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Old 11th Sep 2017, 4:04 pm   #1
WME_bill
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Default Measuring half wave voltages on Valve Testers.

The recent postings about measuring the grid voltage on a Taylor 45 valve tester has got me puzzled, as I cannot get the mathematical relations to work out.
Taylor 45 tester voltages.
Can others help. I have recently sold my 45C, so speaking from the records I have retained and comparing with my CT160.
1. Grid Voltage pot. Appears to be an open wirewound with a linear scale, giving a panel calibration 0 to 15V linear. Others talk of a 3 stage pots, as used on the Avo CT160.
2. My circuit diagram says the grid winding on the transformer is 20v, which must be rms, as I have a note of 60vpk-pk (presumably from my oscilloscope).
3. The grid voltage is rectified half wave, and the dial calibration is presumably the mean grid voltage applied to the valve.
4. The moving coil meter will read the mean of the voltage.
5. Now the grid 20v rms is (1/root2) peak = 30v pk, which is as noted above. This is the familiar x1.4 rms to peak.
6. The mean of half a sine wave is (2/pi)x peak. (x0.64).
The average over a whole cycle of half wave rectified, mean = (1/pi) x pk. (0.32).
7. I would therefore expect the grid mean voltage as the dial to be pk/pi (30/pi) = 10v.
8. Yet the dial is calibrated 15v linear.
9. Can anyone tell me where I am wrong please.

10. Another point, a normal moving coil meter with rectifer (as Avo 8) measures the full wave mean voltage (0.64 x pk). The rms value is (pk/root) = 0.707 x pk. So the scale of rectifier meters is fudged (0.707/0.64) to show rms values. (x1.1). Fine with sine wave, but in error for a square wave or a distorted wave or our half wave rectified.
11. So you cannot calibrate the voltage on your grid control by directly reading from your Avo8.
12. The Avo VCM servicing says use a valve voltmeter. These use a rectifier, and measure the peak voltage. The Calibration is fudged again to show the rms value.
Same applies for most digital meters.(Fluke 70 series, Fluke 8000series). If it says that max AC input is sinsusoidal, then it is rectifier, and worried about peak ratings.
A superior digital will measure the true rms.(Solartron7150). It will tell you that prominently, as it is a selling point.
13. Yet an earlier posting (17 Sept) quotes "Use the Avo Electronic Testmeter (or equivalent DC mean Valve Voltmeter)" to measure the grid voltage, which should be half that set on the dial. For the Avo VCM/ CT160.
14. I have a1959 copy of the factory set up instructions for VCM MkIII, which says for checking, use a standardised DC mean valve voltmeter, and for setting up, a standardised DC valve voltmeter.
15. I suggest this is an error in the Avo VCM setting up instructions. The Avo Electronic Test Meter circuit is a peak reading rectifier, calibrated to rms (x 0.707 or 1/root2). I have two editions of the ETM manual and the CT38, and they all state that it reads peak, and fudges the scale to show rms. not the mean.
16. I would welcome comments from the Avo experts.

17. As a final comment. The Taylor uses a single rectified half wave for the grid voltage. So measuring it will be valid.
18. The Avo VCM and CT160 use two out of phase half wave voltages, one valid when the other electrodes are positive, and one greater. They are different peak values. Hence the need to open the link on the Avo VCM when measuring the grid voltage, so you get just the actual grid peak voltage.
19. I was interested to see the comment that this feature is in the original Avo patent, and intended to keep the grid well negative during the dormant phase. Not a feature though of the Taylor.
20. Still does not clarify to me what the actual voltage is which you should be measuring to correspond to the calibration dial on the Tester.
The Avo VCM says that grid voltage should be 0.52 of indicated DC. But not clear what sort of meter being used to measure. As it talks of anode voltage = 1.1 indicated voltage, it suggests a rms reading meter.
21. How is the 0.52 of indicated DC calculated.
22. More helpful suggestions please.
I can post summary circuits of the Taylor and VCM circuit when measuring Gm, and the Avo ETM and CT38 when on AC. See also Scroggie on Rectification, Wireless World Oct 1954 and Scroggie Radio Laboratory Handbook, 6th and 8th edns, Ch 5.
wme_bill m0wpn

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Old 11th Sep 2017, 5:07 pm   #2
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Default Re: Measuring half wave voltages on Valve Testers.

I look forward to the replies and only can comment on point number 1 which refers to 3 stage pots. As far as I know these are only used a the A pot.
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Old 11th Sep 2017, 6:25 pm   #3
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Default Re: Measuring half wave voltages on Valve Testers.

It only gets worse, Bill once you dig into it.

The anode current is a function of both grid-cathode voltage and anode-cathode voltage and both are changing at once. The anode current waveform is distorted by this and also by the characteristic of the valve being tested. So what sort of meter response is appropriate for the Ia reading? and what fiddle factors are needed?

I'm afraid I can't take these machines seriously as measuring instruments. Their pointers point to a number, but it's very difficult to determine what it actually means. I think AVO put in fudge factors so that middle of the road valve types produced numbers of about those in their manufacturer's data sheets.

Trying to relate settings of grid and anode volts to what will be read on a bench meter of any characteristic is more involved than I want to do.

I keep a Taylor 45D here and it does service screening valves at the radio club to weed out definite duds and low emission ones, but that's about all I trust it for.

If I wanted to measure valve characteristics with any hope of traceability, I'd build a DC tester like David Simpson's or else a Sussex. With real DC all these issues are blown away.

I still think the AVO VCMs are attractive bits of classic kit and one would look great in the shack, and they are interesting restoration projects in themselves.

David
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Old 11th Sep 2017, 7:06 pm   #4
Bazz4CQJ
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Default Re: Measuring half wave voltages on Valve Testers.

When David Simpson offered to supply standardised valves a few weeks ago, my impression was that the feedback from people with Avo VCM's generally showed pretty good agreement with his DC measurements. I don't know if David or anyone else compared the data in detail and came to any firm conclusions?

B
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Old 11th Sep 2017, 9:23 pm   #5
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Default Re: Measuring half wave voltages on Valve Testers.

I hope I answer some of your questions below and don't complicate things even more.

If you read my document here: https://www.vintage-radio.net/forum/...ad.php?t=86262 on the AVO VCM internals you can read about the 0.52 fudge factor in chapter 2.3 for the CT160 and Appendix 3 for the Mk IV and you can also read about the two negative half periods on the grid control in chapter 2.1.4 and why the circuit was changed in the CT160/A.

The "fudge" factor is nothing else than a way of choosing components to produce a delta voltage on the grid that corresponds to the mA/V setting on the scale, it has nothing to do with AC, DC, Mean or RMS voltages at all, it is just a factor. This delta voltage should position the needle on the 1mA/V position on the scale if the valve under test has a mA/V reading that corresponds to the setting on the mA/V control.

The two negative half wave periods, which were split up in the CT160/A to two separate signals, is used for calibration purposes as well as for the Insulation and Leakage tests.

In the Taylor series of valve testers the cathode resistor is shorted, after the standing Anode current has been backed off, when the mA/V reading is taken. The Taylor testers use two different cathode resistors, one for each range, and they have been chosen to produce a mA/V reading on the scale. You can read about this design in the Taylor 45D and 45D2 manuals where this is described in chapter 11. Taylor talks about changing the bias of the grid volts/cathode circuit when shorting the cathode resistor to produce a grid volts change, this bias change is done since the grid volts control is in series with the cathode resistor, there is also a circuit that shows this. The AVO VCMs do it in a similar way but the circuit is more complicated.

Using a scope to look at the voltages will let you see how they are changed when making a mA/V measurement in each tester, don't forget to use an isolation transformer or battery operated scope so you don't form a ground loop that destroys the scope or tester. You will have to connect the ground electrode from the scope to the correct common ground point in the tester get correct readings though!
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Old 11th Sep 2017, 10:21 pm   #6
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Default Re: Measuring half wave voltages on Valve Testers.

David Simpson's valves gave a good indication that various people's valve testers gave reasonably consistent figures for the transconductance. This is good.

However, the transconductance measurement uses a sort of bridge circuit and compares measurements made on the same meter having the same dynamics, so, as Martin describes some of the fudge factors vanish and the measurement results turn out to be useful in practice.

The difficulty with these instruments comes if you try to use ordinary lab instruments to measure the voltages the scales on their knobs imply are being applied. So even with a freshly calibrated lab meter, the best you can do is to get a calibrated valve to check the VCM against. That's what AVO supplied back in the day. From the metrology viewpoint, it's very messy.

David
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Old 11th Sep 2017, 11:13 pm   #7
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Default Re: Measuring half wave voltages on Valve Testers.

I'm working on a Solid State Calibration Vacuum Tube (SSCVT) right now, I have one design that works on my Roetest (www.roehrentest.de) as well as on my VCM163 but I still haven't tested it on my AVO Mk IV since it is in storage and will take some time and effort to dig out plus it will need to be calibrated to show correct measurements. The VCM163 and the Mk IV measures a tube in different ways so I will have to make sure that it works on both. I have another design that works on the Roetest but I will see if I get it to work on the VCM163, still some bugs to kill, this design might work better on the Mk IV though.

I'll publish something here on the forum when I am finished testing it on my Mk IV but that might unfortunately take some more time since I am in the middle of changing jobs right now.
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Old 12th Sep 2017, 10:54 am   #8
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Default Re: Measuring half wave voltages on Valve Testers.

If silicon rectifiers were around back in the old days, would they still have used the AC testing method?

Lawrence.
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Old 12th Sep 2017, 12:17 pm   #9
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Default Re: Measuring half wave voltages on Valve Testers.

In my opinion the AVO testers were mainly designed to be simpler than the DC testers as they didn't need any voltage regulators for the Anode, Screen and grid voltages, just different tappings on a transformer and the correct ratio for these voltages.

AVO still needed rectifiers in the grid volts circuit though, in the Anode circuit the tube itself worked as the rectifier. Later AVO added diodes in the Anode and Screen circuits and according to their documents they did so so that the tube wouldn't conduct current in unwanted ways.

AVO would still have gone with their design no matter if there were silicon diodes available or not I think.
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Old 12th Sep 2017, 8:22 pm   #10
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Default Re: Measuring half wave voltages on Valve Testers.

Bill, AVO's, Taylors, etc only work on 20mS pulses from 50Hz sinewaves applied to valve electrodes. Basically - Full AC on the Anode, +ve h/w pulses on the Screen, and -ve h/w pulses on the grid.
Yes, they have an RMS component, but that's only used for calibration. Scoping the waveforms is the best way to understand. CT160's for example derive pulses from 2 separate t/f windings - 55V & 66V.
Just keep looking back over heaps & heaps of info via "Search".

Regards, David
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Old 12th Sep 2017, 9:02 pm   #11
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Default Re: Measuring half wave voltages on Valve Testers.

The AVO CT160 is specified for an operating frequency between 50-500 c/s as per the manual, the CT160/A also adds +/-10% for these frequencies. So pulses with a time between 1.8182mS up to 22.2mS works very well in these two models.

The AVO Mk III and IV are specified for 50Hz use but work well on 60Hz and I've tested them on 440Hz and they work equally well there even though the transformers might not be specified for that frequency (the CT160 series mentions extra shielding and also has some extra capacitors for use in higher frequencies on some tests).

I ran my AVOs from a dynamotor (motor/generator) for aircrafts which I could run from 40 Hz up to 440 Hz and they performed equally well between 45-440 Hz (I didn't go lower than the specified -10% for the CT160/A that I own). There was a very slight distortion of the sinusoidal waveform at higher frequencies on the Mk III and IV but since it affected both the Anode and the Grid voltage the effect was almost completely gone from the measurement result.

The maths works at any frequency, so as long as the transformers don't distort the sinusoidal waveform too much the measurements work very well.
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Old 12th Sep 2017, 10:22 pm   #12
Craig Sawyers
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Default Re: Measuring half wave voltages on Valve Testers.

The theory behind the way that the AVO II, III and IV work is in their patent number 606707 (1948) attached. The AVO two-box AKA AVO I operated by a different (but still AC) principle described in their patent number 480752 (1938) also attached.

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Old 13th Sep 2017, 1:34 am   #13
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Default Re: Measuring half wave voltages on Valve Testers.

Quote:
Originally Posted by Dekatron View Post
so as long as the transformers don't distort the sinusoidal waveform too much the measurements work very well.
I have a 2kW constant voltage transformer which works by some ferromagnetic effect which I don't present recall in detail, but it produces a distinctly non-sinusoidal waveform. Now, a constant voltage suppy for my 163 would be nice, but when I tried it on the 163, that particular unit had quite a significant effect results obtained; unuseable!

B
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Old 13th Sep 2017, 5:58 am   #14
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Default Re: Measuring half wave voltages on Valve Testers.

Quote:
Originally Posted by Bazz4CQJ View Post
I have a 2kW constant voltage transformer which works by some ferromagnetic effect which I don't present recall in detail, but it produces a distinctly non-sinusoidal waveform. Now, a constant voltage suppy for my 163 would be nice, but when I tried it on the 163, that particular unit had quite a significant effect results obtained; unuseable!
B
Somewhere in their manuals AVO mentions that you shouldn't use constant voltage transformers due to the effect they have on the measurements, but unfortunately they don't write it in all of their manuals.

The VCM163 is sensitive to outside noise from switching power supplies, constant voltage transformer which produces overtones due to the non sinusoidal waveform and other equipment that distorts the waveform like solar panel inverters.

I've used constant voltage transformers that works like a variac but with a motor that controls the shaft of the variac, where the motor is controlled from a voltage sensing circuit. They work just fine but they didn't have much of an effect since the voltage is quite good where I live, but for people living in areas with more fluctuations they might be good.

I sometimes check my measurements on the VCM163 by using a small isolating transformer as that filters out some of the noise on the power lines and I always use it if I run the VCM163 with open panels for some tests. Especially now that I am developing the Solid State Calibration Vacuum Tube - I just have to remember to remove it for some tests as the design might be sensitive to the noise too and that has to be taken into account!

/Martin
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Old 13th Sep 2017, 7:13 am   #15
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Default Re: Measuring half wave voltages on Valve Testers.

Some years ago I looked into why a VCM mkIV was giving odd readings and it turned out to being used in a part of the factory with a very badly distorted mains waveform with flat topping and phase synchronous spikes and rings. There was about 2MW going into fluorescent strip lighting and electronic equipment (no PFC) and a machine shop on the floor below with lots of phase-controlled variable speed drives.

I ran it from 50Hz from an HP 651A wien bridge oscillator and a huge 2kW 'Elgar' amplifier/mains simulator and all was well.

The other thing distorted waveform does is change the amount of heat delivered by the heater.

Bad mans waveforms aren't a factory thing, modern housing areas are rather bad too, especially as less and less of the base load is resistive. All these energy efficient bulbs and smart motors are usually below the power level where PFC becomes compulsory, but there are now so many of them....

David
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Old 13th Sep 2017, 9:37 am   #16
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Default Re: Measuring half wave voltages on Valve Testers.

Bill, I reckon from the answers you've received, and the heaps & heaps of banging-on that myself & others have done over the years, that you can be confident in using a Taylor 45 or an AVO VCM or CT160, or a Mullard. They'll all test valves reasonably well if they are up to cal.
Please don't worry about the theoretical RMS values of the sinewaves & half sinewaves used. As long as the Manual's calibration procedures are followed, these AC testers will plod on for years - giving good results. As you will see from Craig's post concerning AVO's patents, a helluva lot of maths is concerned. When you think about it - the valve under test is only subjected to passing Ia during the 10mS of the positive halve cycle using 50Hz. But even then, only at the crest of the waveform, does the full potential effective peak Ia flow. Two or three mS at the most. If we all had stupendous eyesight we could see the valve tester's meter's needle vibrating 50 times a second. Hence its best to study the waveforms using a scope. To delve into the maths of "crest factor" involves a serious amount of calculus.
As David & others have said many times, Taylors & Mullards are great at screening out duff valves in your collection. A "Green Zone" reading will suffice for any valve being used from 1920's TRF's up to thumping big radiograms of the 50's. However, if you're a serious audio amp designer, then the ability of an AVO VCM to tabulate Ia against Vg is what you need.
I've said this a million times - if you've set Vh, Va, &Vs correctly, then the only thing that controls Ia is Vg. Always set Vg a bit higher than recommended before the Ia test - thus avoiding "whanging" of a tester's meter.Then reduce to the recommended value.
As well as recently testing a number of standardised valves, I've also been testing quite a few valves for local vintage chums. From the standpoint of 17 years into the 21st century, looking back at the vast variety of valves from 90/80/70 or whatever years ago - there is sometimes a helluva difference in manufacturer's specifications, compared to AVO's VDM, Taylor's Valve Charts, or Mullard's cards.

Regards, David
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Old 13th Sep 2017, 2:33 pm   #17
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Default Re: Measuring half wave voltages on Valve Testers.

Quote:
Originally Posted by WME_bill View Post
The recent postings about measuring the grid voltage on a Taylor 45 valve tester has got me puzzled, as I cannot get the mathematical relations to work out.
Taylor 45 tester voltages.
Can others help. I have recently sold my 45C, so speaking from the records I have retained and comparing with my CT160.
1. Grid Voltage pot. Appears to be an open wirewound with a linear scale, giving a panel calibration 0 to 15V linear. Others talk of a 3 stage pots, as used on the Avo CT160.
2. My circuit diagram says the grid winding on the transformer is 20v, which must be rms, as I have a note of 60vpk-pk (presumably from my oscilloscope).
3. The grid voltage is rectified half wave, and the dial calibration is presumably the mean grid voltage applied to the valve.
4. The moving coil meter will read the mean of the voltage.
5. Now the grid 20v rms is (1/root2) peak = 30v pk, which is as noted above. This is the familiar x1.4 rms to peak.
6. The mean of half a sine wave is (2/pi)x peak. (x0.64).
The average over a whole cycle of half wave rectified, mean = (1/pi) x pk. (0.32).
7. I would therefore expect the grid mean voltage as the dial to be pk/pi (30/pi) = 10v.
8. Yet the dial is calibrated 15v linear.
[B]9. Can anyone tell me where I am wrong please.
So far as I remember the RMS out from a half wave rectifier = RMS(in)*0.707.

Based on your 'scope measurement then 30vpeak*0.707 = 21.21VRMS into the rectifier therefore out from the rectifier will be 21.21VRMS*0.707 which = 14.99 volts.

Or am I totally off beam...

Lawrence.
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Old 13th Sep 2017, 4:58 pm   #18
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Default Re: Measuring half wave voltages on Valve Testers.

Ignore what I wrote, brain not engaged woz to late to delete.

Lawrence.
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Old 13th Sep 2017, 8:15 pm   #19
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Default Re: Measuring half wave voltages on Valve Testers.

Lawrence, Bill & Folk - You have to stop imagining that a valve under test in a Taylor or AVO or Mullard, is under the same conditions as it would be in a superhet's or amplifier's DC circuitry. I.e. a steady smoothed DC HT, a steady smoothed -ve Vg from a battery or smoothed fwr.
The voltages marked on the Va, Vs & Vg switches & pots are just shown in RMS terms for calibration & simplification. In real terms - only during the first quadrant of a mains derived peak to peak sinewave, (when Va & Vs are in phase, & Vg is in antiphase) does all those electrodes receive a constantly rising working voltage, for just 5mS of a 50Hz's 20mS period. Then everything drops rapidly back to zero in the 2nd quadrant. No current flows for the last two quadrants at all.
Circuit design & clever mathematics by AVO & the other tester manufacturers (involving calculus - dy by dt, etc, or in vcm terms - dIa by dVg), ensure that AC operating testers achieve GM results which are pretty close to pure DC testing conditions. This is shown by the jolly good graphed results that a number of folk posted in my "Standardised Valve" thread. Many thanks, by the way, to all those contributors.



Regards, David

Last edited by David Simpson; 13th Sep 2017 at 8:34 pm. Reason: Ad'l info.
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Old 13th Sep 2017, 9:05 pm   #20
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Default Re: Measuring half wave voltages on Valve Testers.

To be fair, I never imagined it as a steady smooth DC if using AC to supply the anode, screen/ cathode voltage.....

My old Hikock based I-177b valve tester used full wave rectification for the HT and full wave rectification for the grid bias supply, the HT via a mercury vapour rectifier to keep the supply impedance low to the DUT, there's no backing off control in that one as such, the circuit design ensures that meter ballistics takes care of eliminating the effects of the standing current so far as the meter display goes, the meter displaying the differential current resulting from the applied AC voltage to the grid during the gm test.

The AVO two panel tester used AC for the anode, screen/cathode supply and AC only for the grid, no DC supply except for the meter backing off supply.

Lawrence.

Last edited by ms660; 13th Sep 2017 at 9:17 pm. Reason: missing words + addition
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