What you describe is/are the telltale signs of a tube that simply not conducting. *IF* it was conducting, it would be drawing current. *IF* it was drawing current, you would see (most typically) a couple of volts across the (usually) small 1.5K cathode resistor...like the ~~2 volts you see in the preamp stages....and, in the same way, you would see a voltage drop across the plate resistor.
The most common size of 12AX7 plate resistor is 100K, usually we think of about 1-2 mils going through a given 12AX7, so if we just guess that 1.5 mils is passing through the tube (either half) then across that 100K plate resistor, we should see E = IR = .0015 * 100,000 = 150 volts.
This is exactly what you see on your preamp stages: 2 volts on the cathode, and two tubes which are supplied from a 410 volts source, their plate resistors are 100K's, lo and behold, they have 270 volts on their plates, looking at the schematic. So, there you go, the plate resistors drop 140 volts each. Close enough.
That's for your generic 12AX7. The vibrato circuit is a little different, but the principles are the same. On the first half, we have that (about 2 volts) on the cathode and 280 on the plate. On the second half, the cathode sits a little higher, closer to 7 volts.
It looks to me like your trem circuit is not oscillating. Obviously that tube (both halves) are getting good B+, but it seems like you have trem osc failure. Recheck wiring & parts there. Alternatively, measure grid voltages on the first half-tube (the one with three caps in series---that's the identifying characteristic of the phase shift oscillator.) That tube gets a negative voltage from the bias circuit. If that grid is driven very low for some reason, that tube will be hard-shut-off.
I think you just have an omitted part or connection.
And finally....is the tube lit up?
Topic: I have trouble with a Cathode voltage range (Read 2469 times)