Reducing Transformer Voltage

[Dave.H]

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.

 

[John Fields]

On Mon, 17 Dec 2007 06:04:33 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

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.

---
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"?


--
JF

 

[Dave.H]

On Dec 18, 2:22 am, John Fields <jfie...@austininstruments.com> wrote:

On Mon, 17 Dec 2007 06:04:33 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
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.

---
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"?

--
JF

I have a multi voltage DC wallwart, but it wasn't what I was referring
to.

 

[John Fields]

On Mon, 17 Dec 2007 07:26:52 -0800 (PST), "Dave.H"
<the1930s@googlemail.com> wrote:

On Dec 18, 2:22 am, John Fields <jfie...@austininstruments.com> wrote:
On Mon, 17 Dec 2007 06:04:33 -0800 (PST), "Dave.H"

the19...@googlemail.com> wrote:
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.

---
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"?

--
JF

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

 

[Jamie]

Dave.H wrote:

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.
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.



--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

 

[Ray]

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:iAE9j.72$%v.45@newsfe06.lga...

Dave.H wrote:

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.

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.

 

[Steven Swift]

"Ray" <argyle1nospams@hotmail.com> writes:

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:iAE9j.72$%v.45@newsfe06.lga...
Dave.H wrote:

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.


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

 

[gearhead]

On Dec 19, 6:45 pm, "Ray" <argyle1nosp...@hotmail.com> wrote:

"Jamie" <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote in message

news:iAE9j.72$%v.45@newsfe06.lga...

Dave.H wrote:

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.

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.

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.