Modifying to 220V operation. How to make sure?

Herman's cautions regarding popping caps are certainly valid. Try powering it up on a variac at half-voltage first, making the measurements as planned.

Herman, I have textbooks and I'm able to read the circuits.
It's much tougher to do it without a circuit diagram knowing only basics. I tend to know as much variables as possible in order to start or not-to start at all.
Twl's responce seems to be the safest. All it needs to be done(to avoid electricution) is to newtralize caps by simply shorting them(using isolated screwdriver) when power is off and than start to examine transformer output voltage without even taking off the tubes.

Even with the tubes removed you still have the potential to do some serious damage. If you do it wrong and plug it into 220, you could have twice the normal DC voltage in the amp. That means twice the normal DC voltage across the filter caps. In my amp that would mean going from a B+ voltage of about 500 volts to 1000 volts. This is sure to be more than the maximum rating and since electrolytic capacitors tend to explode when they have too much voltage across them, you may have a real mess. No telling what else would be destroyed. Perhaps you.

Marakanetz, you seem to know just enough to be dangerous. I mean that in the nicest way. I admire your inquisitive nature and your desire to improve your system. But you are going into areas that will result in disaster unless you know what you are doing. Perhaps you could get some textbooks on basic electronics or take some classes at a community college. Once you have a better understanding of what is involved then you will be better equipped to redesign your amp.

I like the idea of first removing the tubes for safety's sake, seems like a very good idea. When you power up, try measuring the filament voltage at a tube socket. If you get twice the reading that you expect then you're probably applying the 240V to a unit actually wired for 120V. If you get the correct filament voltage (may be slightly high because there's no filaments loading the transformer output) then it appears you're already setup for 240V. Cathode & anode voltages could vary significantly, especially with no tubes in the circuit; it's hard to say what you should expect to see there.

Thanks Twl!
It's simple and genius.
Whattadumb i am! Bridge, Cathode voltage... it's far more simplier.
Thanks for warning working with high voltage. I take all precautions not to get electricuted.
You might also be right about VTL people that by default are not sure if I can deal with high voltages(AC is even more dangerous than DC) and I don't blame them. In all other cases they provide an excellent customer service.

Marakanetz, I am not familiar with your VTL, but in general, the transformer is the only issue you have to deal with. All rectification/regulation and everything else takes place after the secondary winding of the power transformer. The 110 to 220 change all takes place in the transformer. If there is a provision(set of wires on the tranny) that is intended to do this, then all you need to do is hook them up with a correct power cord with 220VAC plug. The current will be 1/2 of the current used at 110VAC. You don't even have to worry about getting the polarity wrong. But you have to be sure that these wires are made for that purpose. And you have to be sure that you use the right wires. The ones I have seen have 3 wires on the primary side. Leg A, Neutral, Leg B. When hooking up for 110VAC, you select a Leg(A or B) and neutral. When hooking up for 220VAC, you select Leg A and Leg B. Leaving the neutral out. Generally the colors for Leg A and Leg B are Red and Blue(but may vary), neutral is always white. Always connect the ground. If you can disconnect the secondary winding connections to the amp, then you can measure the output voltage of the tranny with a voltmeter while attached to 110VAC, and then change the primary wires to the 220VAC configuration as outlined above(at your own risk) and again measure the output voltage to see if you still get the correct voltage at the secondary outputs. This is what I have encountered, but it is up to you to verify what is correct for your gear. I find it hard to believe that the people at VTL couldn't tell you exactly what to do, unless they didn't want you in there messing around with it. Remember, this is high voltage stuff and is dangerous. If you have any hesitation, don't do it. You could hurt yourself, or the amp. Or even be killed.