Post by agsabq on Jul 19, 2023 9:29:21 GMT -7
Yesterday I pulled the chassis for my 2014 Maz 18 with the Mark II mods in order to lubricate a scratchy master volume pot. While I had it open I measured some voltages out of curiosity and found the anode and screen voltages for the power tubes are 348 V and 333 V, respectively. These values exceed the maximum plate and screen voltage ratings of 300V for EL84 tubes. I do have 6n14n power tubes in the amp, which I understand from a video Dr. Z posted 5 years ago, are more robust than normal EL84s, and can take these higher voltages. My concern is whether these high voltages are normal for a Maz 18, and what would happen if I needed to replace the 6n14n tubes with JJ EL84s. Has anyone else measured these voltages and if so, what did you find?
Some technical details follow for those interested.
I found a 6n14n datasheet at tubedata.jp/sheets/113/6/6P14P.pdf, which indicates that the 6n14n can handle 400 V provided the plate dissipation is less than 8 W, but only 300 V if the dissipation is greater than 8 W. I therefore calculated plate dissipation as follows. I measured the cathode bias voltage to be 11V. With a 150 ohm cathode resistor, this should generate 11V/150 ohm = 73 mA of total cathode current, or 36 mA per tube. Assuming the screen current is 10% of the anode current gives 33mA of anode current. The anode to cathode voltage is 348 V – 11 V = 337 V, giving a plate dissipation of .033 mA x 337 V = 11.2W, which is 80% of the maximum of 14W. However, this dissipation exceeds the 8W maximum for anode voltages over 300V so I’m still concerned about the anode voltage.
Some technical details follow for those interested.
I found a 6n14n datasheet at tubedata.jp/sheets/113/6/6P14P.pdf, which indicates that the 6n14n can handle 400 V provided the plate dissipation is less than 8 W, but only 300 V if the dissipation is greater than 8 W. I therefore calculated plate dissipation as follows. I measured the cathode bias voltage to be 11V. With a 150 ohm cathode resistor, this should generate 11V/150 ohm = 73 mA of total cathode current, or 36 mA per tube. Assuming the screen current is 10% of the anode current gives 33mA of anode current. The anode to cathode voltage is 348 V – 11 V = 337 V, giving a plate dissipation of .033 mA x 337 V = 11.2W, which is 80% of the maximum of 14W. However, this dissipation exceeds the 8W maximum for anode voltages over 300V so I’m still concerned about the anode voltage.