Ive just discovered I have a 470 meg cathode resistor instead of a 470 ohm resistor for the 12ax7...
I'm gonna guess you meant to say 470k instead of 470M. 470M is a hard to find value, and would have turned off all tube current.
... Im going to swap it out but I dont understand the effect this will have on plate current or tone.
The 470 ohm resistor in the long-tail pair sets the bias of the tube, because it defines the difference between the grid voltage and the cathode voltage. If you used a too-big resistor, then switching to 470 ohm will make tube current rise. The value of this resistor in a phase inverter is not really selected for "tone", but to define how much current flows through the tube, and where the operating point is. Going too high or low can reduce the size out the output voltage the phase inverter is able to make. That could limit you being able to drive the output tubes to full power.
Until you hit the point where you want to think about designing tube stages, it is best to copy KOC's values, which are themselves copied from older, proven circuits.
Im sitting here learning how to measure pre-amp and phase inverter pin voltages.
The grid voltages of every long-tail pair will look strange to you, when you compare their value to the cathode voltage. The grid resistors returning to the bottom of that 470 ohm cathode bias resistor have an effect called "bootstrapping" which makes the impedance at the grid look crazy-high. As a result, the impedance of almost every meter you attempt to use to measure the grid voltage will be too low to get an accurate reading.
Something I haven't tried, but which ought to work, is this: Attach your meter's negative lead to ground, and measure first at the junction of the tail resistor, cathode bias resistor and grid resistors (you might call this the "bottom" of the cathode bias resistor). Note the voltage. Next measure at the cathode, which is also the "top" of the cathode bias resistor. Note the voltage. If we're lucky, you might get readings within a few volts of each other, rather than 20-30v difference like you're seeing now.
The 20-30v difference doesn't really exist; it is just an illusion caused by bootstrapping, and the tube really only feels a couple-volt difference. To see what it should be, measure the voltage across each plate load resistor, turn the amp off and measure the actual resistance of each plate load resistor. Voltage/resistance = current. Add the currents calculated for each triode section of the phase inverter, and you'll know the total current flowing through the cathode bias resistor. Multiply the current calculated times the value of the cathode bias resistor, and you'll know the actual voltage across it. You should wind up seeing that it's a few volts.
Topic: 12ax7 Phase inverter voltages (Read 9904 times)