High voltage startup unit for tube PA

The below described circuit is more-less 30 years old, just a little bit modified for present project. Both mains input realys (on the primary winding of HV transformer) are controlled by 24V DC. Power supply stabilizer 24V DC is a part of this unit and beside startup unit is used for other PA circuits, like cooling ventilators, BIAS supply, sequencer and for PTT and an antenna coaxial relays. Although the era of tube PAs is over, still it could be helpful for someone, I hope. If you are looking for PCB, let me know, please.

The unit board is a part of HV power supply – see the schematic diagram. The unit is supplied by 26VAC from one winding of small (40W) separate transformer, which is used (by separate winding of course) as well as for tube filament. After PA switch on the tube starts preheating (within this time the green power LED is flashing). When the preheating time is off, the unit activate startup relay and over serial resistor 47Ohms (25W size) are loading filtration electrolyt capacitors in HV supply. It takes a few second. When voltage on the capacitors reach ¾ of nominal high voltage level, the unit get reference voltage from HV resistor divider and swich on the comparator, which activates the second relay. It makes a short connection of start up resistor. The LED light change to stable shine and the PA is ON and ready for operation.

In case of some problem in HV voltage unit – for example some short connection, or some of the electrolytic capacitors are not healthy, the unit switch OFF the startup relay within cca 5 sec and try the same procedure after 40 seconds. In case of any failure the next test of switching sequence is repeated after 3 minutes again.

On the top of it – in case of operation failure (for example by electricity outage)  the PA cavity is hot and by 1k NTC resistor the unit knows, that the tube preheating is not needed and the start up process is accelerated to cca 90 sec.

Schematic diagram of complete HV unit, control unit and as well as the PCB and board component place is below.

Please do not forget, that any High voltage power supply may kill you! Be very carefull!

Let me wish you many nice QSOs with PA supported by this HV control unit!

73 de OKVPZ

PS: the PCB size is 90 x 82.5mm and on the picture of complete unit is the version just before last change - so there are few variations between that pic and final (updated) PCB components setting plane on right pic.

(click on the pics for full resulution)