Q: Sphere of Resistance timing

[jdl]

Scenario #1:
My opponent has in play 1 City of Brass and 5 moxes.  He has no other sources of mana in play.  He has a Sphere of Resistance in his graveyard.

I have an active Goblin Welder in play.

My opponent announces that he wishes to cast Triskelion.  He taps his 6 mana sources.

In response to the triggered damage from the City of Brass, I weld one mox of his for the Sphere in his graveyard.  Will the Sphere now require him to pay 7 mana for the Triskelion, or is it already too late?

Scenario #2:
Replace the City of Brass and 1 mox with an Ancient Tomb.   I think that in this case, I will have no opportunity to get the Sphere into play in time, because I have nothing to respond to.   The damage from Ancient Tomb doesn't go on the stack as an ability like the City's, right?

[rvs]

Once a spell is announced, the mana cost is locked in. Welding out his Mox (which he can obv. tap in response) won't do you must good in stopping the spell.

[jdl]

Once a spell is announced, the mana cost is locked in. Welding out his Mox (which he can obv. tap in response) won't do you must good in stopping the spell.

He tapped everything to begin with.  My question has nothing to do with the mox.  I'm just not clear on how the existence of the City's triggered effect changes my options for welding in a Sphere in time to modify the cost of the Triskelion.

[Limbo]

If he taps his mana without announcing that he will play a triskelion, you are right, and you can weld the mox into a sphere preventing him to cast a triskelion.

If he announces his triskelion and starts tapping mana after that, you are basically ****ed and can't weld the sphere into play to prevent him playing the triskelion.

The thing is, that the city is a mana ability, and thus the trigger of the cities damage will go on the stack after the triskelion is put on the stack. A city's damage is not the same as the LED's discard effect, which can only be played on instant speed, not on "mana ability speed", where "mana ability speed" implies being able to use the ability after a spell has been anounced.

[ProZachar]

The thing is, that the city is a mana ability, and thus the trigger of the cities damage will go on the stack after the triskelion is put on the stack. A city's damage is not the same as the LED's discard effect, which can only be played on instant speed, not on "mana ability speed", where "mana ability speed" implies being able to use the ability after a spell has been anounced.

The City's damage (technically the triggered ability that will make City deal damage to you) will go on the stack above Trike only if the player playing Trike anounces and then pays for it.  

Ancient Tomb's damage is part of the mana ability.  It will not use the stack.

The mana ability of Lion's Eye Diamond follows the normal mana ability resolution procedure (resolves immediately, doesn't use the stack); you just can't play the LED ability unless you could play an instant.  Comparing this situation to LED is bad because of the timing restricion on LED's ability.  The Oddyseey Eggs are better here.

Bottom line:  With a City, if your opponent is smart, you won't have a chance to weld in the Sphere to prevent the Trike from being played.  If he plays incorrectly, you can.

With Ancient Tomb, it doesn't matter how your opponent plays; you won't be able to stop the Trike by welding in the Sphere.

City of Brass
Land
Whenever City of Brass becomes tapped, it deals 1 damage to you.
: Add one mana of any color to your mana pool.

Ancient Tomb
Land
: Add to your mana pool. Ancient Tomb deals 2 damage to you.

[Limbo]

The mana ability of Lion's Eye Diamond follows the normal mana ability resolution procedure (resolves immediately, doesn't use the stack); you just can't play the LED ability unless you could play an instant.

I just mentioned the LED because there is a major difference between them. Although both create mana, the one (the city) puts a trigger on the stack but doesn't prevent you to announce a spell before you use it, while still using it to pay for the announced spell. The other (the LED) does not put anything on the stack (mana ability only) but does prevent you to pay for something you are putting on the stack.