Resistor wattage

[lgehrig4]

Hey!

This is more of a general question, but since I'm building a Dumble I wanted to post here. I'm looking at a layout of the smaller boards (Relay, Bias Supply, FET Preamp, etc) and a few resistors specify the wattage (.5W, 2W, 1W), but most do not list the wattage. Is there a default value that is understood when the wattage is not shown?

I would think to default to 1W, but since one of the resistors specifies 1W in the layout, I'm a little unsure.

[sluckey]

I can't speak of the Dumbles but the old Fender schematics had a note that stated "All resistors are 1/2 watt 10% if not specified".

[ViperDoc]

I asked the same thing to a very reputable Dumble builder who used to be on this site before I joined. He said that mil spec power ratings for passive electronics are generally under-rated, meaning a part that is sold as a 1/2 watt part can handle two to four times the load before failure for safety purposes. So whereas most people would generally default to 1W spec particularly in the preamp, if your cork sniffing demands a part you only have 1/2 W stock for, proceed without concern. You'd take extra caution in the power supply, obviously. I'd be interested in the forum's opinion, but that's what this particular maestro told me.

[xtian]

Another concern, with modern components, is size. Modern resistors are SMALL per watt. So, for hand built amps, I go for 1 watt for general use.

[pdf64]

It’s also worth noting the a resistor’s voltage rating can be important.
Consider transient and dynamic conditions of each application before dismissing it as not being a concern, eg due to a low voltage across a resistor whilst idling.

Also worth pointing out is that this is in regard of the leg to leg voltage, not the leg to circuit common (ground) voltage.

Low wattage resistors typically have a lowish voltage rating, eg 250V, but beefy looking power resistors might be even lower than that.

[lgehrig4]

Thank you for all the replies so far!

On a less technical note, what are you Dumble users using for your builds in general? Aside from the few that require 2W+, are you using 1/2W or 1W all around?

[martin manning]

1/2 W works everywhere except for a few locations. The RN65 plate loads are rated at 1/2W @ 70C. Larger resistors are seen in photos of real Dumbles, for example the NFB series R, but 1/2W is sufficient there. The some of the power string droppers are a bit oversized too, but DIY and low-volume boutique builders are not pinching pennies. If there is any doubt, do the math, and use the standard 100% safety factor rule of thumb.