02-15-2013, 07:31 AM
Jerry tried to explain, but I will try also.
The old-time meters used when these sets were new were rated at 1000 ohms per volt. These put a load on the circuit under test so that the readings were not really accurate, and displayed a voltage lower than it actually was due to the loading effect of the meter. But it was all they had in 1930, so they had to go with it.
Today's modern DMMs have input impedances in the megohm per volt range, which does not load down the circuit under test as did the old style 1000 ohm/volt meters, so the voltages read higher than published data of the time shows.
Factor in today's higher line voltages, and this is why your voltages are reading higher than they did in 1930.
If you had an older VOM with an input impedance of 1000 ohms/volt and used a Variac to reduce the line voltage going into your Philco 77 to 115 volts, and then measured the plate voltage of your 45 tubes with the old 1000 ohm/volt meter, you would get readings that are at - or close to - the published 1930 readings. These VOMs rated at 1000 ohms/volt were still being sold as late as the 1970s by such cheapie manufacturers as Calrad. By the 1970s, 20,000 ohms/volt had become the standard for VOMs. I remember seeing ads for these when I was a teenager and a beginner in this hobby. As I said, today's digital multimeters are rated at one - or several - megohms/volt.
The old-time meters used when these sets were new were rated at 1000 ohms per volt. These put a load on the circuit under test so that the readings were not really accurate, and displayed a voltage lower than it actually was due to the loading effect of the meter. But it was all they had in 1930, so they had to go with it.
Today's modern DMMs have input impedances in the megohm per volt range, which does not load down the circuit under test as did the old style 1000 ohm/volt meters, so the voltages read higher than published data of the time shows.
Factor in today's higher line voltages, and this is why your voltages are reading higher than they did in 1930.
If you had an older VOM with an input impedance of 1000 ohms/volt and used a Variac to reduce the line voltage going into your Philco 77 to 115 volts, and then measured the plate voltage of your 45 tubes with the old 1000 ohm/volt meter, you would get readings that are at - or close to - the published 1930 readings. These VOMs rated at 1000 ohms/volt were still being sold as late as the 1970s by such cheapie manufacturers as Calrad. By the 1970s, 20,000 ohms/volt had become the standard for VOMs. I remember seeing ads for these when I was a teenager and a beginner in this hobby. As I said, today's digital multimeters are rated at one - or several - megohms/volt.
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Ron Ramirez
Ferdinand IN
