Big power resistors on a PCB, bad idea?

[Snicksound]

I'm working on a PCB layout that will support multiple options for my future builds. And I was considering adding spots for big cement box power resistors, one as an optional "sag" resistor, another one for when I'm not using a choke (between plate and screen + preamp supply)

Then I see pictures of commercial amps where such resistors have burned the PCB and I'm not so sure.

Of course I'd uprate them, but 3W through a 10W resistor still gets hot, especially inside a tube amp. Putting them a little high off the board helps, but then you risk the leads breaking from the vibration (happened on a big axial cap once).

So should it be avoided?

My options:
1- Uprate by at least a 3X factor and leave a little room between the resistor and the board. Maybe even some slits under it in the board.
2- Use turrets soldered to the PCB so the resistors have a solid foundation while being off the board (instead of relying on accurately bending the leads)
3- Just leave it off the board, mount them on terminal strips else where.

[martin manning]

Elevate them sufficiently and you may be ok. I would say put them at least one body diameter/width above the PCB, and yes you could open a hole in the board under the resistor for better convective airflow. increasing the wattage rating lowers the body and surface temperature for a given dissipation.

[chief mushroom cloud]

I've been using these: https://www.mouser.com/en/ProductDetail ... sreg%3D%3D
They come in shorter sizes, as well....
But you could use tall turrets soldered into the board.
In addition to an opening in the board for air flow, as Martin mentions

[nuke]

There are cement, wirewound resistors with built-in standoffs. The Bourns Riedon for example.

The spacers are a good idea.

Also, there are lead-forming pliers that will put a kink in the leads to stand them off the board for lighter resistors.

The choice of board material is also a factor. There are heavier, higher quality board substrates, and there are cheaper ones that aren't so great.

On the other hand, I've seen plenty of SMD high power resistors too, and they were expected to live in industrial and automotive, long-life applications.

If you do mount a heat-generating component on the board, spread the leads out as far as practical, space it from the board, and the give the lands where the leads are soldered a generous amount of copper pad to spread and dissipate heat.