The bias probe that I use a lot recently died again. Specifically the switch which toggled between the two probes failed again. I replaced it last year. Crappy switches – although not supposed to be. I could have simply put in a new switch but I don’t like the design of the old probe as it puts the 1 ohm resistors in the box with the switch which is at the end of a cable. I much prefer to have the series resistor right at the tube socket. So I decided to retire the one I’ve got and make my own.
I bought the major parts from Tube Depot. They have 8-pin octal bases that fit right inside the 8-pin octal tube ‘plugs’ that they also sell. The plug part is pretty much what you see on the bottom of an EL34 or 6L6 tube etc. The two relevant part numbers are SK-8BASE-1 which is the deep base part (the plug), and SK-ST8-810 which is the octal socket that happens to fit inside the base. I also bought a couple of decent quality banana plugs (P-BP-J) and a small DPDT switch (P-108-MIMI-2) from them. The resistors, wire and sleeving were all things I already had.
Here is a sketch of the essential bits. Pins 1 through 7 get wired top to bottom but pin 8 on the socket on the top is connected to pin8 of the plug at the bottom via a 1 ohm resistor. The resistor can be 1/4 W it’s not dissipating any real power. For a tube running at 100 mA ( happily red plating at that point) the resistor is only dissipating 10 mW. But do use a 1% resistor as we want accuracy in the current measurement. Wires are connected to pin 8 of the socket (red) and pin 8 of the plug (white) as we’ll see.
Since the gap between the bottom of the socket tags and the base of the plug is tight, and the socket tags have dual holes, I clipped them off so they only have one hole. That makes it easier to get it all fitted together later.
The first thing I did was make up a couple of 2-core cables which will connect the probe to the switch, and then bend the resistor leads and extend the straight lead with some red wire. Heatshink over the joint to protect it of course.
Now we get the the fiddly bits. I drilled some hols in the plugs near the base for the wire to pass through. I measured the diameter of the wire and chose a drill that would just pass it. On the ends of the wire that would be inside the plug I added a short pice of heat shrink tube which I also super-glued in place. This is to prevent you from pulling the wire out later. You need to strip half an inch or so on the ends of the white wires that go down into the plug pins.
Next we insert the white wire end into plug pin 8 and then put the resistor in the center cavity of the plug with its bent lead going down through pin 8. Make sure both wires peek out the bottom of the pin.
Solder the two wires carefully into the pin. Do not get blobs of solder on the outside of the pin. Let capillary action suck the solder down into the pin between the wires once it’s hot. Now you should have just the two red wires sticking up.
Of course your colors may be different. I used red and white because I had some 2-core screened audio cable with those colors.
Next we take the socket with its chopped off tags and attach both the red wires to pin 8.
To make the next bit a lot easier, cut some bits of tinned copper wire (24 gauge is great) about three or four inches long. Then bend over one end to make a small hook that will go through the socket tag holes. I stripped down a couple of feet of 24 gauge hookup wire to make mine.
For each pin in the plug, starting with the pins either side of pin 8, insert the straight end of a wire down through the pin, hook it into the socket tag hole and squeeze the hook closed. Then solder the wire to the socket tag. Take your time and be tidy with the wiring.
Now we push the socket down into the body of the plug. Line up the key between pins 1 and 8 on the plug and socket. You pretty much have to do this anyway to get the wires to go in smoothly. If it gets stuck somewhere maybe you’ve kinked a wire. Pull it out slightly and look. Try not to pull it all the way out as it will then be very difficult to get the 7 wires back down the right holes.
When it looks like it’s seated correctly apply some super glue around the seam and press it together until it’s solid. You might want to clamp it but be very, very careful not to push too hard on the plug center pin or you will break it. I just used my fingers. Many cyanoacrylate will set a bit faster in a moist environment so you can huff on it a bit. Let’s try not to stick fingers, lips, or tongues to the probe body.
The glue I use is Locktite Super Glue professional Liquid. The ‘Professional’ bit is a marketing joke. What I like about this glue is that the screw on lid stops it from going hard inside so you can use the same container for months of occasional use.
When the glue is dried, cut off all the wires about 1 mm above the end of the pin. You can even cut them flush with the end. It doesn’t matter. Then solder each one carefully ensuring that the solder goes down inside the pin (just a bit) and not all down the outside. I use Kester 0.8 mm solder for all my work as it’s quite fine and solders delightfully.
Do the second one the same way. Or 2, 3, and 4 if you are making a super 4-way probe for your 100 W amp.
If you want you can run the wires to a little plastic box and put the switch in there. My old one was like that but I decided not to bother so I wired everything to the switch using lots of heatshrink tube to insulate the connections and provide a bit of mechanical support. I also cut my probe wires pretty short. You can make them longer with no problem since the sense resistor is down in the probe. I used silicone test cable for the red and black leads to the volt meter. I also soldered them into the banana jacks since I never trust the screw connections.
To test the probe I set up my bench power supply to 5 V with a current limit of 50 mA. Then connected a voltmeter to the test leads (using the banana jacks). With 50 mA from the power supply you should get 50 mV on the voltmeter. The pictures don’t show the power supply LEDs well as they are multiplexed. A small philips screwdriver makes a good test plug to go into the socket side.
For bonus points, set your multimeter to continuity test mode and buzz out the other pins. pin 1 through pin 7 should go from socket to plug.
Now you’re all set for some bias measurements.