I have been busy soldering the last couple of weeks. All six amplifier boards are complete apart from a small number of components that have not yet arrived and I have two EQ boards yet to build. As I have a full complement of amplifier boards I thought this would be a good opportunity to check that the little 12V 3A SMPSU, that I got to power the heaters, was going to work. I hooked up the PSU output to the motherboard, plugged in one board and turned on the power supply. The tubes lit up. So far so good. So I plugged in a second board and turned on again. Again the tubes lit up. So I carried on adding boards. This is how far I got:
The SMPSU refused to power up more than four boards which is a nominal consumption of just over 2 amps. Four it will do, five it will not. It seems to go into some sort of short circuit shut down mode because with five boards in the voltage across the heaters is zero volts. It seems to be completely unharmed by this as you can go back to four boards, turn it on again and it is fine.
I suspect this is an inrush current problem. Cold heaters have a much lower resistance than when they are powered up and hot, just like any incandescent bulb. It appears the SMPSU can cope with the inrush current from four boards but no more. I shall next try a SMPSU with a much higher reating, say 6 amps to see if that works.
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