I'm planning on building a battery radio kit, that I plan on making into an AC set through the use of trnsforers. I've found a transformer that gives out 9 volts, but I need to add a resistor to step it down to about 1.5-2 volts. What value and wattage must this resistor be? The transformer will be powering two IT4 vacuum tube filaments.
Reducing Transformer Voltage
Dec 17, 2007
7 Replies
Not a good idea.
The thermal mass of the filament is low enough that the AC will
modulate the emission and you\'ll wind up with hum in the output, I
think.
Or do you have a wall-wart AC to DC converter with a 9VDC output
that you\'re improperly referring to as a "transformer"?
I have a multi voltage DC wallwart, but it wasn't what I was referring to.
Then you'll want to turn that 9VAC out of the transformer into
1.4VDC for the 1T4 filaments if you connect them in parallel, or 2.8V if you connect them in series.For parallel 1.5V filaments: (View in Courier.)
9VRMS FWB 11.3VDC LM317 \\ +----+ \\ +-----+ 120AC>--[F1]--+ +--|~ +|-----+--VIN| |VOUT-+--->1T4 FIL 1/2 AMP | | | | |+ +--+--+ | P||S | | [1000F] ADJ | R||E | | | | | I||C | | +--[30]--+--[240]-+ | | | | | 120AC>--------+ +--|~ -|-----+--------------------->1T4 FIL +----+For series filaments, change the 30 ohm resistor to 300 ohms.
The capacitor should be rated for 16VDC.
With the filaments wired in parallel the LM317 will be dissipating about a watt, so you may want to heat sink it if it gets really hot.
With the filaments wired in series it'll only be dissipating about half a watt, so it shouldn't need heat sinking.
JF
You would be better off getting a xformer that is designed for the heater voltage but if you insist, you can use that wall wart if it supplies enough current for the heaters. You need to know the current specs of the heaters when hot. All leads going to the heaters should be in twisted pairs to null unwanted noise emitting through out, near by components and even then you may have a little problem. Isolated xformers are a good idea for heaters how ever, you could also use DC but that too, would cause noise if it filtered.
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As I recall, the filaments in those 1T4 tubes, and the rest of the tubes used in old ac-dc portables were very delicate, and in series mode, installing a tube with the power on could blow all the filaments in the string. You would be safer to get a transformer with a closer voltage.
Yes. You have be careful not to overvoltage them. Also, the filament is the cathode in most of them and AC hum will be all over your set.
Steven D. Swift, novatech@eskimo.com, http://www.novatech-instr.com
NOVATECH INSTRUMENTS, INC. P.O. Box 55997
206.301.8986, fax 206.363.4367 Seattle, Washington 98155 USA
e
With a low-impedance power source you might blow the filaments, but placing a resistor in series with a higher voltage will actually result in current limiting. As concerns inrush current, it might be safer than the low voltage source.
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