Airbrake Original vs III

[Littlewyan]

The original Airbrake was deliberately designed to not match the impedance of the cabinet to the amp. However there was a new design (Mk III) done by a few forum members here (sorry not sure who it was) which does a pretty good job of matching the impedance.

For anyone wanting to read why the original Airbrake doesn't match the impedance I've attached a txt document with an article from Ken Fischer!

Now I believe a few members have built both versions, so my question to you is, which do you think preserves the tone better?

[Colossal]

Pretty sure it was Professor Emeritus Mark Huss that designed the MkIII improved Airbrake. If not, well, Mark's a great guy anyway.

[Littlewyan]

After looking through many posts I believe it was, as you said, Doc Huss.

Interesting that no one has commented on here about these two designs. I'm sure a few forum members have built both.

[M Fowler]

I've only built the original AB.

[Littlewyan]

I'm glad you posted Mark as I've been studying a photo you posted of the Kendrick AB and I've managed to draw out the circuit. It seems Gerald Weber modified the AB so the amp sees roughly 4Ohms no matter what speaker cab you plug in.

[RJ Guitars]

Ryan,

Edit 10/25/15 - I have only built the original Airbrake and a handful of other iterations but not the MKIII. However, it is my conviction that purely resistive attenuators are all going to have a similar impact on your tone with maybe the exception of how 40 ohms of impedance might interact with your amp as opposed to more or less ohms depending on the design of the circuit.

I like the performance of attenuators better than master volumes or VVR's because I think you can get decent tones over a wider range of volume. However, there is still nothing that ever seems to sound as good to my ears as loud. The attenuated signals never sound exactly like miniature carbon copies of the original, but they are more environmentally friendly and have merit.

I've only had a few of the various inductive units and in comparison with the pure resistive units my ears can't sort out a real cost/effectiveness payoff... YMMV. I have started building in my own version of a resistive attentuator not all that different from an airbrake and using it in my little combo amps. http://www.ampgarage.com/forum/viewtopic.php?t=25510. Like the airbrake it mixes some resistance with some speaker action and I think the results score very well on the cost/effectiveness scale.

I ran some calculations and found a combination of parts that keeps the load impedance in the safe range and has given me the confidence to put one into place and use it routinely. I've got about a couple years on the prototype in a Rockster amp and everything is still going strong.

FWIW - we all know that the load impedance is not going to be that nice smooth curve since the speaker will be changing impedance values constantly, but for arguments sake - it's close enough.

Hope this is helpful and relative...

rj

[Littlewyan]

Thanks for this RJ.

I have done a lot of reading on the different types of attenuators and it does seem you are far better off with resistive attenuators such as the Airbrake. They are cheaper and safer (in most cases).

Although there seems to be conflicting info where Ken says his Airbrake is safer for an amp due to it's higher load impedance, but others state that this can cause issues such as arcing or screens failure. I'm sure someone even said their amp was red plating with the Airbrake until they matched the impedance correctly.

I have modified the Airbrake III schematic to show my version of the Airbrake. I started with the Kendrick Airbrake and modified it slightly. It matches any speaker load to 4Ohms. The only downside to this design are the huge jumps in attenuation. I've attached a spreadsheet to show the attenuation levels. The resistor values can be played around with to shorten the jumps between the settings.

[RJ Guitars]

It's a well known standard for tube amps that if your load impedance value goes to zero your output tranny dollar value soon follows. No mystery there but maybe a few things we can take from various sources that might indicate 40 ohms isn't especially bad for OT health.

Many tube amps (all of mine) come with a shorting output jack just in case you forget to plug in a speaker. It's not an ideal load but Fender did this long ago with the assumption that a dead short is better for your amp than no load. Lots of old fender amps out there with the original transformers so that's a good practice at the least. How long can an OT take a dead short - so far that seems to be a data point that has rarely been sought after?

Gerald Weber gives some rule of thumb that in a typical tube amp you can be off by approximately 100% in matching your speaker load impedance to your OT and you'll not suffer any harm. We've probably all done that routinely as we plug into various cabs with anywhere from 4 to 16 ohms.

I might not be surprised if a big mismatch is hard on the tubes but I haven't studied it. I'd be inclined to think that if you are pushing everything to the ragged edge it might be more likely that a big mismatch would cause problems when you crank the amp through an attenuator...

Running an amp through an attentuator really works it hard and it's more likely that you'll discover and cause more weak links the harder you push the amp. An attenuator lets you work an amp in a way that normally (using a proper speaker system) your ears, your wife, and your neighbors would say is not making the world better.

[vibratoking]

Not sure if this has been mentioned, but adjustable resistors are only rated to the max power spec when the whole resistance is used. If you take a 4 Ohm tap off a 25 Ohm/100W resistor, the 4 Ohm tap is rated for 16W.

[Littlewyan]

I think you're right RJ. Valves that are dealing with a nice clean signal aren't as affected by an impedance mismatch compared the valves that are being blasted by a square wave. Randall Aiken explains why on his website.

I thought of that earlier Vibratoking and the new design I posted uses 2 of those tapped resistors. As a result it can probably only handle up to 50W Peak. I've come up with a slightly different design which I will post soon that matches all speaker loads to 4Ohms and can handle 100W peak. So you could run a 50W Marshall into it flat out but a 100W Marshall would be too much.