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stevedee · 14th Mar 2018, 11:52 am

Yes, you are absolutely right. Different instrument types will give different results due to the way they load the circuit under test. But what is the magnitude of the differences compared to the 'within tolerance' variations of the circuit components?

To be honest, I didn't know. So I have just worked through an example using the circuit data for my Bush VHF54.

For the EABC80 in my Bush circuit, it shows a 180k anode resistor fed from 215V, with an anode voltage of 85V. So the calculated anode current is 0.72mA. I think this gives an equivalent anode to chassis 'resistance' of approximately 118k, which naturally includes the parallel AVO 7 on 1kV range (i.e. 20M Ohm).

- Substitute a 1M Ohm DVM and the calculated anode voltage drops to: 79.6V
- Substitute a 10M Ohm DVM and the calculated anode voltage drops to: 84.7V

So it looks (from this example only) like a 10M input resistance DVM would be a suitable alternative to an AVO 7. Otherwise (if it is important to you) either avoid the 1M variety or do the maths (I'm just plugging values into my spreadsheet). Generally, a circuit with higher resistance values may be more sensitive to the 'shunt' resistance due to the application of test equipment.

Now if we leave the 20M AVO in place and change the 180k anode resistor to its limits (in this case its a +/-10% resistor):-
- With 180k -10% the calculated anode voltage rises to: 90.5V
- With 180k +10% the calculated anode voltage falls to: 80.2V

This is approximately +/-5Volt, but I don't know what the additional variation (range) could be by comparing one 'good' valve to another, or the variation due to cathode resistor tolerance.

There is also the calibration of your meter to consider.

As my service sheet says, in relation to test point voltage readings: "The figures quoted are approximate and variations may occur without impairing the performance of the receiver".

I don't know whether there is a useful 'circuit voltage' guideline we can give newcomers to wireless servicing (+/-5%, +/-10% ?) because it mostly comes down to detective work.

14th Mar 2018, 11:52 am	#38
stevedee Retired Dormant Member Join Date: Feb 2018 Location: Horsham, West Sussex, UK. Posts: 19	Re: Bush VHF 81 Restoration Yes, you are absolutely right. Different instrument types will give different results due to the way they load the circuit under test. But what is the magnitude of the differences compared to the 'within tolerance' variations of the circuit components? To be honest, I didn't know. So I have just worked through an example using the circuit data for my Bush VHF54. For the EABC80 in my Bush circuit, it shows a 180k anode resistor fed from 215V, with an anode voltage of 85V. So the calculated anode current is 0.72mA. I think this gives an equivalent anode to chassis 'resistance' of approximately 118k, which naturally includes the parallel AVO 7 on 1kV range (i.e. 20M Ohm). - Substitute a 1M Ohm DVM and the calculated anode voltage drops to: 79.6V - Substitute a 10M Ohm DVM and the calculated anode voltage drops to: 84.7V So it looks (from this example only) like a 10M input resistance DVM would be a suitable alternative to an AVO 7. Otherwise (if it is important to you) either avoid the 1M variety or do the maths (I'm just plugging values into my spreadsheet). Generally, a circuit with higher resistance values may be more sensitive to the 'shunt' resistance due to the application of test equipment. Now if we leave the 20M AVO in place and change the 180k anode resistor to its limits (in this case its a +/-10% resistor):- - With 180k -10% the calculated anode voltage rises to: 90.5V - With 180k +10% the calculated anode voltage falls to: 80.2V This is approximately +/-5Volt, but I don't know what the additional variation (range) could be by comparing one 'good' valve to another, or the variation due to cathode resistor tolerance. There is also the calibration of your meter to consider. As my service sheet says, in relation to test point voltage readings: "The figures quoted are approximate and variations may occur without impairing the performance of the receiver". I don't know whether there is a useful 'circuit voltage' guideline we can give newcomers to wireless servicing (+/-5%, +/-10% ?) because it mostly comes down to detective work.