current is flowing through the VCM meter (on the valve tester) and whatever the value is on the VCM meter, the value should be within +/- 2% of the current reading measured on an external meter connected between the current link terminals. Got this.

You must ensure that the external meter reads 0.5 times the respective full scale anode reading of the VCM meter +/- 2%. Not sure what this means. Would if it just said "anode reading of the VCM meter", but it says "respective full scale anode reading of the VCM meter".

[radiozero]

Firstly, How does the VCM work? Ia is "measured in terms of DC current". Well, what does that mean? Well, any anode current in any type of valve has to be direct current. In the VCM it's varying DC current, not steady. Va then has to be a varying DC voltage. Va is, I believe, half-wave rectified AC in all tests. When rectifiers are tested, it's the actual valve under test which is rectifying the AC applied.

I think what AVO did in patent 606707, was simply to measure Ia, which is DC current, in the usual way, by putting an ammeter is series, but to use rectified AC for the voltages. In other words, the VCM meter isn't an AC meter, as such, it's a regular DC meter.

Now, as far as I know, a regular DC ammeter gives you the average value of current when the waveform is varying. The average value of a sinewave is 0.637 x Vp. Given that half wave rectification is used, I guess then, compared to steady DC voltage, the average voltage for the anode is 0.3185 x Vp.

If this is the case, then I suppose you make the AC voltage, to feed the anode, 3.139 times the steady DC voltage actually required. So, if the table says Anode Voltage 100V, the AC voltage is 313.9V which is then half-wave rectified and applied to the anode.

Is this reasong correct, or not? My math is probably screwy.

EDIT. Regular DC ammeters do read average, but marked to show RMS.

[radiozero]

I guess much hangs on how the DC ammeter is marked up.

I'm trying to figure out why the external meter should read 0.5 times VCM meter.

[radiozero]

A regular DC ammeter is calibrated to show RMS current as far as I know.

You can arrange for the meter to read average current.

RMS current is 1.11 times avearage current.

If you had two ammeters in series one reading RMS and the other average, and you were looking at the averaging meter, to get Ia in RMS terms you would times by 1.11.

If you had two voltmeters in parrallel one reading RMS and the other average, and you were looking at the averaging meter, to get Va in RMS terms you would times by 1.11.

Is this why in the MK11 manual on page 7 we read?:

"Va RMS = 1.1 Va indicated DC"

[radiozero]

This is what I think is true:

A regular AVOMETER 8, set to read AC current (or voltage) only gives a true reading when the current or voltage is in fact alternating. And the meter is calibrated to read RMS values under these conditions.

If the current only flows one way, or the voltage is either posative, or negative, but does not alternate, as in half-wave rectification, the meter will show half the RMS value.

In the setting up details, you put say an AVOMETER 8 across the current link of the VCM. The meter is supposed (I think) to read 0.5 x the VCM meter reading. The details don't say whether you set the AVOMETER to AC or DC current.

Assuming this is correct, about the AVOMETER needing to read 0.5 x the VCM meter, then my only explanation of this is that the current being measured is half-wave, pulsing in only one direction, and we are using an external meter set up to measure RMS (set to read AC current). Therefore, it needs to read 0.5 x the VCM meter, because that meter is not calibrated to read AC current, just pulsating DC current, as in half wave rectified current.

Am I right? Thanks.

[radiozero]

The AVOMETER has to be on DC current. Therefore as far as currents are concerned, the actual current passing through the anode of a valve, or what is being measured, is an average current and the VCM meter is reading average DC current. But, the VCM shows the current flowing at twice what the average current actually is. Under the test conditions then, if a valve needs to show as passing 100mA, it's actually passing 50mA on average, but it would pass 100mA on average if voltages were DC and steady. That's my understanding of what is happenning with the AVO VCM MKII.