Resistor wattage available

[dude]

I never see anything more than a 2 watt rating for Metal Oxide Resistors, except the big stores like Mouser, etc. why is that? Are 2 watts acceptable in the power rail of most amps with 400 to 450 B+, most of the power R layouts here are rated at 3 watts, screens too..?don't want to use big cement R's. None of the smaller amps parts stores like Doug, AES, Mojo, etc. carry anything more than 2 watts, except cement R's...?

[sluckey]

Hoffman sells 3 watt metal oxide resistors.

[dude]

Yeah, I see that, sorry.  Doug's the only one smaller guy he's closed till after new years, Mouser doesn't carry other amp parts like the smaller amp suppliers. Do you know of anyone else besides Mojo, AES, Angela, Amplified parts...?


I guess ebay

[SILVERGUN]

I'm not far from you. Let me know what values you need. I'll meet you at the Wawa. 

[sluckey]

https://www.hawkusa.com/product-groups/m000?f%5B0%5D=field_series%253Atid%3A7406

I use 3W metal oxide in my power rails because that's what Doug sells. I bet many others on this forum do likewise. 2 watts is likely good enough for most of what I've built. Do the math to be sure. I like to double the power rating. For example, if a resistor actually dissipates 1 watt I will uprate the resistor to 2 watts for safety. If Doug has the value I need I'll buy his 3 watt resistors. If not, I'll buy a 2 watt from AES.

[pdf64]

It wouldn’t be helpful for Hoffman designs to call for parts not sold here. So all suitable power resistors are likely to be noted as 3 watt; that doesn’t mean they need to be.

Another often overlooked consideration is resistor voltage rating; all else being equal, higher power resistors have a higher element limiting voltage. This is in regard of voltage surges / low mark/space ratio signals. eg a power up voltage surge across a HT dropper resistor, ie before caps have charged.

[dude]

I'm not far from you. Let me know what values you need. I'll meet you at the Wawa. 

Thanks SG, I have a few AO43 chassis's I hadn't started yet for 6V6 builds. I'm not stuck right at the moment for anything but as it's Cyber Monday I was going to order some stuff now that I planned on using over the holidays and after New Years. But that's a very kind offer, BTW, I'm in Media, I believe you're in South Philly?


al

[SILVERGUN]

I PM'd you. If you're ever in a pinch just let me know, I'm right around the corner.
Nothing worse than figuring out you're one resistor short of greatness and now you're gonna have to pay shipping, again.

[PRR]

> Are 2 watts acceptable in the power rail

What is it powering? Screens and six small bottles is likely 40mA, probably wants to drop less than 40V, is 1.6 Watts, so a 3W is marginal. OTOH we would more likely aim a screen-feed for 20V drop, which makes 0.8W, and a 3W is ample. If you wisely use a brick for screen feed, it is unlikely the rest of any sane amp needs 20mA, at 50V drop, is 1W, and again a 2W-3W is ample.

That's for all-day running. We like an over-size resistor here because at Start-Up one side goes quick to 400V and the other side slowly charges from zero to 360V. So for an instant we have near 400V across the resistor. And this can compute-out to hundreds of Watts. The surge fades before the resistor flames-out. If you were dumb enough to have your finger on it at start-up (and lived), you might not feel any extra heat. In Carbon-Comp the heavy heat makes micro-arcs inside the powdered carbon, and resistor value changes every start-up. Metal types are tougher.