Hi, I made an 18 watt TMB with MV about 10 years ago and gave it to a friend. The voltage measured at cathode resistor 120R and pins 3 of EL84 is 6.24v and should be 10v. What could have caused it to drop and how to fix. My amp work is very intermittent now, big gaps before I come back so my knowledge base is not what it was on diagnostics.
Plate and screen voltage are high along with the low cathode voltage. I’d check the ground connection of the cathode unless you were checking the cathode voltage at a different chassis reference point than the ground lug for the cathodes.
wpaulvogel wrote: ↑Sat Jan 14, 2023 2:53 pm
Plate and screen voltage are high along with the low cathode voltage. I’d check the ground connection of the cathode unless you were checking the cathode voltage at a different chassis reference point than the ground lug for the cathodes.
Have had a good look, pressed down on the ground with a chopstick. Latest readings:
That's crazy. Your plate voltage dropped about 100V which indicates a big increase in plate current. That same current must flow through the cathode resistor, yet you say the cathode voltage is still 6V. Very odd. Maybe your meter is lying to you.
sluckey wrote: ↑Sat Jan 14, 2023 6:29 pm
That's crazy. Your plate voltage dropped about 100V which indicates a big increase in plate current. That same current must flow through the cathode resistor, yet you say the cathode voltage is still 6V. Very odd. Maybe your meter is lying to you.
That’s a good point. I’ll put a new battery in the metre and re-test.
sluckey wrote: ↑Sat Jan 14, 2023 6:29 pm
That's crazy. Your plate voltage dropped about 100V which indicates a big increase in plate current. That same current must flow through the cathode resistor, yet you say the cathode voltage is still 6V. Very odd. Maybe your meter is lying to you.
That’s a good point. I’ll put a new battery in the metre and re-test.
About the only thing left to do (and should have suggested this sooner) is measure resistance from pin 3 to chassis. If less than 120Ω replace the cathode capacitor.
sluckey wrote: ↑Sat Jan 14, 2023 8:52 pm
About the only thing left to do (and should have suggested this sooner) is measure resistance from pin 3 to chassis. If less than 120Ω replace the cathode capacitor.
Thank you. I’ll try that in the morning. Appreciate it.