Diagnosis Made on my VTVM!
Feb. 20th, 2018 07:51 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
stormdogI'm 99% sure the VTVM has a bad 12AU7 tube. Voltage at the meter (coming from the tube cathodes) is between 12 and 15 volts depending on how the AC balance pot is set. The AC balance pot is supposed to allow adjustment of voltage to the triode plates to match each other, thus creating matching voltage at the cathodes with no grid bias. This has to signify either a bad pot or a bad triode (thus a bad tube).
I clipped three of my new handy alligator jumpers onto the three contacts on the pot and confirmed with my DMM that it's behaving mostly correctly. (These are old pots and could use some cleaning.) I can adjust it to make the plate-to-ground voltages match, so the difference at the cathodes is appearing within the triode.
I looked carefully at the tubes. The other tube has three active elements, and I could see three glowing heater filaments when it's powered. I could only see one in the 2-element 12AU7 though. This tube has a center-tapped heater making two parallel arms. I removed the tube and checked resistance from center tap pin to each end pin. One was about 7 ohms. The other was infinite. Ah ha! To verify, I tried to Goolge pictures of an operating 12AU7 with no luck. But I did find an old hardware forum where someone was talking about the two glowing filaments he remembered seeing on the 12AU7. Confirmed! A new tube is on the way (for about $12) and I feel damned pleased with myself.
The tube I have is a 12AU7A. The A suffix indicates a tube with a different heater warm up time. I don't know that it matters much, but the label by the socket says a plain 12AU7, so that's what I bought.
The writer of the wonderful article on VTVMs that I've been frequently referring to (http://www.angelfire.com/electronic/funwithtubes/VTVM.html) says that replacing the tubes is the *last* thing you want to do when troubleshooting one of these, which is why I've been holding off until I better understand the operating theory. I think I'm there now.
I clipped three of my new handy alligator jumpers onto the three contacts on the pot and confirmed with my DMM that it's behaving mostly correctly. (These are old pots and could use some cleaning.) I can adjust it to make the plate-to-ground voltages match, so the difference at the cathodes is appearing within the triode.
I looked carefully at the tubes. The other tube has three active elements, and I could see three glowing heater filaments when it's powered. I could only see one in the 2-element 12AU7 though. This tube has a center-tapped heater making two parallel arms. I removed the tube and checked resistance from center tap pin to each end pin. One was about 7 ohms. The other was infinite. Ah ha! To verify, I tried to Goolge pictures of an operating 12AU7 with no luck. But I did find an old hardware forum where someone was talking about the two glowing filaments he remembered seeing on the 12AU7. Confirmed! A new tube is on the way (for about $12) and I feel damned pleased with myself.
The tube I have is a 12AU7A. The A suffix indicates a tube with a different heater warm up time. I don't know that it matters much, but the label by the socket says a plain 12AU7, so that's what I bought.
The writer of the wonderful article on VTVMs that I've been frequently referring to (http://www.angelfire.com/electronic/funwithtubes/VTVM.html) says that replacing the tubes is the *last* thing you want to do when troubleshooting one of these, which is why I've been holding off until I better understand the operating theory. I think I'm there now.
