DIY AIR BRAKE WITHOUT THE RHEOSTAT?

[JIMI HAIRLESS]

I just ordered all the parts I need to build the air brake minus the rheostat. I will never need to play my '79 50watt JMP at bedroom levels. I simply need to drop a few dbs for rehearsals and smaller clubs so I figured I could save a few bucks and eliminate the rheostat by simply adding another tab to the adjustable power resistor in its place. Any thoughts and/or suggestions regarding this idea?

[jaysg]

You're saving roughly half the cost and you'll be happy with it. hairyandy built two into a 1U rack for one of his guys.

[hairyandy]

Hey Jimi,

It'll work fine. As Jay said, I built a "stereo" Airbrake into a 1U rack unit. I didn't even bother to add the other tab. It sounds killer, just like an Airbrake should. Maybe a rheostat-less Airbrake should be called the "Airbrake Pro"?



Good luck,

Andy

[Noel Grassy]

Jimi, My thought is, include some pics of your completed build.

Shux I wish I'd have thought to build one in a 1U container. I kept thinking about how unmanageable that Hammond box would be as I was ordering it.
That thing's as flimsy as wet cardboard only lighter. I think I'll fasten it inside of the speaker cab it gets used with.

[JIMI HAIRLESS]

Hey Jimi,

It'll work fine. As Jay said, I built a "stereo" Airbrake into a 1U rack unit. I didn't even bother to add the other tab. It sounds killer, just like an Airbrake should. Maybe a rheostat-less Airbrake should be called the "Airbrake Pro"?



Good luck,

Andy

Thanks! So you just connected the 6th position of the rotary switch to the far end of the variable resistor eh? I wasn't sure about this which is why I ordered a 3rd tab. Good to know! I'm surprised they don't offer a "pro" rheostat-less version as you suggest since it would be cheaper and would offer another price point! I would personally feel like a retard playing a Marshall head at bedroom levels and wasting tube life! Cheers!

J

[JIMI HAIRLESS]

Jimi, My thought is, include some pics of your completed build.

Just finished my Rheostat-less Airbrake. Here's a pic of the innerds. Really happy with the results. Works/sounds great! I'm using it mainly with a '79 Marshall 50watt JMP head that I had to turn up way too loud to get a happening tone before. Now its perfect for rehearsals and smaller clubs. Thanks to everyone on this forum for sharing their info and experience!

[dehughes]

Very cool! I'd like to do the same thing...do you by chance have the BOM for your build? I have the "stock" Airbrake BOM, but yours looks a bit different (even without the rheostat...).

[JIMI HAIRLESS]

Very cool! I'd like to do the same thing...do you by chance have the BOM for your build? I have the "stock" Airbrake BOM, but yours looks a bit different (even without the rheostat...).

I used the exact same part numbers that were posted on the BOM, I just left out the rheostat.
One thing I overlooked was how close to the switch the resistors ended up being located in my unit. I wanted to mount the resistor brackets to the sides like the real Airbrake but now I see why everybody is mounting them to the top! This chassis is narrower than a real Airbrake. I will probably go ahead and reposition them and possibly add heat sinks to the sides as this thing gets pretty warm! Especially the fixed resistor side!

J

[dehughes]

Sweet. Thanks. What would you recommend doing differently next time? Bigger box? Different parts? Different layout?

[JIMI HAIRLESS]

Sweet. Thanks. What would you recommend doing differently next time? Bigger box? Different parts? Different layout?

I think if I build another I might try to find a box that is wider and/or ventilated. This one which was recommended by forum members is a low-end quality 9"x5" enclosure. A 9"x6" or 9"x7" ventilated chassis might be better. The more room for air circulation around the resistors the better. Otherwise this 9"x5" box is totally adequate. You just need to mount the resistor brackets from the top of the box. I noticed that one builder used heat sinks on the sides and I think that's a great idea. Some folks might be a little unnerved by the heat of this thing when running it with an amp of 50 watts or more. It gets pretty hot to handle especially on the fixed resistor side which I mentioned previously. Of course a real Airbrake has no ventilation but appears to be a thicker gauge, higher quality chassis which probably offers a bit more heat dissipation. Good luck!

[Noel Grassy]

I was thinking of having some louvres punched like the old hot rods had. The other thought was installing a 12VDC mini (apprx. 2" X 2") computer fan and run it off a 9V battery with the intake at the opposite end.

[JIMI HAIRLESS]

I was thinking of having some louvres punched like the old hot rods had. The other thought was installing a 12VDC mini (apprx. 2" X 2") computer fan and run it off a 9V battery with the intake at the opposite end.

Louvers is a great idea! That would look bitchin! A 9 volt inside a box with all that heat I don't think is a good idea. I'm sure that's why all the attenuators I've seen with a fan inside run off of a wall wart. Great idea with the louvers though. I wonder if anyone makes a box like that? I seem to recall seeing vintage amps with steel chassis that had them.

[dehughes]

Cool. Thanks! I'm not getting the mental picture regarding louvers...explain, if you'd be so kind...

[Tubetwang]

No louvers but...those were used as attenuator box a few years ago when we were experimenting with attenuation...

I used to get those from my local surplus store for around 5$...

[dehughes]

Nice. Got it. Thanks.