Too high transformator voltages...
 

Hi,

I have an old amp from the 60' I am trying to fix. The first problem I have found is the secondary voltages on the mains transformer. They are a bit high. 420VAC, 55VAC and 7.2VAC for HT, bias and heaters.

I think there are two reasons: Amp is made for 220V mains and we now have 230V and also that transformator is unloaded.

If still too high, heater supply can be controlled by a series resistor, but what should I do with the HT supply? Would a series resistor here be a good idea also?

Your voltages will go down some once you connect a load.

You can use a series resistor in the HV as well...just pay attention to current draw and the resulting heat dissipation in the resistor (Ohm's law comes to mind ;) ). When your psu uses a choke (common in older amplifiers), just add a RC block after the choke to drop the voltage.
On the other hand tubes can easily tolerate 10% of deviation in the HV supply...

Put in some tubes and measure your voltages again...

There are some good tips on building a buck autoformer here:
http://www.diyaudio.com/forums/showt...hreadid=133843