Airbrake Wattage Rating - Oversight maybe?

[Littlewyan]

The original Airbrake is rated for 75W and is advertised as being able to handle 4/8/16 Ohm Speaker Loads. I've also seen an article from Ken stating that it can handle a 100W Marshall or even a 100W Hiwatt.

When I've been doing calculations for my Modified Airbrake unit I can see that with a 4Ohm Speaker and 100W of power on the 2nd Attenuation setting, the 6Ohm lug on the 25Ohm Resistor needs to dissipate 30W. However as the 25Ohm resistor is rated for 100W at 25Ohm the 6Ohm lug is only rated between 20-25W. Is this an oversight by Ken? A 100W Marshall kicks out 170W at full tilt which means that 6Ohm lug will actually have to cope with 72W! Even with an 8Ohm Speaker it needs to cope with 46W.

In my design I've used a seperate 25W Resistor for the first lug and I'm only rating the unit for 50W RMS. A 50W Marshall can kick out 90W at full blast so it should be fine. I just can't see how Ken thought this would work.

[xtian]

During performance, how long could you maintain peak output from a 100W Marshall? I mean, your figures are for signal generator into dummy load, right? Worst case?

[Colossal]

During performance, how long could you maintain peak output from a 100W Marshall? I mean, your figures are for signal generator into dummy load, right? Worst case?

Xtian,

I think the question is when DON'T you maintain peak output on a 100W Marshall

With apologies to OP...

[Littlewyan]

They are for worst case but surely you should prepare for worst case

Thats why I spent so long redesigning the Airbrake for my purpose. I didn't want to build it thinking it would be fine as long as I don't go full peak output for too long.

I wonder if hes relying on the big impedance mismatch the Airbrake presents. As the Airbrake presents a higher load to the amp, less current will flow. Though saying that I think the calculator already accounts for that. I know he says to set vintage amps to half the impedance of the cab for the first 2 clicks, maybe thats how he gets round it.

[pdf64]

They are for worst case but surely you should prepare for worst case

My experience is to allow a little extra on top of the worst case! Such a bad feeling when the sound fizzling out is accompanied by that toasted electrical smell, be it due to a resistor, speaker or transformer!

I wonder if hes relying on the big impedance mismatch the Airbrake presents. As the Airbrake presents a higher load to the amp, less current will flow.

The highish output impedance of tube guitar amps results in their output voltage tending to rise as the load's impedance value tends to increase, thereby maintaining a fairly constant output power.

I think that tube guitar amps tend to use load impedances that seem fairly low, but that actually work out well, due to a speaker's nominal impedance only tending to apply over the ~200-1kHz band.
Outside that, the impedance tends to be rather higher, maybe >x10 at bass resonance.
So with a real speaker, for a power chord, the average primary reflected load may be higher than expected.

[vibratoking]

I know he says to set vintage amps to half the impedance of the cab for the first 2 clicks, maybe thats how he gets round it.

Yes, but what user is actually gonna do that? Doesn't seem like a work around that works.

[Gibsonman63]

Mine on a 100W 4-hole Marshall smelled like a toaster. I was afraid to run it very long.

[Littlewyan]

Such a bad feeling when the sound fizzling out is accompanied by that toasted electrical smell, be it due to a resistor, speaker or transformer!

Exactly what I want to avoid PDF64.

I thought the higher the frequency the higher the impedance and vice versa? Thats what Ken stated in his article. He said when approaching DC (100Hz for me) that the impedance goes lower.

Its not a good workaround Vibratoking no. You can't rely on users to read that line in the manual stating that the impedance must be set that way. Even if they did read it, what if they didn't understand it?! Maybe this is why Ken only sold the Airbrake to TW customers originally.

Gibsonman63 - Not surprised. How did you have the attenuator set? If you only want 6dB of attenuation then Gerald Weber has a good design you could use which costs very little to build and would take a 100W Marshall no problem.

[Gibsonman63]

Littlewyan, I sold that one last year. I have had a half a dozen Marshall amps and that one by far was the loudest. Way too much power for anything I do.

I still use it on my Express for rehearsals and it works fairly good. At -6dB I still have some clean left. -9dB is pushing it a bit, but still usable. You just give up a bit more of the clean. -12, it starts to not be so good anymore. I also place acoustical foam in front of three of the four speakers, just leaving one of the bottom ones. The combination seems to be about right volume wise. Not very practical for gigging, but I get some decent recorded sounds and that rig never moves.

If you are not too into super clean, a variac can help some. Less supply voltage, throw away a bit of power with the attenuator and move less air. All three are compromises, but not as bad as taking it down with just one method.

[Littlewyan]

Yeah I'm not sure there is much call for a 100W Amp nowadays unless you're in a stadium or festival.

I have to attenuate my Express by 15dB at practice. The cleans are quiet but thats ok as the clean parts are normally quiet in the songs that require them. Or if the whole song is clean then normally the rest of the band goes quieter so it works out ok.

I was testing my current attenuator yesterday to see how much of a drop in volume -12dB and -15dB was to see if my new attenuator would drop enough volume and to cope with the volume in my room I stuck two pillows in front of the cab and pushed it against my bed . I am keen to one day try putting plexiglass screens in front of my cab as once suggested by Guitarmike.

I also plan on one day building an amp with a low HT to simulate a variac. Mike Soldano did this on the Hot Rod 25. 310V on 6L6s.

[pdf64]

I thought the higher the frequency the higher the impedance and vice versa? Thats what Ken stated in his article. He said when approaching DC (100Hz for me) that the impedance goes lower.

No, most all speakers have an increase in impedance around bass resonance; for guitar speakers it tends to be significant.
eg see the grey line of the freq response chart http://www.eminence.com/pdf/EJ1250.pdf

That's measured in free air or with a large open back baffle, in a treated room.
In a sealed cab the resonance will tend to be greater in magnitude and Q.

[Littlewyan]

Ah thats very interesting. I just read back on Ken's article and I will admit I was wrong. It adds up with exactly what you said PDF64. He actually said

For example, a 16 ohm standard 4-12 Marshall cabinet can be as low as 10 ohms as the frequency approaches DC & over 30 ohms at some of the higher frequency harmonics generated by the guitar under distortion.

Which is correct. He just missed out that as you get to DC it rises rather significantly. I guess he was trying to not over complicate things and just give a general example.

So sorry about that!

I do wonder how he designed the Airbrake. Maybe he actually carried out experiments by monitoring the speaker impedance at various frequencies as he attenuated the signal.

[Littlewyan]

Also as the impedance rises significantly at 100Hz (well depending on the speaker of course) then a good test for an amp would be a constant 100Hz into the front end and for it to be hooked up to a reactive load. This would drive the amp very hard and would test to see if there will be any issues with arcing. Am I right?

[pdf64]

..a good test for an amp would be a constant 100Hz into the front end and for it to be hooked up to a reactive load. This would drive the amp very hard and would test to see if there will be any issues with arcing..?

I think this may only be a valid test if the reactive load was a close frequency - impedance match for a regular real speaker, with a bass resonance at 100Hz.
Bear in mind that for a 100Hz overdriven signal, although the fundemental would result in a very high reflected p-p primary load, the next few harmonics 200, 300, 400, 500, etc would reflect back ~ the intended load impedance, and it would gradually rise for the harmonics above 1kHz.
But most of the power for the harmonics will probably be in those lower order ones.

[Littlewyan]

I'll have to do some reading up on all of this. I've heard the Marshall Power Brake attenuators are a very harsh test for an amplifier. I know Randall Aiken uses one as a test for his amps. I wonder what impedance that device reflects back at 100Hz.