DIY Bias Probe

From kee–(at)– Sun Jan 28 18:24:35 CST 1996
From: kee–(at)– (R.G. Keen)
Newsgroups: alt.guitar.amps
Subject: Re: Does Anyone Own the Bias Probe?
Date: 28 Jan 1996 19:05:48 GMT

Brian C. DenUyl – Staff (denuyl–(at)– wrote:
: I was wondering if any of you use the Bias Probe and how it works.
: Can you get the same results by soldering a 1 ohm resistor between the
: cathode and ground on the output tubes and measuring this with a meter
: while adjusting the bias? How is the manual that comes with the Bias
: Probe? Just out of curiosity, what does the manual recommend for the
: Nominal mV and Maximum mV rating for a Fender Deluxe Reverb? Thanks,

The “Bias Probe” is a commercialization of the old socket extender trick.

This is, by the way, a great use for those old burned out octal tubes.

You can easily make your own by getting a pair of octal sockets and the
old bases from some burned out tubes. Unsolder the bases and slip them
off the old tube. Stack the socket about 1″ or so above the octal base on
some stiff wires, soldering the wires into the tube base, then solder the
wires on to the matching pins of the socket. Put a 1 ohm resistor (or whatever
your favorite measurement value is) instead of wire in the cathode pin
position between the two. Make two.

You probably ought to solder a pair of wires at each end of the resistor
so you don’t get fried trying to stick voltmeter leads in there when
you’re using the thing.

To use this, remove the output tubes from the amp, stick the socket
extenders in the sockets, and stick the tubes into the sockets on the top
of the extenders. Now you can fire up the amp and measure cathode current
directly as teh voltage across the resistors.


From kee–(at)– Wed Jan 31 21:10:27 CST 1996
From: kee–(at)– ()
Newsgroups: alt.guitar.amps
Subject: Re: “Biasing Devices”
Date: 31 Jan 1996 15:18:43 GMT
Distribution: world
Originator: kee–(at)–

In article <310D8A92.253--(at)>, Bruce J writes:
> I was wondering about “easy” bias sockets sold by a couple of mfgs.
> Do/CAN they work? (I get the feeling there may be some strong opinions
> about this.) I think one I saw was by Audio Glassic and it is a socket
> you just put into the tube base and make biasing easy. For those of us
> who will admit to having more amps than we have tube electronics skill,
> it would be nice to have an easy way to bias our own amps. I’d really
> like some opinions or warnings – but esp. by anyone who has used one of
> these.
You can make your own. These devices usually are an octal base on bottom
to plug into the amp’s tube socket, and an octal socket above. All pins are
usually wired 1 to 1, 2 to 2, and so on with heavy copper wire, except that
the cathode lead has a resistance of 1 to 10 ohms in it and leads coming
out to measure the voltage drop across the resistors.

You can unsolder the octal socket from a burned out tube. If you want to make
a dedicated meter, you can get digital panel meters with a +/- 199 millivolt
range for under $20 these days, so you could actuall put one of these
on each socket if you make a pair of socket. One ohm is 1 millivolt per
miliamp, 10 ohms is ten millivolts per milliamp.

Warnings? Be very careful how you wire them and be sure to insulate them
carefully once they are made so you can’t get hold of the high voltage
leads – which may be any of them.



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