David Simpson MM6ACI
Some folk might ask why a "Standardised Valve/Tube requires "pure" DC applied to all electrodes for the standardising process. Answer - because the manufacturers have always tested their valves with DC. Reason - from DeForrest's early WW1 era triodes through to the early 1930's, all valves just used battery sourced DC . I.e. an LV battery or cell(usually 1.2V or it's multiples) for "Filament"(heater + cathode), and HT's from a dry cell pack(60V,90V, 120V, etc.). Grid Bias could either be a single cell, or an LV dry battery. (No "cathode bias" back in those old days.
So, early pre-WW2 valve testers were also DC battery supplied. However, once AC/DC &/or fully mains rectified radios appeared in the early 1930's, HV valve testers were needed for later types of higher Plate(Anode) & Screen(G2) voltages. The major valve tester manufacturers then changed to the full P-P mains sourced voltages for testing. Maily because the Capacitors(Condensers) & Chokes required for FW or HW smoothing of rectified voltages would've been excessively large & heavy for their testers. I'm not going to go into the "meter backing off" methodes & circuitry, because the likes of AVO, Mullard, Hickok, Jackson, etc, all used slightly different methods & relied heavily on the stability of the electricity mains supplied to radio shops & domestic premises.
To sum up - decent AC working valve/tube testers will give pretty good testing results compared to DC working testers as long as they are regularly (weekly or monthly) tested with a "standardised valve/tube", And, (most important) their AC mains supply is checked for each valve/tube every few minutes. AVO call it "Set AC" & is displayed on the tester's meter. DC Testers are better, though.
Returning to the Tek570, if I ever get my hands on one, I suspect that its testing results would be beyond reproach. For folk with the technical know-how - Jac's excellent ideas on 576's are really something to aspire to. Here in the UK, the "Sussex" project now has a decent following. Then for folk with very deep pockets & exceptional software skills - the "RoeTest" kit project is the "bee's knees" I suspect.
Me - I just built my DC Tester out of parts from other vintage but redundant items of analogue test equipment from yesteryear. Probably cost less than £200. Why would I want to acquire a 570 - because the UK's most prolific & highly respected valve tester restorer/collector (over a hundred items), now approaching 90 & with poor eyesight - - Dennis Tabor - - told me that the Tek570 had been the best tester he'd ever had.
PS. Does anyone know the email address for Dick Anderson from Ohio, whome I mentioned 2 days ago ?