My first MC240, when power on, Woong~~ sound in 2 seconds.

[Sookwang Cho]

Hello,
Recently I bought old MC240, all original condition and I've replaced the all electrolytic capacitors.
And found some of the bumblebee capacitors are cracked so I've replaced all bumblebee capacitors as well for the next 50 years.
Without tubes, I've checked the B+, it's 440V so cool.
And then I placed all tubes and connected the speakers.
When I power on, there is Woong~~ sound in 2 seconds came from speakers then it's disappeared.
After then no hums or noise, the sound is very nice.

Question: where does this sound coming from? what should I do?

All the best and thanks in advance. Sookwang.

 

[Pio1980]

Were the electolytics replaced with similar recommended values? For instance. It is recommended the value of the replacement reservoir filter capacitor C27 12uF on the bias supply be increased to 22 uF.
C23 and C24, regardless of replacement values must be identical value, some restorers will increase their value to increase power reserve.