Bias Tool ?

[casey]

Does anyone have a schematic or diagram to make a
biasing tool for tube amps ?  You know, the kind you
can plug in to a socket and the other end goes in a
multimeter....?

:lol:

[Peter Snowberg]

The easiest way is to just include a precision 1 ohm resistor in each cathode. Break the connection to ground and add it there. Measure the voltage across it and multiply by 1000 to get the milliamps.

.042 volts on the meter = 42mA being consumed by the PLATE AND SCREEN GRID together.

If you want just the plate current, you'll need to put the resistor in line with the plate, but now your meter is sitting at plate voltage :shock: ; this is why I suggest the cathode method.

You can always build this into a box with a tube socket and plug, but adding two resistors with three touch points (1 for a good ground) on the rear of the amp allows you to bias without opening anything or having to take life off the tube socket.

I know somebody will have a great link about this.

-Peter

[Peter Snowberg]

Of course R.G. has a design at GEO.

http://www.geofex.com/FX_images/biasprob.gif

Check out his tube amp tech pages too!

[casey]

wow, that's pretty cool.
thanks.

[Mike Nichting]

Groove Tubes sells a bias tool. Antique electronic supply sells a xcouple too but they are very expensive.