6.3V 5A filament & B+ delay supply for car vacuum tube
Some of diyers facing a problem when using my PSU for car tube amp. They connected filaments in series and supply from 12V directly from car battery, with 2 diodes to drop a bit voltages when the car is on the road. The voltage between each tube's filament is not the same, one recorded 6.8V and another tube measured 6.2V.
I promised to design a simple 6.3V supply with B+ delay for next kit but I found that this one is easy to build, so I include this design and not going to launch the kit by now. The schematic can be seen below: (click for larger schematic)
The 6.3V supply using Motorola's MC34167 switch mode chip that can supply up to 5A of current, with only a small heatsink on it. Farnell Components sell this wonderful chip at reasonable price. The schematic looks simple, just a few parts to do the job. The MC34167 is actually a 5V switching regulator with fixed frequency of 75Khz, by applying feedback resistor 6.8k and 27K, the voltage can increase to 6.3V. I drive a bulb from motorcycle, rated 25W with only a bit of heat on heatsink. The inductor is not easy to obtain, I used crossover ferrite core winding SWG18 45 turns. I have some ferrite rings with unknown specs, I try it but it can't work properly due to core saturation.
The remote function of the whole system using a small relay, which I think it will supply more current for the 6.3V supply and the delay timer as well. The timer using simplest schematic that only using a single transistor. The 39K resistor need to be lowered to 33K or 27K if it can't work, this depends on sensitivity of different relay. Delay time will varies too. I build mine using lower capacitor value and 27K to get 15 second of delay because my brother complaint that the delay is too long, can't wait to listen to music when he turn on his car stereo.
Some might ask why not using 555 for the job? I think it is not necessary. The timer might suffer from shorter time constant if the set is turn off and then turn on again fast. That is not a big problem at all, the filaments that had just turned off and on again is still hot, the timer no need to delay the B+ for so long. In fact, This is an advantage for faster turn on if the tube amp turn on and off rapidly.
The delay's SPDT switch is connected to 12V+. not the supply from remote. This is to ensure full 12V of voltage can feed to B+ switch mode PSU module, to supply the TL494 so that it can drive the MOSFET fully on. I had finished my prototype and run it in my 6P3P SE amplifier, I don't have any photo right now but will include some photos in my coming article about 6V6 Stereo car amplifier that I'm planning to build for my brother. Stay tuned for more info.