Reactive attenuator project

[balazs.bencs]

Lately, I've been messing around with attenuators because at some gigs I was not able to get the right tone at the desired volume with my JTM45 clone and I wanted to do something about it. I came across a forum post on the marshall forums about reactive attenuators: http://www.marshallforum.com/threads/simple-attenuators-design-and-testing.98285/

There are lots of information and experiments done, and there are two designs on the first page that are kind of stable. I built the smaller one cause I felt that I don't need the bypass switching and the resistive load options, and it's pretty simple. Volume is not really an issue anymore, I can even reduce the volume to bedroom level (though it does not sound good, cause the speakers are not really moving air how it supposed to). If you guys need lower volume, I think this is an easy build to do - thought some people would be interested in this here.

The -3.5dB step is missing from my build, I do not need that much switching options, so I omitted it from my build, you can easily do that too.

Schematics:






My build:

[Mulekicker]

Unrelated but related. What kind of jtm45 clone do you have? I've been wanting to dip my toes into amp building and have been looking at jtm45 kits.

[balazs.bencs]

Well, my JTM45 clone was not a kit, I bought the parts myself from different sources, so I'm not sure if I can help you in what kit is good or not.
My whole process was made using the Ceriatone JTM45 layout (http://www.ceriatone.com/ceriatone/wp-content/uploads/2017/08/JTM45-1-August-2017.jpg). Basically I made a BOM list and bought stuff from places where I could find parts easier and started building :)

You can check some pics about the build process in this topic: http://guitar-fx-layouts.42897.x6.nabble.com/First-t00b-amp-build-td33400i60.html

[Mulekicker]

Awesome! Thanks. Ceriatone is where I'm leaning.

[Travis]

What country are you in Mulekicker? I did the same with my tube builds (sources parts from various locations)

If you’re in the US, Mojotone has really nice chassis and Heyboer transformers

[Mulekicker]

I'm in the US. I may end up sourcing parts from Mojotone actually. I was leaning towards a kit with good instructions just cause ive never built an amp (hundreds of pedals though).

[Travis]

If you are comparing chassis/kits designed to use the original layout then the instructions would be interchangeable.

Mojotone is great I recommend them without hesitation. I have bought similar chassis from Mojotone and Weber, and to me Mojotone seems a little more vintage accurate. Weber makes fantastic speakers but their amp kits come with very cheap components.

Since you have a lot of experience it should be relatively easy but very fun and rewarding

Let us know if you have any questions, I absolutely love this stuff and am excited to talk about it

[Mulekicker]

I will for sure. Thanks! Sorry for the thread hijack...

[nonost]

Ey! I'm pretty interested, thanks. I've been thinking about building a reactive load for a while but the rheostat prices are not very friendly. This schematic with switches rules.

Do you know any site to get the inductors from?

Since I have an Orange ad30 I will build a 2 switches unit as you did.

Thank you for sharing this here :)

[balazs.bencs]

Yeah, I had the same issue, cause I didn't know much about coil brands, or types, what to get, where to get it, etc...

The easier thing is to go to your local electronic parts distributor and get an air-core coil with the right values. The wire diameter should be somewhere around 1mm and the DC resistance as low as you can get.

I'm in Europe (more precisely Hungary, Budapest) and locally I found a store that sells a german coil brand called Visaton and they were not expensive, really nice looking coils also seemed well built, so I went with those. I saw some on Amazon too: https://www.amazon.com/Visaton-SP-Coil-1-0-VS-SP0-47MH/dp/B00CI6507M/ref=sr_1_18?keywords=visaton+coil&qid=1574154263&sr=8-18

Oh and another thing that I read: Do not use iron mounting bolts when mounting the coil to the chassis, cause it will change the inductance.