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Re: A Solution for Measuring Bias Current

11/26/1997 12:01 PM
Reid Kneeland
Re: A Solution for Measuring Bias Current
Check your loclal surplus place for octal  
relays. You get the plug plus a cool  
little case . I picked one up for $1,  
took out the relay guts, put a socket in  
the top, wired the pins through (with a  
1 ohm 1% 10W resistor in the cathode  
wire, ran a couple of test leads out the side,  
and ended up with a nice litlle bias probe for  
an investment of about $3 and two hours of  
11/26/1997 12:55 PM
Tony T

Now that's a creative solution to problem. Thanks for the advice!
11/23/1997 6:09 PM
Cosmik Debris

Yup, these are cool alright. Difficult to clip onto circuit board traces though. Another advantage of point to point wiring eh?
11/25/1997 10:16 AM

I dug through my Newark Electronics catalog (1-800-463-9275) and found a "non-contact clamp-on milliammeter, DC/AC true rms" from F.W. Bell Co. for the meager asking price of $463.82 (gulp). I imagine this would be rather handy if you seviced a number of different amps, but it's out of my price range.  
Steve Prouty
11/25/1997 7:01 PM
Elsewhere in the Newark catalog are the octal plugs that will fit output tube sockets ( stock no. 42F034, $3.93 each ). The backshell w/strain relief is also available. Luckily for us, the octal socket is still commonly used for relays.  
The bias probe is the only method I can think of that allows bias current measurement without opening up, drilling or otherwise modifying your amplifier. The first one I saw was in an amp shop here in So. Cal., and it was very similar to R.G's. This one had four sockets, tip jacks to connect an external DMM, and it used a rotary switch to connect the meter to one of the four 1-ohm cathode resistors.  
There's another bias probe available, called the Bias King. You can read about it on the Svetlana home page, under "Guitar and Music".
12/5/1997 3:24 PM

From the amount of interest, I decided to draw this up. I'm putting a drawing together for how to wire up a four tube probe. I'll post here when it goes up on GEO.  
Looks like it can be pretty economical if you don't get fancy with packaging.  
By the way, Mouser has 1% 1W, 3W, and 5W resistors for about $1.90 each.

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