Before I Drive Myself Nuts!!

[Yorkie]

In THIS case an AVO 7 is the best thing to use as that's what the measurements quoted on both the Trader and Bush sheets were taken with.

- Joe

So it was Joe, so Rob’s choice of the AVO 7 was the right thing to do in this instance! A bit puzzling that they used an AVO 7 in 1956 when the set came onto the market.

I guess that on the 10V range, when checking the cathode voltages on say V2/V3/V4 (1.3V - 1.4V) putting the AVO7 on the 10V range across the 150R resistor wouldn't have much effect, in that it would parallel 10K with 150R and would only reduce the 150R to 147.78R – a reduction of only 1.5%, and in any event, the resistor is unlikely to be spot on 150R anyway. But were the AVO7 on the 10V range to be placed across a 1 Meg resistor in a potential divider, it would reduce the overall resistance at that point to just 9,900 Ohms with drastic effect on the reading. The important thing is to know what the effects are in any given situation and whether they’re likely to be negligible or significant. In the case of Rob’s VHF90, then clearly, when the set is working, the AVO7 should give results for current and voltage similar to those on the data sheet.

Interestingly, the Trader Sheet (1293) says that for alignment purpose, either an AVO8 or a Valve Voltmeter should be used.

So does the maker’s sheet for VHF alignment, so it pensions off the AVO7! Seems a bit inconsistent.

None of which is of the slightest help to Rob in his trials and tribulations.