• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Too high transformator voltages...

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Skorpio

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Joined 2004
#1
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?
 
#2
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...
 
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