DIY AIR BRAKE WITHOUT THE RHEOSTAT?

[JoeyE]

Great thread.

I will be ordering parts soon and the things i plan to do different was to get 50w resistors instead of 100w. It is only 4" long and i could fit it in a compact die-cast aluminum box (mouser part 400-4593). I will also skip the rheostat.

I wonder how big of an amplifier i can use it if using 50w resistors? How hot do you think can it get with the 6.8X4.7X2.2 die-cast box?

[JIMI HAIRLESS]

Great thread.

I will be ordering parts soon and the things i plan to do different was to get 50w resistors instead of 100w. It is only 4" long and i could fit it in a compact die-cast aluminum box (mouser part 400-4593). I will also skip the rheostat.

I wonder how big of an amplifier i can use it if using 50w resistors? How hot do you think can it get with the 6.8X4.7X2.2 die-cast box?

I would not recommend using 50w resistors. With my 50w Marshall after about 10 minutes the 100w fixed resistor gets blisteringly hot and the 9"x5" box gets uncomfortably warm to the touch. It is always better to err on the side of caution! The 50w resistors may be fine if you have done the correct calculations for the power and impedance of the amps you are using but the heat generated by the lower wattage resistors in a smaller box could melt wiring and the plastic switch. The last thing you want is to go to all the trouble to build this thing and have it melt down on you and possibly do damage to your amp. Remember this Airbrake is a proven design that has passed the test of time. I always try to remember the old adage: "If it ain't broke, don't fix it!" Cheers!

[RockNRollDr]

Can someone send me the layout for this rheostatless brake. Thank You.

zoso503@hotmail.com

[dehughes]

Yes, I as well would like the schematic for the rheostat-less Airbrake.

Also, I'm looking for feet for the unit...something from Mouser, preferably, as I'll be ordering the parts from them anyway.

Thanks!

[JoeyE]

Here's what i used for an enclosure. As per Jimis sugestion, i used 100w resistors and the enclosure is roughly 9"x5"x2" and it is sturdy and the circuit guide fins at the side might help with heat dissipation too. I made two speaker jacks so that i can mix speaker types and impedances. I will observe if the plastic bevel will hold up to the heat but it can be removed. I hope the plastic jack survives too.

[mhuss]

I found that by using two "slider" 100 watt resistors and an added 20W resistor, I could keep the load impedance as seen by the amp much closer to the load value (8 ohms in this case) vs. the stock Airbrake design.

Note that this is a simple resistive L-Pad design, no rocket science. Perhaps using the wire-wound resistors adds some inductive mojo.

In practice I never use the "-1 db" setting, which is the worst mismatch of the bunch; You could skip this one altogether.

You could do even better with extra sliders, I was working within the constraints of the four sliders I had on hand.

N.B., PTFE-insulated wire or careful layout is a good idea in a high-temp project like this.

--mark

[JoeyE]

How transparent is the sound compared to the airbrake layout? What setting do you start to loose tone or color the sound?

[dehughes]

I found that by using two "slider" 100 watt resistors and an added 20W resistor, I could keep the load impedance as seen by the amp much closer to the load value (8 ohms in this case) vs. the stock Airbrake design.

Note that this is a simple resistive L-Pad design, no rocket science. Perhaps using the wire-wound resistors adds some inductive mojo.

In practice I never use the "-1 db" setting, which is the worst mismatch of the bunch; You could skip this one altogether.

You could do even better with extra sliders, I was working within the constraints of the four sliders I had on hand.

N.B., PTFE-insulated wire or careful layout is a good idea in a high-temp project like this.

--mark

Whoa...very cool....you'll have to explain that schematic a bit for me....I'm not getting it. 25R stands for 25 ohm? What do you mean by "adjustable" resistors? Are they they different from the standard ones in the Airbrake design? Do you by chance have Mouser part numbers for what you've built?

I'd be using my Airbrake for mostly 8 and 16 ohm loads, with the occasional 4 ohm load, so something that would keep impedance mismatches reduced would be great....
Thanks!

[Roe]

when you leave out the rheostat, should you also leave out the .1uf cap?
how much would this thing attenuate if you connect to the end of the 25ohm resistor - 8db?

[mhuss]

25R stands for 25 ohm? What do you mean by "adjustable" resistors? Are they they different from the standard ones in the Airbrake design?

Yes, 25R = 25 ohms. I got the parts from a "kit" someone (perhaps here?) was selling at one point. IIRC, the original AB had one adjustable and one fixed.

The adjustable wirewound resistors have the windings exposed along one side, and a slide-able contact (screw-tightened in place) to set the resistance somewhere between 10% and 90% of the entire resistor. I believe the adjustable resistors come with one slider, you have to order additional ones extra. I don't think I've seen these anywhere except for Allied.

--mark

[zedar]

Yes, I as well would like the schematic for the rheostat-less Airbrake.

Thanks

[dehughes]

25R stands for 25 ohm? What do you mean by "adjustable" resistors? Are they they different from the standard ones in the Airbrake design?

Yes, 25R = 25 ohms. I got the parts from a "kit" someone (perhaps here?) was selling at one point. IIRC, the original AB had one adjustable and one fixed.

The adjustable wirewound resistors have the windings exposed along one side, and a slide-able contact (screw-tightened in place) to set the resistance somewhere between 10% and 90% of the entire resistor. I believe the adjustable resistors come with one slider, you have to order additional ones extra. I don't think I've seen these anywhere except for Allied.

--mark

Cool. Thanks. That helps. I've seen those same resistors at Mouser as well...a bit cheaper there, IIRC.

As for the schematic....I still would like to see a schematic for the standard, rheostat-less Airbrake, as I'm not understanding the one you have posted here, Mark. Most of my lack of understanding is related to how to properly connect the rotary switch....

[mhuss]

Most of my lack of understanding is related to how to properly connect the rotary switch....

The wires just connect in a circle around the switch, top to bottom on each deck. The numbers and letters indicated are the actual lug labels from the switch I used.

--mark

[Noel Grassy]

Can someone send me the layout for this rheostatless brake. Thank You.

zoso503@hotmail.com

To eliminate the rheostat & capacitor, the lead from lug #7 gets connected to the far lug on the resistor with the taps. The spot where the rheostat was terminated.

[dehughes]

Most of my lack of understanding is related to how to properly connect the rotary switch....

The wires just connect in a circle around the switch, top to bottom on each deck. The numbers and letters indicated are the actual lug labels from the switch I used.

--mark

Ah, I think I get it. So you have two of the same larger resistors (adjustable ones...), the same switch (Lorin...), and an extra 8 ohm 25w resistor. I studied the schematic you posted in comparison to the "regular" layout, and I think I get it now. Many thanks.

So do you recommend this "improved" version you have over the regular one? Keeping the resistance closer to 8 ohms seems reasonable to me, but I wonder why the original ones didn't do this....I mean...it was made by Ken...and I think he'd have thought about that. Not to be pesky...just curious to know, before I start ordering parts and building my own.

Thanks!