Preamp tube maximal signal level

[dobrota]

Hi, as knows, tube is "almost" linear for small input signal, when it increases, tube starting to compress it in some range and then compress it and distort it for high signal level. And question now - is formula to approximately calculate maximal signal level on anode of totaly overload preamp tube depending on supply voltage ?
I measured RMS voltage in last stage anode of MB Recitifier like clone (but without cathode follower, gain full up) and it was about 40V so say about 112V peak-to-peak voltage for 350V power supply. So it seems maximal peak-to-peak signal value is about 1/3 of power supply voltage (for typical 100k anode resistor and 1k5 cathode resistor)

Thanks,
Fr.

[tubeswell]

Yeah, if you have a higher HT/B+ voltage, you can get a bigger clean output signal swing - all other things being equal (assuming you have a 'clean' input signal on your grid). For a conventional stage, you normally figure on putting the plate/anode voltage at about 1/2 of what the HT/B+ voltage is (in that locality), for maximum clean signal output.

[J M Fahey]

Hi, I find your measured value somewhat low, although not impossible.
Please post the schematic. Not need of the full one, but of the surrounding stages.
On a clasical 100k/1k5/bypass cap and 250V or more, I have measured more than that.
A rule of thumb would be: *unloaded* Vpp approx: Vcc minus around 80V ( say 30V lost on top and 50V lost on bottom). Real world says the following stage loads it, often a 220K resistor, thus "eating" around 1/3 of the available signal.
If the next stage grid/cathode diode clips heavily, no series resistor, it will lop off a good chunk of your signal.
If you are driving a 1M log Master Volume, set around "5" (a very common situation), the practical load is around 900K, and your 12AX7 plate practically does not "see" the following grid, *then* you'll get closer to 80V RMS on that plate/anode.
As you see, clipping, level, waveshape, etc. vary a lot depending on how the amp is set or used, what you are playing, etc; I guess that's a good part of the "dynamic" sound we associate with tubes.
By comparison, a +/-15V fed op amp will always provide close to 25Vpp (around 8V RMS), with a very predictable and boring squarewave clipping, quite unaffected by next stage conditions, unless , of course, you start adding different kinds of diodes here and there to liven things up a bit.

[dobrota]

Signal level

Hi, schematics is identical to MB Recti lead channel, but there isn't cathode follower (only 4 stages, no 5 as in original), so tone stack is connected to anode of the last valve stage. I guess it explaines lower signal level because of tone stack is big loading for valve. I found my notices and it was a little more: about 47V RMS no 40V as a write before.

[J M Fahey]

Yes, tone stack is heavy and complex load.
It would be interesting if some of our Simulating friends traced and posted the input impedance vs. frequency curve of classical tone stacks.
Don't count on me, I'm too old school for that.