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Help W/Homebrew Bias Meter

7/18/2000 2:27 PM
Ethan	Help W/Homebrew Bias Meter Hi, I built a bias meter from a schem. at Geo but it doesn't seem to be working right. Here's what it is: I have a box w/an octal socket wired to an octal plug. The pins on both are wired directly together w/600v. wire, except pin 3 has plug to wire to 1 ohm resistor to socket pin 3. A wire is tapped on either side of the 1 ohm and each goes to a plug for meter leads so you can read millivolts w/your meter. I tried it on my BF Princeton and the tube in meter seems to be operating but the tube in amp starts to GLOW and amp makes weird sounds. New Sensor sells a single plug unit so I figured mine should work. How did I screw up something this simple? I can send a pic of it if anyone is interested- Ethan

7/18/2000 2:27 PM

Ethan

Help W/Homebrew Bias Meter

Hi, I built a bias meter from a schem. at Geo but it doesn't seem to be working right. Here's what it is: I have a box w/an octal socket wired to an octal plug. The pins on both are wired directly together w/600v. wire, except pin 3 has plug to wire to 1 ohm resistor to socket pin 3. A wire is tapped on either side of the 1 ohm and each goes to a plug for meter leads so you can read millivolts w/your meter. I tried it on my BF Princeton and the tube in meter seems to be operating but the tube in amp starts to GLOW and amp makes weird sounds. New Sensor sells a single plug unit so I figured mine should work. How did I screw up something this simple? I can send a pic of it if anyone is interested-

Ethan

7/18/2000 3:03 PM
Doc	The only thing I can think of, if you've wired the adapter correctly, is to go over all the solder joints. Make sure you have no resistance between each socket pin and it's corresponding base pin, and only 1ohm resistance between socket & pin on #3. Sometimes, in using vinyl insulated wire, the vinyl melts into a joint and prevents good electrical continuity. It could have melted down into a tubular base pin, preventing the solder bond.

7/18/2000 3:03 PM

Doc

The only thing I can think of, if you've wired the adapter correctly, is to go over all the solder joints. Make sure you have no resistance between each socket pin and it's corresponding base pin, and only 1ohm resistance between socket & pin on #3. Sometimes, in using vinyl insulated wire, the vinyl melts into a joint and prevents good electrical continuity. It could have melted down into a tubular base pin, preventing the solder bond.

7/18/2000 5:30 PM
Ethan	Thanks Doc, I think I got it going. I resoldered the connections on the plug tho they tested OKand used a different set of tubes, I had swapped 6v6 w/an untested pair, this may have been the problem- Ethan

7/18/2000 9:41 PM
Ponsarelli	I was curious about the construction of this Bias Meter. Unless I am mistaken, pin 3 would be the plate of the tube for 6v6 or 6L6. So I am confused because I was under the impression that you would want to connect the 1 ohm resistor to the cathode (pin 8) of the tube to derive current drawn by the tube. Then you would adjust negative bias voltage until you had the desired cathode current. Does this meter work differently by taking a measurement off the plate?

7/18/2000 9:41 PM

Ponsarelli

I was curious about the construction of this Bias Meter. Unless I am mistaken, pin 3 would be the plate of the tube for 6v6 or 6L6. So I am confused because I was under the impression that you would want to connect the 1 ohm resistor to the cathode (pin 8) of the tube to derive current drawn by the tube. Then you would adjust negative bias voltage until you had the desired cathode current. Does this meter work differently by taking a measurement off the plate?

7/18/2000 10:28 PM
jason	Actually biasing an amp is adjusting the plate current, it's just safer to install a 1 ohm resistor in series with the cathode and measure it there. Only draw back is that the cathode current is the plate and screen current so it'll probably be off a few mA. By putting it in series with the plate he won't have any screen current in his reading but his meter will be floating at around 350 to 450v (depending on B+). In the cathode circuit is probably safer. jason

7/18/2000 10:28 PM

jason

Actually biasing an amp is adjusting the plate current, it's just safer to install a 1 ohm resistor in series with the cathode and measure it there. Only draw back is that the cathode current is the plate and screen current so it'll probably be off a few mA. By putting it in series with the plate he won't have any screen current in his reading but his meter will be floating at around 350 to 450v (depending on B+). In the cathode circuit is probably safer.
jason

7/19/2000 5:51 AM
Ponsarelli	What you say makes perfect sense. It turns out that after reading your reply I realized that what threw me off about this Bias Meter was the original post that indicated that he was reading millivolts from pin 3. Actually what you say is correct, it will be closer to the ball park of 350 - 450v. I suppose that is why the recipe for the Meter calls for 600v capable wiring. Taking into account the difference in the numbers that screen current adds when reading the cathode circuit, couldn't you just connect the wires that extend from the adapter tube-socket to the cathode pin and read that current with your Digital Meter set to Amperage? I realize that the 1 ohm resistor reading is to allow you to easily convert the reading to amperage, why not just read amperage directy, assuming that your meter is capable?

7/19/2000 5:51 AM

Ponsarelli

What you say makes perfect sense. It turns out that after reading your reply I realized that what threw me off about this Bias Meter was the original post that indicated that he was reading millivolts from pin 3. Actually what you say is correct, it will be closer to the ball park of 350 - 450v. I suppose that is why the recipe for the Meter calls for 600v capable wiring.

Taking into account the difference in the numbers that screen current adds when reading the cathode circuit, couldn't you just connect the wires that extend from the adapter tube-socket to the cathode pin and read that current with your Digital Meter set to Amperage? I realize that the 1 ohm resistor reading is to allow you to easily convert the reading to amperage, why not just read amperage directy, assuming that your meter is capable?

7/19/2000 12:44 PM
R.G.	I put the 1 ohm resistors in the plate for a more direct reading, excluding screen current, as you guessed. Actually, the setup is flexible enough to include 1 ohm resistors in the screens as well and read screen current directly. You could, of course, use the same setup to read cathode current by placing the 1 ohm resistors in the cathode leads. I did it that way because the dedicated voltmeter modules are cheap (under US$9 in many places) and because that way I could avoid having any of the high voltages exposed. The switching allows the probe to be connected to any of the tube leads you want to measure, and the tube connection is never opened, always just has a high impedance voltmeter across it. This avoids any switching/wiring changes as you would have to make by inserting an ammeter, and also avoids using an external ammeter with the attendant shock hazards. By the way, most DMMs read current the same way - voltage across a low-ohms resistor. If you wanted to go deluxe, you could easily put in two of the digital panel meters and do matching directly without having to flip switches.

7/19/2000 12:44 PM

R.G.

I put the 1 ohm resistors in the plate for a more direct reading, excluding screen current, as you guessed. Actually, the setup is flexible enough to include 1 ohm resistors in the screens as well and read screen current directly. You could, of course, use the same setup to read cathode current by placing the 1 ohm resistors in the cathode leads.

I did it that way because the dedicated voltmeter modules are cheap (under US$9 in many places) and because that way I could avoid having any of the high voltages exposed. The switching allows the probe to be connected to any of the tube leads you want to measure, and the tube connection is never opened, always just has a high impedance voltmeter across it.

This avoids any switching/wiring changes as you would have to make by inserting an ammeter, and also avoids using an external ammeter with the attendant shock hazards. By the way, most DMMs read current the same way - voltage across a low-ohms resistor.

If you wanted to go deluxe, you could easily put in two of the digital panel meters and do matching directly without having to flip switches.

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