I've been doing some tube learnin's, and I'm probably at the "knowledge level = danger" point.
I put in a 12AT7 in V1 in a HRD to knock down the volume, which it does quite nicely.
It still has the standard HRD plate resistor of 100K and cathode resistor of 1.5K.
With no input, I measure 2.49V at the cathode with both inputs unplugged, reverb at zero, all other controls at noon.
This means the quiescent current through the tube is 1.66mA, with around 166V across the 100K plate resistor (0.27W dissipation), and 177V across the tube.
Is that current too high? Am I burning out the tube too quickly?
The values reported by Fender for an 12AX7 in V1 are 1.72V across the cathode resistor, which is a current of 1.15mA, 115V across the plate resistor, and about 229V across the tube.
Looking at the datasheet, though, it appears that a quiescent current of 1mA would put the grid voltage at -1.95V, and the voltage across the tube at 100V. So I suppose that would require a 2K cathode resistor and 240K plate resistor.
You're not putting the 12AT7 under any particular stress at all with that, rather it's just loafing along. They are beefy, the BF reverb drover uses them as a 1 watt power amp, pushes them way harder. Tinker with its operating conditions (plate and cathode resistor values) if you like, but be warned that the pcb pads and traces will probably be damaged and detach from the board, with repeated reworking. Much better to prototype it out to your satisfaction on some turretboard etc first.
Thanks, that's exactly what I was wanting to know. I got a little worried when I saw how much the voltage changed across the 1.5K. Of course, compared to the 345V supply voltage, it didn't really change very much at all! I guess I should have thought about it that way.