Tom
Jacques is right on the money, with a positive triode grid voltage on pin 5 of the 6K8, just about all of the other voltage readings will be off except for perhaps the heater voltage.? It is worth noting that the 117 volt convention for the power transformer primary voltage is actually an average expected voltage.? The tolerance at a specified 117 line volts is about +/- 7.5%.? For 115 line volts convention, the tolerance is approximately +/- 10% and the modern convention for a 120 volt line voltage tolerance is +/- 5%.? In all of the above conventions, the maximum line voltage is about 126 volts.? The only voltage that has actually changed over all these years is the lower voltage limit.
The ohms-per-volt loading of a circuit by a VOM is essentially how much current is required to deflect the meter pointer to full scale deflection.? The 1kohm-per-volt rating implies that the meter movement is 1mA since 1 volt / 1kohm is 1mA.? Early VOMs were inexpensive so they used less sensitive meter movements.? In like manner, 20kohms-per-volt has a meter movement that requires 50 microamps for a full scale deflection.? I have seen VOMs with 50kohm-per-volt ratings.
Most VOMs and some VTVMs have an accuracy of +/- 3% at full scale deflection.? This implies that the meter accuracy is worse (actually twice the FSD) when the meter pointer is below mid range.? At the same time, the ohms-per-volt loading of the VOM on the circuit under test is dependent on the range multiplier.? For instance, the Simpson 260 has a 20kohms-per-volt rating so on the 250 volt range, the shunt resistance of the meter on the circuit is 5 megohms.??
This is sufficiently close to the reported shunt impedance of a VTVM or DVM of approximately 10 megohms.? This shunt impedance is independent of the range switch and is only dependent on the voltage amplifier in the VTVM and the range multiplier circuit or the method of conversion used in the DVM.? Some VTVMs have even higher loading resistances such as the Keithley 601 which is 100 trillion ohms with an accuracy of 1%.? Truly amazing for a mechanical meter movement.
Getting back to the line voltage measurement, if the 117 volt primary sees 120 volts, the error is ((120v - 117v) / 117v) =? +/- 2.6%.? Since the VOM or VTVM has a +/- 3% full scale error, the combined tolerance is ((3^2) + (2.6^2)) ^1/2 = +/- 4%.? When you add in the line voltage tolerance of +/- 7.5% the measurement error is increased to +/- 8.5%.??
When the increased error caused by less than half scale deflection of a meter movement is added to the mix, the reading error can exceed +/- 10%.? So when comparing the manufacturers voltage measurement with your meter reading, if you are below a 10% disagreement then you are golden.??
The remaining issue is that hallicrafters didn't include an expected voltage reading at the tube sockets in the S-20R manual which is at the heart of this problem.? In this case, the tube manual will give you reasonable voltage levels for the tube in question. But remember, the tube manual ratings are bogey numbers that are, in fact, more average values.
Regards,
Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
Jim Whartenby