How does class AB2 sound?

[Craig405]

Hi y'all

I would be very interested to know the difference in sound between AB1 and AB2 output tube configurations.

Is it better for Hifi than AB1, does it sound better for guitar use than AB1?

I know you can get a lot of power from it provided you have a stiff enough driver stage with low Zo ( i would use MOSFET followers)

I am at a turning point with a guitar amp design where i am deciding.

Cheers
Craig

[SY]

I've done several AB2 amps. I don't think you can generalize and say, "AB1 sounds like THIS and AB2 sounds like THAT." But with the right output tubes, the gains in dynamic range can be very worthwhile. In my experience, 6L6 types are particularly good here. A MOSFET follower is an excellent choice for drive.

[Sch3mat1c]

Presumably, it sounds quite good if the design is worth anything at all.

I doubt it matters to a guitar amp. In the name of harmonics, Eh?

Tim

[KSTR]

Originally posted by Sch3mat1c
I doubt it matters to a guitar amp. In the name of harmonics, Eh?

IHMO it matters a lot with MI amps. With cap coupling it gives that "fart out" effect with hard overdrive, because the positive grid currents de-bias the output stage and the caps can only slowly charge back to the nominal voltage via the biasing network. Also, with a weak driver, it gives additional distortion when the driver cannot handle the pos. grid current. E.g., say, 4xEL34 driven with a 12AX7 cathodyne p/s, when overdriven it gives nice asymmetric distortion and a certain "thump".

When AB2 is used one must keep a close look at grid dissipation and runaway issues....

- Klaus

[Craig405]

So generally AB2 is a good scheme, why does it generate more harmonics? are these even or more pronounced odd orders?

KSTR i assume you mean runaway with the solid state drivers? i wasnt aware that tubes suffered from current runaway.

Cheers
Craig

[SY]

why does it generate more harmonics?

It does?

[Craig405]

Maybe not, just re-read Sch3matics post and i dont think thats what he meant.
Ignore that

[Miles Prower]

Originally posted by Craig405
So generally AB2 is a good scheme, why does it generate more harmonics? are these even or more pronounced odd orders?

When there is no grid current, there isn't any voltage drop across the Zo of the driver. When current flows, that Zo causes a voltage drop. Therefore, the positive half cycles have a lower Vp than the negative half cycles, and so receive less amplification. This is also why a Lo-Z driver is so important, and why a MOSFET source follower is such a good grid driver: it has a much lower Zo than any hollow state device.

KSTR i assume you mean runaway with the solid state drivers? i wasnt aware that tubes suffered from current runaway.

Well, they do. This wouldn't happen if all tubes were filled with vacuum, but, of course, they aren't. Ionization effects of the residual gas can lead to runaway.

[KSTR]

Originally posted by Craig405
KSTR i assume you mean runaway with the solid state drivers? i wasnt aware that tubes suffered from current runaway.

Grid current is only one of the aspects here, in guitar amps. I've experienced runaway (not so severe as to lead to self desctrucion, but still noticable) also with fixed bias output stages which happened to run on a little low heater voltages... when driven hard, overall temperature increases, increasing emmission (and currents). The problem really starts when the PSU xformer saturates with excess current draw, then usually the bias drops, leading to even more current draw -- with often results in burn-out (same type we see with short power-downs of a few seconds, many amps are not failsafe in this regard).

In AB2, with low impedance drivers which can also sink enough current (by use of fairly low cathode/source resistors or adequate current sources) the risk of grid runaway is not very high. Dissipation is a little more problematic, but gold grids can handle a quite a bit of power, though. Also, sometimes the grid wires change the correct alignment when they expand under thermal stress, which can lead to unstable behaviour of the tube characteristics...

A great book on tube reliability etc:
http://www.pmillett.com/Books/intro_...cuum_Tubes.pdf
from Mr.Millets extremely well sorted online library, a favourite of mine among the online tube related resources.

- Klaus

[Craig405]

Miles how does the plate voltage get lowered?
im interested but i dont quite follow.

I can see how the current (normally zero) supplied by the driver stage will begin to develop a voltage accross any z's in the circuit and therefore the voltage at the grid of the 807 will be divided by some ratio.
Forgive my basic knowledge im still learning a lot, this project was for me to have a go at design.

Thanks for the link to pmillet KSTR ill have a look for reading material!

Also what are good MOSFETS to consider for the driver? i read in a thread somewhere the driver benefits from being run at a lower supply voltage thn the 807's plate voltage, i think to reduce dissipation when current is being drawn by the grid. So the drivers will not need to withstand the 500ish volts im going to give to the 807's.


Cheers
Craig