Ch 1 Sensitivity: 1mV for clipping
Ch 2 Sensitivity: 1mV for clipping
Ch 3 Sensitivity: 16mV for full output
...
I have come to the conclusion that this means if you crank the gain knob up to 10, a signal as low as 1mV can be made to clip.
Those look sensible to me - my TW Express style amp will give full (clean) output for ~3mV input. This is with volume on max and tone pots centred. With the guitar and amp volume up full, it's practically impossible to get a clean sound out of it (but that's kind of the point
) - the sustain, though....
. While it's pretty high gain, it isn't into metal territory, so I can believe those numbers are ~3x more sensitive.
... what is the hottest signal I can shove in the front end that WILL NOT clip (i.e., gain on 0).
The input stage in valve amps is very often a cathode biased 12AX7/ECC83 with a voltage gain of about 60 (~35dB). For the values of anode(plate) and cathode resistors used, this stage is pretty much impossible to overdrive with a guitar signal - it needs several volts, and even then, it just compresses the signal a little rather than clips (with common cathode resistor values, anyway). You can overdrive it with... "Overdrive" pedals
The max signal you can put in usually depends on what happens after this - if it goes straight to a gain pot, then you can turn the gain down and can lose as much signal as you like.
The input arrangement in Scooters' post is incredibly common:
R1 is 1M ohm grid leak resistor which sets the input impedance and provides a DC ground reference to the grid on pin 7;
R2 is a 10k grid stopper that (together with stray capacitances inside the valve/tube) sets the roll-off of gain at high frequencies and prevents this stage oscillating or picking up radio signals (tailor value to suit - 68k is common). The 'Low' jack in Fender style 'High' / 'Low' input arrangement usually has an input impedance of ~136 k ohms, made by switching 68k grid stopper resistors through the jack contacts.
R4 is the cathode bias resistor (another common value is 820 ohm);
R3 is the anode/plate resistor (occasionally I see 220k used for higher gain);
C1 is the cathode bypass cap that sets the low frequency roll-off of the gain for this stage.
If you're designing amps, effects, etc. you need to cater for peak values, not RMS, btw.
There is all sorts of excellent information on the
Valve Wizard site, especially the introduction to triodes which is chapter one of his new book (
pdf file)
I do quite a bit with this stuff if anyone wants to dig deeper.
Topic: Pickup amps/volts (Read 13894 times)
