How to measure LM317 output voltage help!

[reyrios]

This is a 6N3 tube buffer, one of the few instructions that came with the kit is a couple of test that have to be done before turning on the buffer with the tube on, one of the test is to measure the output voltage from the LM317 which is the tube filament and adjust it with the variable resistor to 6.3v, without the tube, ok I know that the center pin is the out, so my question is, in order to measure with the multimeter one probe goes to the middle pin and the other where? Ground? which ground? Sorry, I know it is a very basic question, but I can't find an answer anywhere, thanks in advance

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[rayma]

in order to measure with the multimeter one probe goes to the middle pin and the other where?

The lowest horizontal line in the schematic is the reference node for the LM317 output voltage.
Adjustable regulators have a resistor from their adjust pin to the regulator's reference node,
unlike fixed output voltage types. Here the reference node is not "ground" however.
In fact, it is the regulator's output that is grounded in this circuit.
Note that the filament is connected between the regulator's output and its reference node,
as it should be to regulate the filament's voltage.

 

[battalevren]

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[reyrios]

Thanks but I still don't understand, my bad.

 

[battalevren]

Symbol under r2 resistor is ground
There is no ground pin on lm317

Sorry for my bad english 🙂


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[rayma]

Thanks but I still don't understand, my bad.

Ok, a fixed regulator is more specialized and has the "adjust" terminal grounded,
so the regulator's output voltage is between the output terminal and ground.

An adjustable regulator requires two feedback resistors, one between the output and adjust terminals.
The other resistor is between the adjust terminal and "ground". The regulated voltage is measured
between the output terminal, and what the regulator thinks is "ground", which is the bottom of the
second feedback resistor.

 

[reyrios]

That would be R14, right? Thank you

 

[rayma]

That would be R14, right? Thank you

Yes, the bottom of R14 is the regulator's reference point for measuring its output voltage.

 

[Joschl]

Just measure the voltage right at the tube socket. It should be pin 1 and 9. If your probes cant reach the contacts in the socket use short pieces of wire as extension.

 

[AndrewT]

Just measure the voltage right at the tube socket. It should be pin 1 and 9. If your probes cant reach the contacts in the socket use short pieces of wire as extension.

Yes.
Measure at the load.

And check again when the load is connected.

 

[dotneck335]

What is the specific advantage of regulating the heater voltage? I would think it would be to reduce the hum in the output signal, but does it really make a difference?