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Old 18th July 2004, 03:43 PM   #1
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Default DIY Power Brake?

Hey guys,

I play a lead guitar. The amp is a Marshall push pull tube amp from the 80's. Even with two tubes pulled and the amp running at 50W, it's impossible for me to get it running hot enough to cause a nice smooth power tube distortion.

I've had a Marshall power brake in the past, but it used some kind of questionable braking method that really made the amp sound horrible. A lot of people had these brakes fail and take out their amp as well.

With lead guitars, the idea is to actually cause distortion. I wanted to swap my speakers out for some Greenbacks, that distort in a nicer way, but I'm still left with the volume problem.

Checking out how much Celestions like those in my cabinet go for second hand, I began to think a bit, since it's not a great deal. Each of the current speakers is rated for 75W and I have four of them.

How about pulling the cones from one or two of them? This idea is already sold preassembled on the net as The Weber Mass. It works by just replacing the cones with some squishy springs.

And so, no real volume of air is moved; the springs soak up the energy, allowing the amp to go into distortion at lower volumes.

I think I could manage that mechanically, I'm curious about controlling the degree of braking though.

What would be the best way to control the amount of power the mass brakes?
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Old 19th July 2004, 01:33 AM   #2
MIKET is offline MIKET  United States
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Default Restricting cone travel

You can also restrict cone travel by placing the driver in a small sealed enclosure. This change is reversable, I am not so sure about the other. Option 2 use inefficent drivers and this will alow the amp to distort at lower SPL. 5" MCM's work great as a small guitar speaker. Here are the specs from MCM's site.
www.mcmelectronics.com

Woofer MCM 55-1870 Price 1 to 9 $12.05 US

5" Aluminum Cone Stage Monitor with Rubber Surround
•Dimensions: 130.5mm (5") •Impedance: 8ohm •Input power: 50W, Max 100W •Sensitivity: 87±2db/W/M, 2.83V •Frequency range: 50~17KHz •Voice coil diameter: 25.4mm •Weight of magnet: 15.6 oz. Thiele-Small parameters: •Total Q factor Qts: 0.401 •Electrical Q factor Qes: 0.511 •Mechanical Q factor Qms: 1.87 •Vas: 0.35 cu. ft. •Effective piston area Sd: 0.0075m2 •DC resistance Re: 6.7ohm •Linear displacement Xmax: 2.0mm •Maximum impendance Zo: 31.2ohm •Suspention compliance Cms: 0.00126m/N •BL Product BL: 5.56 •Driver piston diameter D: 98mm •Moving mass Mms: 6.99g •Resonance frequency Fs: 54Hz
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Old 21st July 2004, 11:52 AM   #3
MIKET is offline MIKET  United States
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Default here is another possible solution

You have a 100W amplifier. You have a speaker which gives you the tone you want but is too efficent. Using ohms law it would work out to 28.3 volts into 8 ohms accross you speaker of choice at full output. You wish to reduce the voltage accross the speaker but not the amplifiers output. Get a Matching trnsormer for PA use. By hooking the amplifiers output to the 70.7 input of the transformer and the speaker to to output tap of the transformer you can dramaticly reduce the output to the speaker and still max out the amplifier. Matching transformers are inexpensive. Example:
http://www.partsexpress.com/pe/showd...number=300-039
This will set you back $5.75 and alow you to crank the amp and have the speaker see much less power.
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Old 21st July 2004, 12:42 PM   #4
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Thanks for the help Miket,

I've never heard of this idea before. I actually have four speakers, this may become two - but that's just maths anyway.

Looking at the transformer you posted the link to, the output has taps at 5W, 7.5W, 10W and 15W. If I was only driving one speaker, say I was to use the 5W tap, where does the other 95W of power go?

Sorry if this is a stupid question, I haven't been awake long!
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Old 21st July 2004, 12:55 PM   #5
MIKET is offline MIKET  United States
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The power of the amp is based on the voltage swing it can produce accross a given load, raising the resistance of the load doesn't change this much in a tube amp. If you wish to load the amp with a lower resistance after you have tried this experiment place an audio dummy load accross your existing transformer where you hook up the primary of the matching transformer.
Example:
http://www.partsexpress.com/pe/showd...number=019-030

This will allow the power to be disapated from the output of the transformer, probably not needed but can be used.
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Old 21st July 2004, 01:24 PM   #6
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Hi again Miket,

I'm still not totally sure about how this works, not that I don't believe it doesn't.

Could you explain what is happening with the line transformer arrangement?

Is the line transformer just reflecting a different resistance back at the output transformer? Kind of like impedance mismatching?
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Old 21st July 2004, 01:30 PM   #7
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"Is the line transformer just reflecting a different resistance back at the output transformer? Kind of like impedance mismatching?"

That is exactly what will happen. It can be dangerous. It's possible to damage the output transformer if the mismatch is bad enough.

Have you thought about reducing the screen voltage on the output tubes?
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Old 21st July 2004, 03:42 PM   #8
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Extreme care should be exersized when modifying the loading on a output coupling transformer based vacuum tube power amplifier stage. By reducing the voltage to the load by fractioning off less of the power by placing a higher effective impedance load on the speaker winding of the transformer the peak to peak voltage in the output transformer primary and the plate voltages of the output tubes could begin to swing to unsafe levels causing internal sparking in the tubes, and interturn transformer breakdown resulting in expensive repair bills. It is the inductance of the output transformer that causes this potentially destructive voltage rise. The worst case scenario is operating a valve amplifier at high volume setting without a speaker load at all, like may happen if a cable is tripped on pulling a speaker connector out of circuit.

A safety spark gap should be installed across the plate-to-plate connections of the primary of the output transformer to limit high voltage excursions from reaching dangerous levels. The gap distance should be set by experimentaion but 1/16"-3/32" might be a good starting point. When the gap fires there will be a heck of an unpleasant noise emitted from the speakers but at least the amplifier will continue to operate without having to replace expensive components.

If you need to run the amp flat out to get the tone you wish but don't want all the power going to the speaker then make sure you put a load resistor of adequate wattage rating on the output transformer secondary to maintain proper amplifier loading.

To make your own power brake try to find a large wirewound power resistor with say 8 ohms resistance rating (actually it should be exactly the same as your impedance of the speakers)at 100 to 250 watts dissipation rating that has an adjustable tap strap on it. Connect the outside ends of this resistor to the regular amplifier speaker connections. Connect the speaker to the ground (chassis connection) end of the resistor and the tap connection. The closer the tap is located to the ground end of the resistor the lower the acoustic output to the speaker even with the amp safely operating at high power levels.
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Old 21st July 2004, 04:37 PM   #9
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Quote:
To make your own power brake try to find a large wirewound power resistor with say 8 ohms resistance rating (actually it should be exactly the same as your impedance of the speakers)at 100 to 250 watts dissipation rating that has an adjustable tap strap on it. Connect the outside ends of this resistor to the regular amplifier speaker connections. Connect the speaker to the ground (chassis connection) end of the resistor and the tap connection. The closer the tap is located to the ground end of the resistor the lower the acoustic output to the speaker even with the amp safely operating at high power levels.
This may be good.. but maybe not... couple of reasons.

say you have speaker conected across resistor tapped at half way. You have four ohms resistance before the speaker tap, and four ohms in // with the speaker... what does this mean???

say you have 8 ohm speaker, // resistance law says

R = 1/[(1/R1)+(1/R2)+...]

so, we have 4 ohms in // with 8

R = 1/[(1/8)+(1/4)]
R = 2.7

so our 4 ohms before the tap is essentially in series with aprox 2.7 ohms, giving a total resistance of 6.7. of course, tapping at half way will only give a 3dB drop in sound level, and you will probably be wanting more than this, so you will probably be closer to your 8ohm mark, but you will have to be carefull that this system is only being used for higher amounts of attenuation, say in the order of 12dB where you will only be paraleling 0.5 ohms of the resistor with the speaker.

depends on what you want.

Also, even if using this, you will now need to remember, the majority of the load your amp sees is the resistor which is pretty much as close to a non inductive load as you'll get, but a speaker presents a reactive load to your amp as it's an inductive load. Baisically, even though the speaker says it's 8 ohms, this is only 8 ohms at the frequency used to measure its resistance (i think they use 400Hz, not really sure though). In reality, this resistance changes at different frequencies because a speaker is an inductive load. What this does is reflect a different impedance on the primary for every frequency. This effects the response of the power stage, and in effect, it will effect the sound and feel of the amp. Not only does it effect frequency response, it will also effect distortion charachteristics. this resistor method is a relatively cheap idea, and worth giving a shot as long as you do it safely because you may like the sound, and if you don't like it, then atleast you didn't waste a lot of money. personally, if i was using the resistor idea, i'd want to use something like a 10 ohm resistor, or buy a 15W (min) 2ohm resistor (for 50W) and conect in series with the 8 ohm one. you only need 15W rating for the 2 ohm one because with this, you'll have a total of 10 ohms, and 2 ohms is one fifth of this, so you'll only be dropping one fifth of your power across this resistor, so 10W max, but overrate it to avoid excess heat.
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Old 21st July 2004, 08:45 PM   #10
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Default but does it sound the same?

Apart from the safety problems with some of the suggestions above, the main problem is that each of these suggestions changes the operating conditions of the output stage and therefore changes the sound of the stage and it's response to the the way the guitar is being played i.e. they change the very thing that caused the amplifier to be wound right up in the first place... Attenuating the sound level of a guitar amplifier and speaker set without changing the character of its sound is actually quite difficult. Aiken has studied the rpoblem extensively and produced some of the most transparent 'power brakes' that I have heard or rather not heard His site is a good source of guidance for building your own: URL is Aiken technical articles .

hope this helps

James
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