Varying 2 tube amp characteristics: SE treble and richness

[otto88]

Tube amp tone richness:
Iirc, if the tube plate load is only 2.5 to 4 times the rp of the tube (vs say 5 – 6), 2HD and 3HD increase; the lower the ratio, the greater the 2HD and 3HD.
Have I got that right?

And the open sound of treble from single ended tube amps:
It’s probably more than “simply high 2HD” - what actually, measurably is it, and when we scratch design and build an amp, and find there’s not as much as we want/ too much - how can we vary its degree?

Thanks

[Jeb-D.]

You want to vary 2nd harmonic? There are a few ways to do this without switching tube type. Id suggest tweaking the driver stage and leaving the output stage as close to optimal as possible.

-You can add a feedback loop and vary the amount of feedback.

Or if designed so the driver stage has a fair amount of headroom to drive the output to full power you can take one of the following approaches for a capacitively coupled driver to output stage.

-Use a resistor+pot combo for the cathode resistor of the driver stage to vary it's biasing point. The lower the idle current, the more 2nd harmonic your likely to get. The cathode load should be bypassed with a cap so that gain variation with adjustment is minimal. You will want the resistor in series with the pot to limit the range in which it is adjustable. Make sure all maximum ratings are not exceeded under any condition of the adjustment.

-Or you can use a resistor+Pot combo for the grid pull down resistor of the following stage. This will adjust the driver tubes AC load line but leave the DC load line+bias point, alone. As a side effect, when adjusting, the gain of the circuit will vary(if no feedback is used) as will the low frequency extension. The variance in low frequency extension will not matter if you size the coupling cap appropriately.

If you want to vary treble extension adjust the grid stopper value. If on the output tube it should roll-off harmonics generated by the driver tube in the higher frequency range. Or you can adjust the driver's grid resistor which will just adjust the high frequency extension of the amp. The output stage will get all harmonic content of all frequencies generated by the driver stage. Make sure the maximum grid circuit resistance isn't exceeded under the largest value setting.

[Jeb-D.]

Besides the characteristic of adding harmonics to music. Harmonic distortion causes your input vs. output becomes non-linear. This effects the sound in some other ways. The tonality of the amplifier will vary with volume setting. This also alters sound between instruments mixed on a track at dis-similar levels. It causes some sounds to pop out more than they normally would.

[otto88]

Thanks Jeb. In the cap coupled output stage, I like your methods of varying the 2HD content.

Does a feedback loop decrease only 2HD, all harmonics equally, or the higher orders more?

> The tonality of the amplifier will vary with volume setting

Are you referring to the frequency response, or something else?

Cheers

[Jeb-D.]

Does a feedback loop decrease only 2HD, all harmonics equally, or the higher orders more?

I'll spare you the page long reply of fully explaining feedback behavior and leave it at feedback will significantly reduce low order harmonics. Things become complicated regarding higher order, because frequency, open loop gain and compensation circuity come into play.

Are you referring to the frequency response, or something else?

Something else. A side effect in amplifiers with significant harmonic distortion is that Input vs. Output voltage becomes non-linear. It is often referred to as amplitude distortion but has other names as well.

You may find this interesting.
http://www.pmillett.com/file_downloa...Distortion.pdf

[otto88]

Thanks Jeb

When I get some quality time I’ll have a good look at this

Cheers