Cycle Conversion

...and in addition:
Since the rectified RMS voltage is expected to be a-bit lower than same with 60Hz the lack of 220V/50Hz voltage is undesirable.
The best voltage stabilizers, power-conditioners and noise filters at one are TripLite available at digikey.com or newark.com. They're far more cost-effective than PS products.

Lower freequency will certainly dictate lower RMS of already rectified voltage.
Why am I using the term RMS is because at the output of the full-wave or half-wave rectifier(whichever applies) there's still a pulse that needs to be smoothed by electrolytic capacitor. The value of such capacitor is being selected depending on the timed distance and certainly an amplitude between sine pulses at the output of the rectifier so to make such output as streight as possible. The larger the amplitude of such DC pulse the more precise selection of such value is desired. The larger value of such capacitor will have not enough time to get charged and the smaller value of such capacitor will discharge quicker than it should.
For full-wave rectifiers that rectify the voltage bellow 100V the internal power-supply surgery is less-likely neccessary.
For half-wave and tube rectifiers it most-likely does againe due to the large distance between peak pulses.

Krell's power transformers are already set up for 50Hz. So the real issue is line voltage. If you are already set up for that then there is nothing else you need to do.

Give this discussion a read. It's long, but there's some interesting information to be found.

Would the frequency of the mains supply be important? I would think that the amp would not be sensistive to this. He may need a transformer though - is the mains voltage in Vietnam 240v?

Regards,

Dear Quang,
I live in Scotland (240 volts AC supply, 50 cycles) and also bought US equipment ( 120 volts AC, 60 cycles) The solution is to buy a PS Audio power regenerator. There are 3 models that are suitable for this purpose, and all available on Audiogon for reasonable prices. P 300 is for front end equipment, CD player, pre amp etc, the P 600 is for power amps with a medium power out put and the P 1200 for big amps like the Krell. All of these models will take in the home country's voltage and cycle rate and turn out a variety of voltage and cycle rates to suit. The P 1200 I have is plugged into 240 volt mains and is tuned to step down to 120 volts and 60 cycles.( Other voltage and cycle rates can be applied ) It does a fantastic job of cleaning up the mains supply, your Krell will never have sounded better. I suggest you check out the PS Audio web site and some independent reviews. In my personal experience the customer service dept at PS Audio is terrific and helped me solve my problem despite the fact that the unit was second hand.
Jonathan Hart of PS Audio answered all of my nieve and sometimes stupid questions over a period of weeks with patience, humour and a genuine interest in finding me a solution to my power supply dilemma. He is extremely knowledgeable about his company's products, the current models and the units that are no longer in production, any questions he is definitely the main man. If I can be of any further assistance please don't hesitate to contact me .
Cnocmoy10