Too high transformator voltages...

Skorpio · 29th December 2008, 08:06 AM

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?

Stixx · 29th December 2008, 11:22 AM

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...

Ty_Bower · 29th December 2008, 12:21 PM

There are some good tips on building a buck autoformer here:
How to wire a 'Buck' transformer

29th December 2008, 08:06 AM	#1
Skorpio diyAudio Member Join Date: Mar 2004 Location: Denmark	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?

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29th December 2008, 11:22 AM	#2
Stixx diyAudio Member Join Date: Sep 2003 Location: Southern Germany	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...

29th December 2008, 12:21 PM	#3
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	There are some good tips on building a buck autoformer here: How to wire a 'Buck' transformer