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Bias Jig

From kee–(at)–ustin.ibm.com Wed Jul 17 19:29:09 CDT 1996
From: kee–(at)–ustin.ibm.com ()
Newsgroups: rec.audio.tubes,rec.music.makers.guitar
Subject: Re: Biasing “jig” for 6L6s?
Date: 16 Jul 1996 14:23:49 GMT
Distribution: world
Originator: kee–(at)–een.austin.ibm.com
Xref: geraldo.cc.utexas.edu rec.audio.tubes:12104 rec.music.makers.guitar:100854

In article <4se6jb$4i--(at)--nramp.arc.nasa.gov>, selkir–(at)–mes.arc.nasa.gov (Rennie Selkirk) writes:
> Howdy,
> Somewhere I read a description of an improvised octal tube
> socket which could be piggbacked onto the chassis tube
> socket for, say, a 6L6 for the purpose of hooking up the
> hot lead for a cathode resistor bias measurement OUTSIDE
> the chassis. (I’m getting tired of yanking the chassis
> out of my old bf Fender head. So are the mounting screws.)
> Anybody use one of these safely? If so, would you care
> to post a detailed description of your “jig”?
Hi, Rennie. How are the 7591’s working?

The poster was me.

Take a pair of old, dead, octal output tubes. Note that the octal pins are
soldered onto the wires that actually go back through the glass. use a solder
sucker and some patience and unsolder the octal base. The base is probably also
glued onto the glass, so work that off, too. It may take a number of tubes
to get two good octal bases.

An alternate is to use two old octal base relays and scavenge the bases from

With the bases and two octal sockets, wire from the socket (on top) to the
octal base (on bottom) on all pins with stiff wire like maybe wire scavenged
>from house wiring (romex). On the octal base end, stick the wires down into the
holes of the base from the old tube side, leaving the octal pins free to stick
into a socket.On the cathode pin, use a 1 or 10 ohm resistor
instead of the wire. Connect leads or monitoring points to each end of this
resistor in whatever fashion works best for you mechanically to be able to
connect them up to a voltmeter.

Depending on how the bases and sockets are configured mechanically, you need to
get some kind of insulating tube around those wires and a way to plug and
unplug the thing into a socket. A way I used was to split a PVC plumbing pipe of
1 3/4 diameter, and tape it inplace around the wires between the socket and
base, and then pour the pipe full of casting resin. Of course, this rig will
extend the height of the tube in the chassis, and some chasses will not let
you do this. You can also make the wiring between the socket and base flexible
cable and just remote the power tube outside the chassis altogether, but you will
need to be super cautious about insulating this so you don’t die from touching
the plate lead, and extermely cautious about where you lay that (hot!) tube.

You know, while I’m typing this, it occurs to me that what you’d really like
to do is to take an external box big enough to mount a pair of octal sockets
in it, wire all the leads out of the box on flexible cable to a pair of
octal bases, and mount meter contacts on the external box. That would solve both
the heat and danger problems, for the socket end at least. You could also mount
a pair of small DPMs (about $10 each these days) on the box and have a whole
bias current instrument for under $50. That’s probably what’s in the
Bias Probe ™ instrument, I guess.

I’ve always used my plug-in-the-socket extender unit.


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