DIY attenuator with 8/16 ohms switch option. Need help.

[DirtyDeeds]

Hi there!
I'm planning to build my own attenuator by following these instructions:
https://guitar.com/guides/diy-workshop/diy-workshop-build-your-own-attenuator/

I would add a 8/16 ohms switch I can use it also with both my amps.

I've been thinking about wiring a 16 ohms L-pad 100W with a 16 ohms 50W resistor in parallel so I can select either a 16ohms load (with only the L-Pad) or a 8ohms with both the resistor and L-Pad together but I'm not sure if this is the right thing to do.
https://canada.newark.com/ohmite/hs5016rf/res-16r-50w-solder-wirewound/dp/73AC6467

And do the resistor coupled with the L-Pad will help absorb more heat and give a better watt power (I plan to use a 100W L-Pad with a 50W resistor)?

I appreciate your help here. 

[iainpunk]

if you use the '' L Pad '' you can put a 16 ohm resistor parallel, but you still need to drive a 16 ohm speaker to have it work right, but this does mean that in 8 ohm mode, the maximum power you can get out of it is only 50%, not more than that.

what amp do you have that only accepts an 8 ohm load? 9 out of 10 solid state amps don't mind running at a lower (higher resistance) load, and almost all tube amps have multiple output taps.

cheers

[GibsonGM]

I'm building one of these soon. This would be perfect with my Marshall 18W into the 2x12.  Much cheaper than I thought!

[DirtyDeeds]

if you use the '' L Pad '' you can put a 16 ohm resistor parallel, but you still need to drive a 16 ohm speaker to have it work right, but this does mean that in 8 ohm mode, the maximum power you can get out of it is only 50%, not more than that.

what amp do you have that only accepts an 8 ohm load? 9 out of 10 solid state amps don't mind running at a lower (higher resistance) load, and almost all tube amps have multiple output taps.

cheers

Thank you for your response. 
My amp has a 8 ohms load (it's a 70's Traynor YBA-1, 45 watts tubes), my other combo is a Peavey Classic 50 2x12 combo.
But now that you told me that there is a 50% decrease of attenuation with a resistor in parallel I might just build a simple 8ohms L-pad attenuator and use it with my impedance matcher box (Weber Z-Matcher). I just wanted to avoid having multiple boxes between my amp and cab but this is not gonna hurt.
The only thing I'm scarred about those DIY L-Pad is the risk of overheating. Maybe adding a DC fan would help. 

[Rob Strand]

Have a look through the archives on this forum.

The power ratings of those L-pads often isn't the power that is written on the them.    The rating is system power when used with a midrange crossover.  The midrange crossover removes a lot of the signal power.  See high-pass curve on fig 7.1, (use the axis on the right)

https://sound-au.com/lr-passive.htm

Look at the size of those L-pads and think how it could get rid of 100W.    Think of the size of the heatsink on a 200W amplifier.

Even those large 100W resistors are not rated at 100W in free air.  It's less than half that.  To get the full 100W the resistor needs to be mounted on a heatsink, quite large.   Often the enclosure panels are the heatsink.

For small amps the L-pads might work.

Here's some info on industrial 100W rheostats.  These aren't L-Pads and don't work like L-Pads.

- Hot spot temp 300C to 340C.

- 1 foot clearance behind

- high-temperature ceramic  base

- open back (to remove heat)

- no plastic knobs or parts.

- no doubt will need to mount on a metal chassis.

https://www.ohmite.com/assets/docs/rheostats203.pdf

http://www.tedss.com/stock/images/1994/1994000346.jpg

Full pics,

https://sbindustrialsupply.com/shop/new-ohmite-0453-rheostat-100w-model-k-ser-a/