The tube should conduct little plate current, if any, at -60 VDC on g1 in a perfect world.
What voltages are you reading at pin 4 (g2 screen)? It should be in the 450'ish VDC range.
You should also measure the voltage drop across the 1K screen grid resistor and calculate the screen current draw.
Hoping that you don't have some type of oscillation outside of the audible range that is driving the tubes over the top.
Do you have access to an oscilliscope?
Dave O.
Why am I not getting a bias reading on my Matamp?
Moderators: pompeiisneaks, Colossal
Re: Why am I not getting a bias reading on my Matamp?
IMO, there is some weirdness in play here and we should check out a few things before going further.
It is difficult to see the color code strips on the resistors. I think the values of the resistors on pin 1/8 and on pin 4 should be confirmed.
It looks to me like I see on pin 4 brown-black-gold (1 ohm), but the middle one looks like brown-black-green (1 meg). Either value is wrong for the screen. 1K is reasonable for the screen, but I don't know what it is expected to be in this amp.
It looks to me like I see on pin 8 brown-black-green (1 meg) or brown-black-black (10 ohms)? I'm thinking it is not 1 meg because that would be unthinkable and it is not a value often used in amp construction. 1K would be brown-black-red and I don't think I'd miss a red stripe even with the poor lighting on those resistors.
Here's what I'm thinking. I am wondering if the wrong resistors are installed. Let's get this confirmed.
10 ohms would be OK on pin 1/8, the cathode, though we'd prefer 1 ohm which really simplifies things, but in practice it makes little difference. If it is something else, it should be changed to the correct value. Read and report the stripe colors. Then, with the tube pulled so there is no load at one end, meter the resistor using gator clips and report the reading. Clips will help you get stable readings. It can be difficult to get stable hand held readings.
On the screens, pin 4, while you have the tubes pulled, meter that, as well. Report the stripe colors and the meter reading.
Also, please take note that pin 8 is the cathode. Pin 1 is unused by the tube. The layout simply uses pin 1 as a convenient tie point as going directly to pin 8 would look sloppy.
I'm not sure if you are taking the cathode resistor reading correctly or not. Again, use clips. Put one lead on the pin side of the resistor and other lead on the ground side of the resistor (probably you need to use the turret on the board.) This will show you the voltage drop across the resistor. Although it's been said above, I'll repeat this. If there is a 1 ohm resistor, the voltage drop is the same as the mA reading because Ohm's Law states that V=I*R; where R=1, V=I! That's why we like a 1 ohm resistor; if it is a 10 ohm resistor, divide the result by 10.
It is difficult to see the color code strips on the resistors. I think the values of the resistors on pin 1/8 and on pin 4 should be confirmed.
It looks to me like I see on pin 4 brown-black-gold (1 ohm), but the middle one looks like brown-black-green (1 meg). Either value is wrong for the screen. 1K is reasonable for the screen, but I don't know what it is expected to be in this amp.
It looks to me like I see on pin 8 brown-black-green (1 meg) or brown-black-black (10 ohms)? I'm thinking it is not 1 meg because that would be unthinkable and it is not a value often used in amp construction. 1K would be brown-black-red and I don't think I'd miss a red stripe even with the poor lighting on those resistors.
Here's what I'm thinking. I am wondering if the wrong resistors are installed. Let's get this confirmed.
10 ohms would be OK on pin 1/8, the cathode, though we'd prefer 1 ohm which really simplifies things, but in practice it makes little difference. If it is something else, it should be changed to the correct value. Read and report the stripe colors. Then, with the tube pulled so there is no load at one end, meter the resistor using gator clips and report the reading. Clips will help you get stable readings. It can be difficult to get stable hand held readings.
On the screens, pin 4, while you have the tubes pulled, meter that, as well. Report the stripe colors and the meter reading.
Also, please take note that pin 8 is the cathode. Pin 1 is unused by the tube. The layout simply uses pin 1 as a convenient tie point as going directly to pin 8 would look sloppy.
I'm not sure if you are taking the cathode resistor reading correctly or not. Again, use clips. Put one lead on the pin side of the resistor and other lead on the ground side of the resistor (probably you need to use the turret on the board.) This will show you the voltage drop across the resistor. Although it's been said above, I'll repeat this. If there is a 1 ohm resistor, the voltage drop is the same as the mA reading because Ohm's Law states that V=I*R; where R=1, V=I! That's why we like a 1 ohm resistor; if it is a 10 ohm resistor, divide the result by 10.