# Where can I find Bias Probes?



## hollowbody (Jan 15, 2008)

I'm looking for something like the $25 ones Eurotubes sells that plug into an existing multimeter. They charge something silly like $25-30 for shipping, so I'm looking for a local/Canadian alternative. Does The Source carrying something like this?


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## Wild Bill (May 3, 2006)

hollowbody said:


> Does The Source carrying something like this?


Not a hope! They make their money on computers and cell phones. I doubt if 90% of their salespeople would even know what you're talking about with something for a tube application.

www.thetubestore.com carries the entire bias probe unit but I don't know of any domestic source for the simpler style you describe.

They're not hard to make, though. You take a dead octal tube and carefully bust the glass away. Then you heat up each pin and clean out the internal wire and any solder.

Once that's ready you take a regular tube socket and you have the two halves necessary to make your own probe!

You solder wires from all the socket pins to run down into the tube base pins. Instead of just a wire you use a 1 ohm 1 watt resistor for the pin 8 jumper. This is your cathode current test point.

You carefully drill a hole in the side to allow a small dual wire cord to get outside. This will go to your meter. Inside you solder it across the resistor.

If you've built carefully you should be able to have the wires all pass down into the tube base pins until the socket rests on the top edge. Be careful you haven't twisted any wires to cause a short. You then can use epoxy glue to bind things together.

The last thing is to carefully wick some solder up each pin where the wires come through.

If you've done it right you end up with an adapter that will fit between any 6L6, EL 34, 6V6 or tube with the same pinout and the amp's socket, with a 1 ohm resistor sitting between the cathode pin and ground and a cable brought out that you can feed into your meter.

Using Ohm's Law, if you get 35 mv across that resistor that exactly represents .035A, or 35 ma. of cathode current. The voltage scales exactly! If you know the plate voltage you can do the math and bias up your tubes.

:food-smiley-004:


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## hollowbody (Jan 15, 2008)

Thanks Bill, I do have a few old EL34s sitting around, so maybe I'll try my hand at that during the holidays.


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## dtsaudio (Apr 15, 2009)

I like your idea Bill. What I have done in the past is to wire 1 or 10 ohm resistors from ground to Pins 1&8 and mount probe sockets on the back of the amp. However, mounting probe sockets may not be necessary as some amps need the chassis removed to adjust bias any way, so just installing the resistors is the only step. Connect the meter to ground and the tube socket pins. The other advantage is that the resistors will act as fuses if a tube shorts, protecting the rest of the power supply and output transformer.
I don't like the looks of the Eurotube. It measures current directly at the plate if I read the instructions right. This can lead to a nasty shock if you don't follow the instructions. At the very least you could damage your meter.


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## Gunny (Feb 21, 2006)

*Single socket type for free!*

First come, first served.

2 yrs ago I purchased the Weber thing with 2 octal sockets, meter etc. That's NOT what this posting is about.

I have a single octal socket home-made one. It is used with your ammeter. There's no internal resistor on pin 8. It measures total current from anode to cathode (meaning it includes screen grid current which we usually disregard).

To obtain this do the following: email me directly - do not post on this board because I don't log in very often. use [email protected]

IF you are first to respond, I'll then ask you for your mailing address. If you're not 1,500 miles away I'll send it for free in a jet pack or equivalent.

that's it. Merry Christmas. I trust that you understand how to use this properly. It's not tricky.


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## hollowbody (Jan 15, 2008)

Hopeful PM sent!!!!


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## Gunny (Feb 21, 2006)

Tentatively gone to Hollowbody! No more emails please.


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