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Discussion Starter · #1 ·
Anybody use a Bias Master from TAD?

Looks like a simple procedure to bias an amp. Other than pulling out a few multi meters and getting Millamp readings you just plug this in in series and get a reading without too much in terms of connections.

My only question is this unit able to give you an accurate milliamp reading?
 

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Yes. They give you a list with ranges for common amplifiers. (pg.2 of attached pdf).
The idle current is only half of the equation, the other half being plate voltage. Plate voltage multiplied by idle current gives the idle dissipation in watts, which is what we are really after.
Plate voltage does not vary as much as current, so general values are known, and we adjust the current. This gets you pretty close, especially if you know the rough plate voltage for your model of amp.
Ideally, a technician would be checking the plate voltage with a meter, at the same time as measuring the idle current. But the bias probe type attachment should get you close enough.
 

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Discussion Starter · #5 ·
I meant double check the reading with something that you can trust like a Fluke meter.

Not too big of an issue for me as I pulled open the 2555x and it has mV points to easily test the voltage. Would be interesting to see the actuall amperage threw the tubes.
 

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I would trust the meter in the TAD unit. Modern meters under $50 are surprisingly accurate. I'm sure that unit will have such a chip built in.
Worst case is it is out by a mA or two. Definitely more than accurate enough.
Can't find a schematic for that newer 2555x so I don't know where they are measuring, but it is probably doing the same as a bias probe. They put a precision resistor in the cathode circuit and measure the mV across it. If you use a 1 ohm resistor, then the mV reading and the mA through the tube work out to be the same number.
 
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