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Nikon1975 30th August 2011 07:09 AM

Ammeter connection for Tubelab SE
Dear All,

I would like to connect two ammeters, one per channel to my Tubelab SE.

By default there is a 10 ohm resistor to measure the bias, on the high voltage leg, just before the OT. Now I should replace this with the ammeter.

Two issues:

1) I read around not to put instruments on the high voltage, for the risk associated with the potential failure.

2) The instrument would be in the signal path, as this is a SE amp. I don't know if this is good or not.

I tested on the bench and it works, not sure if I want it in the final build.

Any input ?



chrish 30th August 2011 08:29 AM

My question is 'why'.

If there is a 10 ohm sense resistor there, why not use it? Have a voltmeter measure the voltage drop across the resistor. Place a switch in the circuit so that you can place it in the circuit to take the measurement, then switch it out when listening. If you are using an analog meter, you can even change the scale printing behind the needle (there is software to do this). You could have one switch do duty for both channels and only require one meter.



Nikon1975 30th August 2011 11:33 AM

I have to confess the meter is almost only for esthetic reason.

The vintage style round meters look good to me, two was for a symmetry reason. But I don't want to compromise the sound and reliability for that. Maybe I should just leave connectors for the meter as I did in another built.


chrish 31st August 2011 05:26 AM

You can still do this, but using a voltmeter (not ammeter) that is switchable solves the in-circuit issue. Remember that an ammeter is really just a voltmeter measuring across a shunt resistor.

Nikon1975 31st August 2011 07:22 AM

The ammeters I have a very small internal resistance, they work just fine just replacing the 10 ohm that is used to do the measure manually.

It can fail open or shorted. In case it fails open one channel will not work, in case will fail shorted the ammeter will not work. So in reality should not be a big deal. But still not sure what to do.

Thanks for the advices.


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