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Interesting Plate current measurement method

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Interesting Plate current measurement method

Post by valvekits » Fri Jan 03, 2014 10:06 pm

Now this is not a method of measuring plate current in your amplifier output tubes that would normally spring to mind.

http://www.youtube.com/watch?v=xYHm1W6pJHQ

Eddie

 
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Re: Interesting Plate current measurement method

Post by Refugee » Fri Jan 03, 2014 10:53 pm

The voltages on the primaries will give a much less accurate reading than shunting the windings due to them being wound over each other and have differing lengths of wire in them. The good thing is that you don't have to cut into a wire to measure it.
I would measure the DC resistance and measure with a volt meter and keep a note of the resistance for future measurements.

 
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Re: Interesting Plate current measurement method

Post by Alistair D » Fri Jan 03, 2014 11:37 pm

Yes Refugee you are correct in stating that the 2 primaries will have different resistances. You elude to 'shunting the windings' I presume that you mean that connecting your meter(on a current range) across the primary of each half of the output transformer? This method is accepted by many audio repairers. There are a couple of provisos when using this method of determining the anode current. You need to ensure that the volume control(s) are set to zero so that there will be no AC content on the output valve anodes since they would effectively working into a short circuit. Do not make or break the meter connections while the amp is live. Goodness knows what voltage transients could be created.

At the beginning of the video he shows a valvebase plug and socket adaptor to allow the measurement of anode current. This is too risky as a wrongly set meter would rob the anode supply leaving the screen grids dissipating excess current. A much better(and commercially available) alternative is to use the adaptor to insert 1ohm in series with the cathode and use the meter to measure the volts drop across it.

Al

 
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Re: Interesting Plate current measurement method

Post by Refugee » Sat Jan 04, 2014 12:31 am

I think the safest way is to measure the DC resistance of each winding and put it on a label under the chassis and use a calculator on the voltage readings.
The guy on the film did not bother to power down before changing the meter from one winding to the other.

I meant shunt the ammeter across the primary winding.


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