Resistor watts

[vendettav]

so for a simple tube design/test suggested by Icaros (thanx again btw) in here i need a more than 5 watt 13 ohm (or close) resistor for the heater... i am thinking what if i cant get it, is it me or really 5+ watt resistors are hard to get your hands on? especially in my country. is there another way to do the thing?? he also suggested a tranny solution but it'll need a heatsink, i'd like to avoid that

[frequencycentral]

7806 voltage regulator + heatsink. Good for 1A.

[vendettav]

still thinking i won't be able to get a heatsink here

edit: what if i put the thing in 9v? i calculated .450*10v=4.5 (for non-regulated power supply, mine gives 10 actually) so a 5 watt resistor will do the job here?

[R.G.]

so for a simple tube design/test suggested by Icaros (thanx again btw) in here i need a more than 5 watt 13 ohm (or close) resistor for the heater... i am thinking what if i cant get it, is it me or really 5+ watt resistors are hard to get your hands on? especially in my country. is there another way to do the thing?? he also suggested a tranny solution but it'll need a heatsink, i'd like to avoid that

Watts is watts. Two 27 ohm/1W resistors in parallel is equal to one 13.5 ohm/2W resistor. Four 51 ohm/1W resistors in parallel is 12.75 ohms/4W; four 13 ohm/1W resistors in two parallel pairs then in series is 13 ohms/4W. Thirteen 1 ohm/1W resistors in series is 13 ohm/13W.

In general, resistors are designed to dissipate their rated power when their surface temperature is 100-200C (!) so a resistor run at its full power rating will generally burn your skin. It's common to use a resistor rated for at least two times the actual power dissipated in the circuit. If I had to have 13 ohms with 5.67 watts of actual power dissipation, I'd use a 12W resistor minimum.

Use series/parallel resistors to make up bigger power dissipation capability.

[vendettav]

OUCH! RG, thanx for the thing should have read it before this, hmm now i'll have to take this 5watt resistor i got and get a 10watt one :/ well that's how the whole thing works... anyways thanx a bunch!

[R.G.]

It does hurt to have to just burn 5W for nothing. Notice that a regulator would still burn the same power (volts times amps).

We used to try to put resistors in series with linear regulators to lower the power dissipation in the regulators because the reliability of one resistor was better than the reliability of a regulator circuit. I haven't looked at the tradeoff for that when the regulator is one IC.

[merlinb]

so for a simple tube design/test suggested by Icaros (thanx again btw) in here i need a more than 5 watt 13 ohm (or close) resistor for the heater...

If you used both of your 6F1P valves in the same project then you could put the heaters in series and you wouldn't need the resistor.

Alternatively you might also be able to find a small pilot lamp that would work instead of a resistor...

Or even use ten rectifier diodes in series!

still thinking i won't be able to get a heatsink here

You don't need to buy one, you can make one. If you use a regulator then it will only have to dissipate about 2.7W. That's small enough that you can bolt the IC to the chassis (if you have the insulating hardware). Otherwise cut a piece of thick (3mm+) aluminium or copper about 30x30mm and bolt it to the IC without insulating hardware (not touching anything else).

Or raid an old VCR / PC power supply / anything with a switchmode power suppl. They always have plenty of sinks.

[vendettav]

thanx for the replies everybody! that's a lot of nice info for me for the future but as of now i found 10 watt resistor (tho it was 10 ohm) and i'll get it tmaro

[petemoore]

 For tube amps I like to use the safe method, which involves smoke/temperature indicator, a piece of tape on the component for which thermal runaway liklihood is suspected.
  Since this is internet, don't touch only the resistor or regulator insulation-only even if the amp is turned off, unplugged and B+ is still draining at 0.~v...
  It's more difficult to safely take temperature readings on high voltage circuits, but the thermal-time relationships can indicate where a components thermal runaway might be an issue.
  Surface area increase generally = increase in ability to dissipate heat. The thermal mass of heat sinks also 'sink' [even out and reduce max temp of] thermal surges.
   As long as an added connection isn't created, or isn't a problem, the heat sink can be the circuitbox.
 

[iccaros]

If you have any old computers laying around, you can get heat sinks of the mother board. For the last few years they have been putting small heat sinks on the chipset. Teh CPU heat sink is overkill but would also work. I take mine off old graphics cards.