Solid state amp with OT?

[J M Fahey]

Bipolars and MosFets behave like *perfect* pentodes with *perfect stable* screen voltage.

Just look at the horizontal Collector/Drain curves:





so, "no sag" behaviour.

[g1]

waveform kinking (which many confuse with crosover distortion) which also adds extra flavour.

Could you expand on this? (pics would help too )

[J M Fahey]

Oh, easy.

Too sleepy, donīt make me hunt for pictures, see it yourself on your own scope.

Crossover distortion is a low level phenomenon, present in the first 5% or so of the waveform, 10% would already be an exaggeration.

So you set your 100W into 4 ohms amp which should put out 20V RMS at clipping, inject signal so you have, asy, 2 or 4 V RMSat the speaker, incrase bias voltage to a very high value, sy -60V, onpurpose, so you squash that poor little waveform out of the amp, and then strt lowering bias voltage until the waveform is again as close to a sinewave as possible.

Congratulations,you have just dialed crossover distortion out.

Now you start rising drive voltage, so at the output you get 5 - 10 - 15 - 20V RMS.

Waveform will start to kink, slope will change and it will take an S shape on both right and left sides of the half wave.

This WILL change sound , remember what I always say about whatever you hear, you will see and viceversa?

Most Techs and even worse, all noobs will get desperate, point a finger at the screen and SCREAM "Crossover Distortion!!!!!!!" .
Not at all, you killed it 5 minutes ago.

This is some waveform kinking, not clipping, not crossover distortion, so I call it by the very inspired and original name of: "waveform kinking" (the same way as I called my cat "Cat" , for utter confusion of visitors).

Most scream and immediately proceed to twist the bias pot, set bias voltage lower and proceed to kill the kink .... fools !!!

You are killing *one* aspect of tube sound.

Another "solution" is to cook the tube to death by biasing it to exaggerated and unjustified 60% or even 70% dissipation, in great part to avoid it appearing.

Notice no manufacturer does such a silly thing?

Unless, maybe some might introduce it and even boast about it, tired of fighting Forums and Internet Gurus .
Oh, and some books too.

On the other side, some advanced SS guys introduce that on purpose (did I mention itīs a *dynamic* process, which adds flavour to sound?) just as a couple examples think Peavey Transtube and Quilter (and yours truly )

Maybe they know someting others ignore, even when seeing it every day?

[pdf64]

I think that the transfer function chart Vp vs Vg on nickb's IVDS page Interactive Valve Data Sheets makes the cause of this 'waveform kinking' apparent (with regular guitar type AB amps it's a kinda 'S' rather than straight line), and also how reducing the bias voltage to low levels can mask it.