High voltage power supply

[Allan1942]

My mains transformer burnt out. This is in a Hallicrafters HT37 transmitter so is fairly common. I sourced three transformers to do the job of the original and fitted them on the chassis. The new high voltage transformer has a 425v-0-425v 160mA winding and I fitted a full wave bridge which gives me 1100v off load. A full wave rectifier using the centre tap would probably give me about 425v at full load which is not enough.
The output valves are a pair of 6146 which need 750v. What is the easiest way to reduce the 1100v to 750v or so? I thought of using a triac or an IGBT within the bridge which is presently four 1300v 3A diodes, but it must be a very simple circuit as space is limited.
Because the RF output must be linear I need the voltage output to be fairly constant. There was a choke input filter with a 10uF capacitor, but the choke broke down when power was switched on. Without the 10uF capacitor the output voltage was 750v but with the 10uF smoothing capacitor the output rose from 750v to near the peak ie.1100V. Quite predictable...
Any ideas?

 

[albert36]

My mains transformer burnt out. This is in a Hallicrafters HT37 transmitter so is fairly common. I sourced three transformers to do the job of the original and fitted them on the chassis. The new high voltage transformer has a 425v-0-425v 160mA winding and I fitted a full wave bridge which gives me 1100v off load. A full wave rectifier using the centre tap would probably give me about 425v at full load which is not enough.
The output valves are a pair of 6146 which need 750v. What is the easiest way to reduce the 1100v to 750v or so? I thought of using a triac or an IGBT within the bridge which is presently four 1300v 3A diodes, but it must be a very simple circuit as space is limited.
Because the RF output must be linear I need the voltage output to be fairly constant. There was a choke input filter with a 10uF capacitor, but the choke broke down when power was switched on. Without the 10uF capacitor the output voltage was 750v but with the 10uF smoothing capacitor the output rose from 750v to near the peak ie.1100V. Quite predictable...
Any ideas?

HI ! Dear OM.
You have a many solution for Your problem :
1. use only 1x 425 v to the bridge-fullwawe rectif. Or may put paralel boath winding i paralel (You must testing the Voltage -sparate winding) two 6146 need for fuul output power abt good 220-250 mA.
2. Also rewinding the secundar winding on right voltage.....
I am also on SKY pe under : s57uwabi or my mail add, for direct chat abt this problem.
Great 73, all the best in 2010. Albert S57UW and now also E77UW

 

[Allan1942]

Thanks for the reply om. The method I chose was to make a series NPN transistor regulator using three 270volt zener diodes in series at its base. This will give me a stabilised output of about 800 volts. The transistor is an IGBT rated at 1200v. To prevent too much base/emitter voltage I'm using a 6.8volt zener to clamp the base to emitter as the IGBT base can rise a bit high.
As the 425-0-425 was connected in series inside the xformer I would lose too much power if I used just 425v.
happy 2010