Building an amp - resistor wattage?

[Octavian]

Since there's no "Building stuff other than a stompbox" forum, I thought I would ask here: I've built a few pedals at this point, and I'm thinking about building an amp. I've been looking at a lot of schematics and guides, and one thing seems to get left out of a lot of them: the resistor wattage values. In some cases it IS specified, but only for a few, in some cases most are specified, sometimes none. Is there a good rule of thumb for how to select resistor wattage? The amps I'm looking at range from 5-25W.

[midwayfair]

Most amps specify. Which amp in particular are you looking at that it doesn't tell you what wattage to use?

The only "rule of thumb" that matters is Ohm's law. You have to determine what the current draw is and whether the resistance and wattage of the resistor are sufficient to dissipate the heat.

[R.G.]

P = V² / R

P = I² * R

P = V * I

A useful inversion of the first two formulas is to calculate what voltage across or current through a resistance of R gets you to power P.

So V = SQRT( P*R) and I = SQRT (P/R).

If you have a 1K resistor rated at 1/4W, what is the maximum current and voltage that it can withstand?

V = SQRT(0.25*1000) = 15.8V. I = SQRT(0.25/1000) = 0.0158A.

And not surprisingly, as Georg Ohm would have told us, 15.8V/0.0158 = 1000 ohms. What a surprise!   

Beyond that, there is derating. Resistors are usually specified to dissipate their specified wattage at a surface temperature of 100-200C. That is the boiling point of water to fry-an-egg temps. It is common (and good) practice to use a resistor of at least twice the power rating that the resistor will actually be dissipating. That keeps the resistor surface temp down to not being a burn hazard.

And there is the fact that there is no advantage to trying to get micropower resistors. If the actual dissipation is 0.5W, you need a 1W resistor. If it's 1/4W, you need a 1/2W resistor. If it's 1/8W or less, use a 1/4W resistor, as that's about as cheap as through-hole resistors get.

[thelonious]

Sometimes in the bottom corner of a schematic you'll find a statement like: "All resistors 1/2W unless otherwise specified." My experience has been that eyelet/turretboard/point-to-point amps tend to use 1/2W (or sometimes 1W) resistors as a default except in locations where there is a need for something larger.

Here's a quick 'n dirty guide for 5W-25W amps:
First and second power supply dropping/filter resistors - 5W
All other dropping/filter resistors - 3W
Power tube cathode - 5W if separate, or 10W if shared by more than one tube.
Power tube screen grid - 1W
Everything else - 1/2W

You will always find exceptions to these, but in general, those wattages should work. Also, you can get away with smaller/lower wattage resistors in many of these locations, but mostly at the expense of more heat, shorter life, and/or running them above rated voltages. YMMV.

[Insert the usual caveat here about how tube amp voltages can kill you, how you need to discharge caps, how you shouldn't mess with them unless you absolve the rest of the world of liability, etc.]

[GibsonGM]

Good rules of thumb.  Aren't many 1/4W resistors only rated to 250 volts?  Thought I'd mention that, too.

I use 1/2 watt for plates and anything in the high or "high-ish" voltage path (coupling networks).  Sometimes I get cheap and use 1/4W for cathode resistors; there are only a few volts there and they are fine.   Ohm's Law really IS the law of the land in this regard, tho.  Ohm it wrong, and fire can be the result.

I only use 1/4's in select places because the little "kits" I get of 1/2W units don't contain enough values to make up the special crap I try out.  The added bonus (if there is one) is that a glance shows me where the plates are and so on...but given the choice I'd go with all 1/2W - they're easier to work with.

Another Usual Caveat: usually it's a great idea to make a known amp with known required parts first, then start doing custom work    Maybe from a kit, so you spend less time thinking about sourcing, ordering and what is needed, and more time building and understanding....

[Octavian]

Thanks for all the advice everyone!

And rest assured GibsonGM - I'll have plenty of practice by the time I get around to playing too much. Was just wondering for an order I'm placing - getting pedal parts, might as well get at least some basic amp parts for down the road, since on resistors and whatnot the shipping is usually a significant percentage of the cost.

[GibsonGM]

Good thinking! Then use Thelonius' rules of thumb. You can't go wrong in getting a whole package of various values from Ebay or something of 1/2 W...you will use those now and again anyway.    Probably could get a nice "set" for under $20. 

R.G.'s discussion about power dissipation is great - it took me a long time to add that to my thinking when designing something.  You want to know what currents are flowing thru what parts - esp. the ones like plates that will do the heavy lifting.  After a while it becomes second nature, and you also gain enough experience to spot where you should look at this closely (power supplies, etc). 

[thelonious]

Btw, for amp parts check out:
http://taweber.powweb.com/store/ampparts.htm
http://hoffmanamps.com/
http://tubedepot.com/
https://www.tubesandmore.com/

[StarGeezers]

  Oct, for further amp building information , check out www.Wattkins.com    It's the last word in amp building with many and varied projects  ...

[krister]

Hey, SG, how's it going Wattkins is a great resource for building amps.

Octavian, 1/2 - 1 watt metal film or carbon film resistors will serve you well for most locations except in the power sections where 2 - 10 watt metal oxide flameproof or cement brick resistors will be needed depending on your use as noted above. Good luck and we hope to see you around.

[StarGeezers]

  Krister, Dude !!!  Sure enjoying your gear blog !!!   

[Octavian]

Thanks for the help everyone!