The battery in an analog multimeter is needed for the Ohm range as this requires the meter to provide a current.
Some analog volt-ohmmeters also power the voltmeter function.
For instance a vacuum tube voltmeter or the solid state equivalent that uses a FET.
For instance a vacuum tube voltmeter or the solid state equivalent that uses a FET.
Yes, that would be the ones with integrated active electronics like buffers and amplifiers.
Quite rare these days with an analog meter.
What is the actual purpose of your meter?
Quite rare these days with an analog meter.
What is the actual purpose of your meter?
The meter is to measure cathode current.
I wanted it such that I could have the sense resistors permanently connected and a switch select which resistor the voltage is being read across.
That way I don't have to worry about a malfunctioning switch allowing one cathode to be open.
If I use 10 ohm cathode resistors and bias for 60mA, that would be 30mA per tube which would be 300mV across a 10 ohm resistor.
So it might be possible to find one 100mA milliammeter that require an external shunt and use a series resistor so that 300mV equals 30mA on the meter scale.
I wanted it such that I could have the sense resistors permanently connected and a switch select which resistor the voltage is being read across.
That way I don't have to worry about a malfunctioning switch allowing one cathode to be open.
If I use 10 ohm cathode resistors and bias for 60mA, that would be 30mA per tube which would be 300mV across a 10 ohm resistor.
So it might be possible to find one 100mA milliammeter that require an external shunt and use a series resistor so that 300mV equals 30mA on the meter scale.
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There will be conflicting requirements.
Shunt or series resistors will always make the meter less sensitive.
E.g. a 100mA meter with an internal resistance of 100R will have a full scale reading of 10V.
For a full scale voltage of 300mV you'd need a 3mA meter.
The actual voltage range of the 100mA meter depends on its own resistance.
The lower the meter's resistance, the more current will it bypass/steal from the sensing resistor.
I was thinking moreso of a regular voltmeter that requires a series resistor. A lot of those require current in the uA range for full deflection. Some have a 0-100 scale and that would work for what I need.
All I'd then need is a DMM to measure the voltage across the 10 ohm resistor to get the mA reading and set the meter series resistor to indicate that value.
All I'd then need is a DMM to measure the voltage across the 10 ohm resistor to get the mA reading and set the meter series resistor to indicate that value.
Yes, a µA meter would give some freedom to adjust the mV range via resistors.
Might be hard to find, though.
I've got two small meters that read full scale across the 1 ohm common cathode resistor. I can use a trimpot to adjust the meter to indicate the correct current value. I might use the 0-20 meter as it could then read to 200mA.
Not sure. Will have to measure it.
For the 0-20 scale meter I found a 6.8k series resistor gave me the proper meter indication when compared to a DMM measuring the voltage across the 1 ohm resistor. I tried 50mA and 70mA. Both read at the 5 and 7 mark with 60mA reading at the 6 mark.
For the 0-1 scale meter (1mA full scale) I found a 33 ohm resistor makes it read correctly. I will likely use the 0-20 scale meter due to how it looks and that it has four mounting bolts whereas the 0-1 scale meter only has two diagonally across from each other which won't work for mounting it with a bracket.
I'll move the 1 ohm resistor to the meter.
For the 0-20 scale meter I found a 6.8k series resistor gave me the proper meter indication when compared to a DMM measuring the voltage across the 1 ohm resistor. I tried 50mA and 70mA. Both read at the 5 and 7 mark with 60mA reading at the 6 mark.
For the 0-1 scale meter (1mA full scale) I found a 33 ohm resistor makes it read correctly. I will likely use the 0-20 scale meter due to how it looks and that it has four mounting bolts whereas the 0-1 scale meter only has two diagonally across from each other which won't work for mounting it with a bracket.
I'll move the 1 ohm resistor to the meter.
So here's where I mounted the meter. A terminal strip under the chassis located near the choke is where the resistors and wiring connect.
Here's the meter reading. Close enough for me. I might stick a label on the meter face right above the zero adjust and below the clear part of the window that says mA X10.
Here's the meter reading. Close enough for me. I might stick a label on the meter face right above the zero adjust and below the clear part of the window that says mA X10.
But if you use only one shared meter, one tube might theoretically draw 10mA and the other one 50mA.
You're right one tube could indeed draw a lot more current, but given they are close to each other far as my tube tester says, I suspect they should age the same and barring any failure of components one tube shouldn't draw a higher current than the other. Also if that were the case I'd likely notice distortion in the audio.
There's no guarantee that tubes age at the same rate.
I don't see a sense in using just one common meter.
Why not just mount test sockets connected to individual cathode sense resistors allowing to connect a DMM?
I don't see a sense in using just one common meter.
Why not just mount test sockets connected to individual cathode sense resistors allowing to connect a DMM?
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I don't really want an external meter.
What I might do is use a DPDT switch and a SPST switch and use a 1 ohm resistor in series with each cathode with the ends of the 1 ohm resistors connected to the current 1 ohm resistor. The DPDT switch will select either reading the cathode current of one tube or both tubes. the SPST switch will select what tube is having its cathode current read.
What I might do is use a DPDT switch and a SPST switch and use a 1 ohm resistor in series with each cathode with the ends of the 1 ohm resistors connected to the current 1 ohm resistor. The DPDT switch will select either reading the cathode current of one tube or both tubes. the SPST switch will select what tube is having its cathode current read.
The way I have things set up is the receiver has a supply for its B+ and also an Astron power supply for the 12.6Vdc the receiver tubes require.
I have the amp and power supply plugged into a smart outlet strip with the heater supply plugged into the load sense outlet.
When I turn the heater supply on it turns the rest on. The nice thing is if there's no load on the heater supply, the smart outlet strip shuts off which is a good safety feature so that if the receiver tube heaters have a fault due to the wiring the whole thing shuts off and I don't get any weird noises in the speaker nor is any potential damage done to the receiver.
Also there is a lack of ground loop hum whereas before there was some hum due to everything sharing one ground from the receiver to the amp which at one point had all the voltages the receiver required.
I have the amp and power supply plugged into a smart outlet strip with the heater supply plugged into the load sense outlet.
When I turn the heater supply on it turns the rest on. The nice thing is if there's no load on the heater supply, the smart outlet strip shuts off which is a good safety feature so that if the receiver tube heaters have a fault due to the wiring the whole thing shuts off and I don't get any weird noises in the speaker nor is any potential damage done to the receiver.
Also there is a lack of ground loop hum whereas before there was some hum due to everything sharing one ground from the receiver to the amp which at one point had all the voltages the receiver required.