# DIY Reactive Attenuator/Loadbox



## mjh36 (Jun 18, 2021)

Built this from a thread by JohnH on the marshall forum. This one is built for 50 watts. On front is a line-out with volume and three -dB switches. On the back is the input and two 8 ohm and one 16 ohm outputs.

It's -7dB when you plug into it, with switches for an extra -7dB, -14dB, and -3.5dB, which can be used in any combination. When all are engaged it's -31.5 dB total and safe to use as a loadbox and unplug the speaker. I added a line out for pedals/IR's.

It's a really excellent thread. I went the spendier but faster route and ordered the resistors and Hammond enclosure from Mouser, plus a few things from Parts Express and Tayda. Cost about $130, a steal for a reactive loadbox!

I have not tested it yet because I don't own a tube amp, but it reads out correct on my multimeter. Added the umbrellas to help with cooling.






						Simple Attenuators - Design And Testing
					

Background  Passive attenuators are wired between the amp output and the speakers. Their function is to absorb most of the output power of the amp, feeding a smaller amount to the speaker itself. This allows the amp output stage to run at higher power, letting the glorious tone of a good valve...




					www.marshallforum.com


----------



## dawson (Jun 19, 2021)

How awesome- I'll have to check out that thread as I'm in need of something similar but much more simple.  Incredibly useful.  Thanks for sharing!


----------



## mnemonic (Jun 19, 2021)

Nice, I love seeing stuff like this.

I tried making a small ‘single step’ attenuator (-7db, if memory serves) based on JohnH’s design but at the time I was focused mostly on taming my Mesa rectifier in modern mode, which has no negative feedback in that mode, and the less negative feedback typically the more sensitive the amp is to the impedence curve of the load.

it did work fine for my other amps but the recto was really the only amp I have that needed taming. I ended up building a reactive load box based on Randall Aiken’s design and it’s absolutely perfect.  A lot more expensive parts though.


----------



## Feral Feline (Jun 19, 2021)

Love the little parasols!

What are they rated at? Degrees C or F or BTUs or...? Sorry, really unfamiliar with this form of cooling. 😹


----------



## mjh36 (Jun 19, 2021)

mnemonic said:


> Nice, I love seeing stuff like this.
> 
> I tried making a small ‘single step’ attenuator (-7db, if memory serves) based on JohnH’s design but at the time I was focused mostly on taming my Mesa rectifier in modern mode, which has no negative feedback in that mode, and the less negative feedback typically the more sensitive the amp is to the impedence curve of the load.
> 
> it did work fine for my other amps but the recto was really the only amp I have that needed taming. I ended up building a reactive load box based on Randall Aiken’s design and it’s absolutely perfect.  A lot more expensive parts though.


Thanks, it was easier than I thought. Never realized how few parts are in these. I was dreading the purchase of a good loadbox.


----------



## mjh36 (Jun 19, 2021)

Feral Feline said:


> Love the little parasols!
> 
> What are they rated at? Degrees C or F or BTUs or...? Sorry, really unfamiliar with this form of cooling. 😹


I use Kelvin units lol. Mistake drill holes so I had to think of something fun. But I have a hunch they won't work.


----------



## Feral Feline (Jun 19, 2021)

Maybe invert them and add liquid cooling?


----------



## Paradox916 (Jun 19, 2021)

Feral Feline said:


> Maybe invert them and add liquid cooling?


I think I might require a little liquid cooling.


----------



## Paradox916 (Jun 19, 2021)

I have been running an old THD hot plate For a long time  and have always wondered if my rectifier would respond better to a different box...maybe I should give this a try.


----------



## Chuck D. Bones (Jun 19, 2021)

Impressive, I'll have to read that thread.


----------



## mjh36 (Jun 19, 2021)

Feral Feline said:


> Maybe invert them and add liquid cooling?


Is that a Blue Hawaiian? It's so hot today that looks amazing.


----------



## mjh36 (Jun 19, 2021)

Paradox916 said:


> I have been running an old THD hot plate For a long time  and have always wondered if my rectifier would respond better to a different box...maybe I should give this a try.


It was daunting at first for me, reading that thread because it's really long and I haven't tried building anything outside of pedals. But it's really just a coil, a few resistors, jacks and switches. There's a few versions you can do, too. An 8 ohm, 16 ohm, or variable input. And you can make it for a 50 watt amp, or 100 watt amp, etc. Pretty easy to order and build, and cheap!


----------



## mjh36 (Jun 19, 2021)

Chuck D. Bones said:


> Impressive, I'll have to read that thread.


Thanks man, there was a lot of technical jargon and graphs which I had to gloss over. But it's really cool to read and see how the design evolved over the last couple years.


----------



## Feral Feline (Jun 19, 2021)

mjh36 said:


> Is that a Blue Hawaiian? It's so hot today that looks amazing.


Been bloody hot here, too. 

Yesterday even the breeze was hot. Too early to start drinking, it's half-six in the morning and there's already a weather warning "HOT!" in effect. 



https://www.hko.gov.hk/en/wxinfo/dailywx/wxwarntoday.htm



Haven't read the thread over at the Marshall forum, I'll read it after I finish up a few other projects first. An attenuator would be great for pairing with future amp projects.


----------

