Kinda OT: Tube heaters for ShakaTube

[eliktronik]

Hey everyone, I'm working on my first tube powered build, a Shaka Tube. I'm a little unsure about connecting the 12vac from my supply to the heaters. The supply is rated 12vac @ 630ma, but reads 14.2 with no load. I found some stats for the 12ax7, and the heaters should pull about 150ma. Will this be enough to get the voltage down? Also, I found a site that says that when using series connected heaters, there should be a current limiting device to limit the current when switching on. Can anyone help me out?

[puretube]

your 14V will very probable go down.
Don`t be too concerned about current-limiting...

[Peter Snowberg]

Hook up the tube and measure the resulting voltage. If you see much more than 12.6, you can reduce it either by putting diodes in series, or with a series resistor.

I wouldn't worry too much about current limiting at startup. As long as you have the voltage at 12.6, the current should be fine. The only tubes I've seen that really demand a slow powerup are fom Ei, but slow power-up will make all tubes last longer. One way to get slow-start is to place a resistor in series with the heater that gets shorted when the standby switch gets moved to operate. If you don't have a standby switch, you could do the same thing with a relay and a 555 timer.

Take care,
-Peter

[Paul Perry (Frostwave)]

The supply is rated 12vac @ 630ma, but reads 14.2 with no load. I found some stats for the 12ax7, and the heaters should pull about 150ma. Will this be enough to get the voltage down?

OK, if 630ma drops the supply by 2.2V, then 150 ma will drop it by 0.5V or so. Down to 13.7V. Which is less than 10% higher than the 12.6 that you ideally want, so I think it should be OK.
But to get it exact, follow Peter's advice.

[Paul Perry (Frostwave)]

Oh, and don't worry about that 'current limiting' advice, that is for the old TV sets that had whole strings of valve heaters in series, which all heated up at different rates, giving transient overvoltages. In this case tere is nothing to worry about on that front.

[eliktronik]

Thanks everyone.