Resistor Wattages

[bandade]

Hi folks, I'm getting ready to start by 3rd and 4th amp builds and I was wondering what kind of wattage rating you are using on general resistors in the preamp; I'm not talking about 3 + watt power resistors? My first build used 1 watt and on the second build I used 1/2 watt resistors. Would it really be worth me buying all new resistors to get the 1 Watt rating?

I have never had a problem with 1/2 watt resistors, the only burn-ups i've had were on screen resistors on a Fender tube amp i used to have and it happed after i changed power tubes.

[David Root]

Depends where they are. In cathode service 1/2W is fine, and in most plate applications too. In those figure out the plate current and apply P=I sqd x R. Mostly 1/2W will be OK. I use a minimum SF of 2 for resistor wattage selection, preferably 3.

Possible exceptions: large tail resistor in a PI, especially if the tube is a 12AT7, power tube grid leaks.

[Super_Reverb]

I'd agree with David. Should be able to use 1/2W just about everywhere with exceptions:

1) I use 1W for all plate loads - it's overkill, but you read about amp rebuilds with cooked 1/2W 100K plate Rs, it's worth a few pennies extra to me

2) I use 1W for preamp P/S resistors and higher wattage resistors for resistor between rectifier cathode and screen grids and also as current limit for screens

I too believe in design margin (100% is not too much). Considering the money you spend for trannies, tubes, caps, knob, chassis, low oxygen copper wire, NOS Western Electric solder, and various and sundry snake oil performance enhancements, ( ) spending a coupla $ extra for resistors in your build makes sense to me.

I have never had to "fix" one on my amps, but my Peavey "Classic 30" spends alot of time on the bench.


cheers,

rob

[Structo]

I pretty much try to get all 1 watt resistors on my main preamp board.
I just like the thicker leads and they tend to be a little longer too.

I don't know if there is any sonic advantages but I just like a more substantial component.

There are several spots where a 1/4w resistor will work but I don't use them.

[Drumslinger]

1W in most areas.

[ChrisM]

1W carbon film for most signal areas. Sprinkle in metal film and carbon comp here and there for some sonic mojo and added tone. Though I've been using more metal film these days, cathodes and other non important signal areas for example.

Power resistors are 2W metal oxide or 5W cement.

Higher wattage resistor less thermal/resistor noise.