Help me understand these oscilloscope readings

[exabrial]

Running a sine wave through my tube preamp here: https://123d.circuits.io/circuits/852190-jan-6418-vaccum-tube-valve-preamp/



Here's the sine on the input, and here is the sine after C2, but before Rpot1:




Looks like the first tube is being pushed a little far. Now, I *am* trying to design an overdrive pedal out of this whole thing (see my other thread). I planned on using parts of this preamp to form the "Drive" portion of the overdrive... The dilemma here is how do I decrease the output of V1 without affecting the input impedance of the circuit?

Again, total noob to analog electronics. Any advice appreciated. Thanks!

[GibsonGM]

Hi,

Any reason you're using a pentode instead of a triode?  Not that it's a bad thing, but they tend to be noisier than triodes. A little harder to work with for someone new.  I suggest reading up on them, and actually doing the load line...the offer quite a bit of gain, sometimes too much, esp. in a first stage.

To drop some gain, try decreasing your plate resistor, maybe to 100k, 68k, 47k....this will lower the gain of the stage...you can use your scope to see what is happening, to get an idea of the amount of clipping you're getting....you may wish to look AND listen after the pot, to see just what you're getting, tho!   

Gain = gm(Ra)    gm=tube transconductance, Ra = plate (anode) resistance.

Be sure to get the signal level low enough so you don't toast your amp, etc....you want to see no DC, and just a few volts AC at whatever you use for an output.   A triode stage would be much simpler, based on what you say you'd like to do...

You might do well to read up, starting here:  http://www.valvewizard.co.uk/pentode.html  Good luck!

[PRR]

> Looks like the first tube is being pushed a little far.

Well, sure. Doesn't that scope-o-meter say "1.04Vrms"? That's VERY hot for a guitar. If you take your vitamins, use a hard pick, and strum with force, you might get 1V on initial attacks, but the level fades quickly.

1V is not wrong, but not too likely. What does it do with 20mV (0.020V)? You usually want "slight" distortion at 20mV. Usually this will come in your second stage; the first is just boosting to where it will overload the second stage. You want rising bent-ness from 20mV to 200+mV. (Actually, declining bentness as level falls from 200+mV pluck to 20mV fadeaway; or as you vary your pluck 100mV 50mV 200mV to emphasize individual notes.)

And of course in the end, we don't listen to the "middle" of the box, we hear what comes out the OUTput.

[exabrial]

I was using a tone generator But that's a very excellent point, and I'm kind of an idiot for not thinking about that first... I'll get my generator off the vitamins and steroids and remeasure between 20mv-200mv