19V laptop on 20V ac adapter?

I doubt it.

20V is just 5% more than 19V, which technically is the far end of the +/-5% tolerance of input voltage. However, your laptop would only leech what it needs from the power brick. If it only needed 3.4 A, it will only get 3.4 A. 6A is the MAXIMUM current output of the AC adapter you use, and it doesn't necessarily mean that it always has to output 6A.

I would not run the notebook off of the 20V adapter. The current mismatch is fine as long as the adapter current is greater than the notebook current, as gracefully said, but voltage does not operate in the same manner. Sure, it may be a hair within tolerance, but what's worse - a $20 adapter, or having to buy a new notebook?

Now that I think about it, frozenlead is right. 5% is too much difference to expect proper performance from the laptop. Although if I remember correctly, there are voltage regulators inside the system itself which makes sure that nothing gets too much voltage.

If it helps: I ran my Asus A8Sc off a 20V adapter once. It didn't encounter any problems whatsoever.

There are voltage regulators and voltage converters on the motherboard. They are normally fine with more than 5% overvoltage, and will perform to spec. Also, most motherboards will give you BIOS warning through beeps or refuse to POST if the voltage is too far off spec.

You can also see inside of your 20V power supply if there is a trimming pot for output voltage. Some power supplies have them. With a Volt meter you can adjust the voltage to read 19V, but most likely you will not need it. If it works, you are fine.

Two things:
1.. 19v does not mean 19.00 volts, it's anywhere from 18.51 to 19.49 . It doesn't hurt to check with a voltmeter. Similarly with 20v - so the difference could be from 0.01 volts or so to 1.99.

2.. Before plugging in an adapter from one thing into another thing, check the polarity. Often, + is the center, but it doesn't hurt to check. Laptops should have protections against wrong voltages and polarities, but why risk anything if you're not sure?

I had the same problem. I have 3 ac adapters for my (now dead) Averatec laptop, and they are "20v". I bought a new Toshiba Satellite, which has one "19v" adapter with it. So, I measured the voltage on the Averatec adapters, and it was 19.30 volts, positive centers, and the new on was 19.09 volts, positive center. The old plugs fit into the new laptop.

There is only 0.21 volts difference between them, which is 1%. This must be much less than any margin inside the power supply.

Now I have four ac adapters for one laptop!