Monday, 22 April 2013
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.
Thursday, 4 April 2013
Today I received two more front panels from Frank. These are my own layout for the Pultec styple EQ. I just laid them all in place on the sub-rack to see what they look like.
The text around the gain control is definitely too big on my layout. The 54dB gain legend is too close to the left hand edge. The font would definitely benefit from being smaller. This problem is made worse by the fact I am using a quarter inch diameter shaft on the gain switch which sets the minimum knob size you can use. In future it would make sense to use the switch with the one eighth inch diameter shaft as this would allow a smaller diameter knob to be used. My horizontal lines look thinner as well but they do line up nicely. I am not sure which I prefer at the moment. Overall not bad. Now I just need to finish all the PCBs so I can complete all four modules.