UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Measuring half wave voltages on Valve Testers.

David Simpson · 13th Sep 2017, 9:15 pm

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

13th Sep 2017, 9:15 pm	#19
David Simpson Nonode Join Date: Mar 2008 Location: Aberdeen, UK. Posts: 2,853	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 9:34 pm. Reason: Ad'l info.