final file EDC1

The Electronic Development Company Ltd.

The thermionic valve matcher.

CLIENTS OBJECTIVES:

Develop a valve matcher for testing and characterising the majority of thermionic valve types.
Requirement to match valves in push/pull amplifiers to eliminate mains hum and balance current draw.
Store information regarding valve data and compare with valve under test.
Hold data from valve under test and compare with next valve.

EDC thought it would be useful to make all the programmable PSU outputs availiable for prototyping circuitry and the ability to download data to a PC or phone via USB or wifi. The client welcomed these ideas and after price negotiation EDC started the design.

Five programmable power supplies were required:

1/ 50 to +400vDC @ up to 100mA for anode
2/ 50 to +400vDC @ up to 100mA for screen(s)
3/ 0 to –30vDC @ up to 20mA for grid
4/ 0 to +12vDC @ up to 3A for heater(s)
5/ fixed 6.3vAC @ up to 3A for heater(s)

In order to reduce transformer quantity and cost EDC, designed and built the transformers.

The whole system is under the control of a main microcontroller and all outputs can be adjusted and turned on and off manually via push buttons, there is also a display which can show all currents and voltages and power device temperatures.

50 to 400vDC - HV PSUs
There are 2 of these each with their own pcb mounted on the heatsink.
Control is provided by a high voltage power MOSFET working as a series pass element driven by an industry standard LM723 PSU IC controlled by a small (14 pin) micro which is optically coupled to the main micro (the small micro floats on the HV output).

0 to 12vDC Heater PSU
This uses an industry standard power op-amp (which has short circuit limiting) mounted on the heatsink with its own pcb.
It is controlled by another 14pin micro which is opto coupled to the main micro as this supply has to be floating relative to the chassis ground and the AC heater supply.
The small micro measures the DC heater current and voltage and sends these to the main micro from which it receives the desired DC heater voltage.

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