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Old 11th Sep 2017, 5:04 pm   #1
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
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

Last edited by WME_bill; 11th Sep 2017 at 5:25 pm.
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