airbrake?

Express, Liverpool, Rocket, Dirty Little Monster, etc.

Moderators: pompeiisneaks, Colossal

rockgod212
Posts: 22
Joined: Tue Jul 25, 2017 5:19 pm

airbrake?

Post by rockgod212 »

could someone help me build an airbrake for a 100 watt marshall? I have a 25ohm/100watt rheostat coming in place of 25ohm/100watt variable power resistor. so how to wire it and such. any help would be great. I think I have a layout for it, not sure if it is correct.
User avatar
romberg
Posts: 501
Joined: Wed Nov 28, 2007 6:10 am
Location: Lafayette, CO
Contact:

Re: airbrake?

Post by romberg »

What schematic/layout are you planning on using? There are several on this site such as the one found here:

http://ampgarage.com/forum/viewtopic.php?f=3&t=8733

Note that the one Ken built is labeled "75 watt maximum input":

http://ampgarage.com/forum/download/fil ... =25280&t=1

Also keep in mind that a 100 watt spec on a guitar amp is usually at the onset of clipping with a sine wave. Or maximum "clean" power. As the gain is turned up, the output becomes square waves. Square waves can have double the average output power of sine waves. So, your 100 watt marshall can be putting out nearly 200 watts. A JCM800 2203 owners manual says:
Typical power at clipping, measured at 1kHz, average distortion 4% 115 watts RMS into 4, 8, 16 ohms. Typical output power at 10% distortion 170 watts into 4 ohms.
So, you may want to be sure whatever attenuator circuit you plan to use can handle this power. If you are just gonna knock a few db off the top then even a 50w attenuator would be fine. Taking a cranked 100 watt fully overdriven amp down to "don't wake the baby in the next room" levels would probably need a 200-300w attenuator.

Mike
User avatar
RJ Guitars
Posts: 2662
Joined: Tue Nov 14, 2006 3:49 am
Location: Los Alamos, New Mexico
Contact:

Re: airbrake?

Post by RJ Guitars »

I build something I call the Power Broker in the parts package form - an improved airbrake (IMHO). http://diyguitaramps.prophpbb.com/topic33.html

However, I use 100 watt resistors and claim mine as a 50 watt maximum. I could build that for you and double all the resistors but there is significant risk so it would definitely take a release from you from any liabilities and the box size and price would also be about double. 100 watts into a small aluminum box makes a ton of heat (sorry mixing units here) and that in itself might be a show stopper... or starter, depending on your perspective.
Good, Fast, or Cheap -- Pick two...

http://www.rjguitars.net
http://www.rjaudioresearch.com/
http://diyguitaramps.prophpbb.com/
Mark
Posts: 2957
Joined: Sat Jan 15, 2005 8:10 am
Location: Sydney Australia

Re: airbrake?

Post by Mark »

How does the Airbrake compare to other attenuators?

The Tom Scholz ones were awful as the amp sounded mid heavy and compressed. I have a 50 watt Weber attenuator which works okay though some amps sound better than others.
Yours Sincerely

Mark Abbott
User avatar
RJ Guitars
Posts: 2662
Joined: Tue Nov 14, 2006 3:49 am
Location: Los Alamos, New Mexico
Contact:

Re: airbrake?

Post by RJ Guitars »

Mark wrote: Fri Jul 28, 2017 4:54 am How does the Airbrake compare to other attenuators?

The Tom Scholz ones were awful as the amp sounded mid heavy and compressed. I have a 50 watt Weber attenuator which works okay though some amps sound better than others.
Not sure you are asking me specifically but I have a rule of thumb that I sorta apply to all volume reducing techniques - MV, Power Scaling, or Attenuators.

IMHO - Amps tend to sound best when they are louder and the more volume you take away the less magic they hang on to. I like attenuators best myself because I think they don't start to suck as early as the other methods. Quantifying that is a challenge but I think it has to do with keeping the output tubes working hard. I will say that you'll likely want to adjust the tone controls as you dial it down because among other things our ears don't seem to hear everything in a linear scale.

One final thought - you do get to use more of your volume control with any of these options and that may be the best outcome from dampening the output a little. This might allow a little more of our guitar signal to get to that output stage... YMMV. rj
Good, Fast, or Cheap -- Pick two...

http://www.rjguitars.net
http://www.rjaudioresearch.com/
http://diyguitaramps.prophpbb.com/
rockgod212
Posts: 22
Joined: Tue Jul 25, 2017 5:19 pm

Re: airbrake?

Post by rockgod212 »

what I was planning on doing is this. it would still have the 25ohm 100 watt shunt resistor, I will omit the 25ohm 100 watt variable leg resistor( I want to use a 25ohm 100watt rheostat I have here,) I will omit the 150ohm 50watt rheostat and use a power resistor here (or I might use it,) omit the switch is the goal. just need advice on wiring it, should I just trace the path and wire it accordingly. is this airbrake not capable for a 100 watt head, I don't plan on diming it- just 3/4 full volume should do, just to get a bit more power thing going on with less overall volume, what would I need to change in order to make it take my amp (170watt marshall cranked.)
AirbrakeLayout.jpg
You do not have the required permissions to view the files attached to this post.
Mark
Posts: 2957
Joined: Sat Jan 15, 2005 8:10 am
Location: Sydney Australia

Re: airbrake?

Post by Mark »

Thanks RJ, that is the sort of thing I wanted to hear. I was concerned that the Airbrake would be similar to the Tom Scholz attenuator which is rubbish.
Yours Sincerely

Mark Abbott
User avatar
romberg
Posts: 501
Joined: Wed Nov 28, 2007 6:10 am
Location: Lafayette, CO
Contact:

Re: airbrake?

Post by romberg »

rockgod212 wrote: Sun Jul 30, 2017 10:59 pm what would I need to change in order to make it take my amp (170watt marshall cranked.)
Take a look at this:

http://ampgarage.com/forum/viewtopic.php?f=3&t=11000

http://ampgarage.com/forum/download/file.php?id=11945

The airbrake is pretty close to a Pi attenuator. So, you can plug in numbers and see the results for various values. For example:

Pin=170w, Zin=8ohm, Zout=8ohm, 12db attenuation
you get
R1=13.4ohm @102watts, R2=14.8ohm @51watts, R3=13.4ohm @6.4watts, PowerOut=11watts

11watts is still way above bedroom levels but it does not sound like you are looking for that. However there is still a concern with R1. These resistors are rated at their full resistance (in this case 25 ohms) for the given power dissipation. If you use a fraction of the resistor then the power rating goes down by that fraction. So, in this case R1 can only handle 100w * 13.4 / 25 = 53watts. So, it will probably fail. With this in mind, I tried using 16 ohm loads for the amp and cabinet:

Pin=170w, Zin=16ohm, Zout=16ohm, 12db attenuation
you get
R1=27ohm @102watts, R2=30ohm @51watts, R3=27ohm @6.4watts, PowerOut=11watts

This looks better because R1 is now higher and thus does not need to be de-rated. However the power dissipation does still exceed it's maximum rating. Conventional wisdom is to use a resistor twice the size of it's expected power dissipation. So, I think (could be wrong) you should use a 200w resistor for R1 and set the amp and cabinet to 16 ohms.

You also might be able to get away with the 100watt parts as long as you don't have the amp turned all the way up. For example keep the amps master volume turned down and or pull a couple of power tubes and run it at 50w.
User avatar
Winder
Posts: 201
Joined: Sat Nov 29, 2008 11:12 pm

Re: airbrake?

Post by Winder »

I use something similar to RJ's Power Broker design, and can't imagine using anything else. Had a "HotPlate" ... tried "Power Dampening", various MVs, etc. You can take them down to loudish conversation levels and still enjoy decent interaction with the amp. Not perfect, but kind to the old ears with tinnitus.
User avatar
RJ Guitars
Posts: 2662
Joined: Tue Nov 14, 2006 3:49 am
Location: Los Alamos, New Mexico
Contact:

Re: airbrake?

Post by RJ Guitars »

rockgod212 wrote: Sun Jul 30, 2017 10:59 pm... I don't plan on diming it- just 3/4 full volume should do, just to get a bit more power thing going on with less overall volume...
This is a fun thing to look into when you consider the non linearity of the relationship of volume in db and power in wattage. I'll be curious how many watts you need to throw away to get where you want to be.
Good, Fast, or Cheap -- Pick two...

http://www.rjguitars.net
http://www.rjaudioresearch.com/
http://diyguitaramps.prophpbb.com/
User avatar
romberg
Posts: 501
Joined: Wed Nov 28, 2007 6:10 am
Location: Lafayette, CO
Contact:

Re: airbrake?

Post by romberg »

At the bottom of this article is a nice little interactive calculator that will convert differences in amplifier output in watts to differences in SPL and perceived volume:

https://geoffthegreygeek.com/amplifier-power/

It is kinda cool. It shows that to perceive a 200 watt amp as being 75% as loud you need to lower the output about 4.2db to 77 watts. The SPL is then down by about 38%. So, this means you are looking at dumping like 120 watts into an attenuator. And the amp will still be (as Ozzy would say) "louder than Satan!".
User avatar
RJ Guitars
Posts: 2662
Joined: Tue Nov 14, 2006 3:49 am
Location: Los Alamos, New Mexico
Contact:

Re: airbrake?

Post by RJ Guitars »

Nice...thanks for that!! It's great to have somebody put the numbers in place for what my intuition was trying to tell me.
Good, Fast, or Cheap -- Pick two...

http://www.rjguitars.net
http://www.rjaudioresearch.com/
http://diyguitaramps.prophpbb.com/
User avatar
romberg
Posts: 501
Joined: Wed Nov 28, 2007 6:10 am
Location: Lafayette, CO
Contact:

Re: airbrake?

Post by romberg »

rockgod212 wrote: Sun Jul 30, 2017 10:59 pm omit the switch is the goal. just need advice on wiring it, should I just trace the path and wire it accordingly.
Hard wiring this so that it has a fixed attenuation should not be difficult. Just look at how the rotary switch connects things at each setting. Then wire things up for that specific setting. One could experiment a bit with this after it is built (not while the amp is running of course) to find the amount of fixed attenuation you like. All the switch really does is select a different tap on the second power resistor and or use the rheostat.
is this airbrake not capable for a 100 watt head
I do not believe that it is. As RJ suggested, I think I would at least double the wattage rating of all the resistors. Other factors probably matter too (heat sinking etc). Before I did anything like this with an amplifier I owned I would also check the math on how much power dissipation to expect in each component (such as using the spreadsheet linked above). You could try it. And it might work for what you want to do. But it could also fail and destroy parts of your amplifier along with it.

I think it would be a better idea to pull two power tubes out of your amp (either the outer or inner pair) and run it at 50 watts. You can search about how to do this on the internet. This attenuator seems like it would be fine with a 50 watt amp. Why bother to run it at 100watts when you are just going to be converting at least 50 watts of that power into heat?

Mike
rockgod212
Posts: 22
Joined: Tue Jul 25, 2017 5:19 pm

Re: airbrake?

Post by rockgod212 »

that's true about 100 watts and pulling tubes, I think I will build it in time for my 1987 head- that's what I really want it for anyway. thanx for the info
rockgod212
Posts: 22
Joined: Tue Jul 25, 2017 5:19 pm

Re: airbrake?

Post by rockgod212 »

ok so here we go..........here is what I have on hand so far to make an attenuator.......... what can I make with these parts, need help with a schematic/ layout ideas.

1- 25ohm/100watt rheostat
1- 150ohm/50watt rheostat
1- 25ohm/200watt shunt wire wound power resistor with heat sink built in
8- 8ohm/50watt wire wound power resistors with heat sink built in
1- 7"x7"x4" electronics enclosure box with vents/ feet

***I still need to get some DPDT switches and bypass caps.

I would like to wire in a fan, anybody know how to do it?
Post Reply