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Can I use a speaker motor to "absorb" energy? Answered

I am looking to use a speaker motor or a high wattage speaker minus the cone to absorb some of the power coming of a tube amp. The setup would be a 4x12 cab rated around 300-400 watts, 120watt tube head, and the speaker motor. I am hoping that the speaker would absorb some of the power and allow me to drive the tube head harder but without blasting the windows out of a building. Power tube saturation is a great sound and not alway easy to get ... even though the cab is capable of handling most of the heads power ... it gets deafening quick. Let me know what you think and feel free to comment if I can explain in further detail.

Thank You!


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8 years ago

I might have left this out in my question. It is a tube head and not a combo so it was never matched exactly like you think a combo to be matched. It is a tube head sitting on top of a 4x12 cabinet that is connected my an aftermarket 1/4" speaker cable. It you can actually add 4 (or more additional) cabinets to just the one head.
I have thought about the L-Pad but I was just wondering if one the old subs I have sitting in the garage would work if I moddified it a little.
Pre-amp saturation is an on going argument with tube head players... there are usually three amp essentials for tone. The tube head ( pre-amp tubes and power tubes), speaker enclosure 1x12-2x12-4x12 and so on, and the speakers themselves ... not just their values and parameters but also how they break up when driven ... example is to listen to the same setup with different speakers for comparison Greenbacks versus say G12t-75. You will hear speaker break up or intentional distortion at a much quicker turn of the dial versus the other speakers at say nearly double the volume. I am not explaining everything here just some of the key points.

Pre-amp tube saturation is often easy to do because you usually have a knob that allows pre-amp volume or gain. The Master Volume or Post is what is driving the power tubes what then pushes the speakers. If I have something that is absorb the increased power I can crank the head a little more and push the Tubes harder ... it wears them out quicker but they sound so much sweeter. I know they make the weber power attenuator, THD hot plate, and Marshall PowerBreak along with others but check the price and you see why I am interested in make something else myself. I know Weber uses and speaker motor and I think they also have some resistive load as well but I was wondering if a working speaker minus the cone would work. As steveastrouk stated it is not a resistive load ... so I would what I can do.

By the way ... I am not certain a 200w L-Pad would hold up. I know a 100w solid state head can power a 100w cab and usually no problems ... but you plug up a 100w tube head and all the speakers will explode within minutes of playing! I am just thinking a 200w L-Pad might do the same ... am I correct???


Any input is valuable input. Thank you!

I posted this just so that I could explain a little more of what I am after.



Answer 8 years ago

I agree totally about preamp vs. power stage distortion. Some people love the preamp distortion sound (Mesa Boogie) but I'm a bigger fan of driving the power and PI (phase inverter) into clipping.

However, there's lots of controversy as to whether using any attenuator sounds as good as cranking the speakers--some of break-up actually occurs in the speakers themselves.

Also, I agree about the 200W L-Pad. The general rule of thumb is the L-Pad should have twice the power rating as the amp--but I repeatedly stated you could wire your own L-Pad using discrete power resistors. Of course, such an L-Pad wouldn't be as easily adjustable, but it could handle any wattage you want...

The 2X rule (guideline, really) -- that's why you always want to have speakers with a combined power rating at least twice the amp's wattage, too.

Measured traditionally, the wattage of a tube amp is not only RMS (lower than peak), but it's measured before the power tubes clip. So a much higher peak wattage is the norm for tube amps. However, I sincerely doubt that most modern amp manufacturers use that metric anymore. Yes, they still use RMS, but they measure it with the amp maxed out. So peaker wattage is higher, but it wouldn't ever exceed 2X, even for a transient signal...

And don't forget: double the # of speakers actually increases the "sound pressure" level, and equates to a 3dB increase (3dB is approx equal to doubling the amp wattage. An amp that's twice as loud (10dB) requires TEN times the wattage.) Its a logarithmic thing.

On top of that, there's the Fletcher-Munson curve thing, which basically states that guitar amps will always objectively sound better when played LOUD.

So, as you say, a different speaker / cab combination will effect the volume (and the overall sound.) You could try a 1X12 cabinet--but like an attenuator, there's no guarantee you will get "your sound."

A speaker motor may not be a pure resistive load, but neither is a speaker!  Inductive loads are still measured in ohms--they just vary with frequency. Plus, speakers do have a DC resistance, as well.

The voice coil on your speaker motor will need to have it's motion physically restricted somewhat (just like a speaker cone would do.) That physical load will effect the inductive load, as an electric motor draws more current when it's working harder...

The conclusion--and you seem to be well informed about the subject--there isn't a "magic bullet" for this problem. The perfect attenuator doesn't exist.

1) Build or buy an attenuator and live with that sound.
2) Don't turn the master volume up to 10, and live with that sound.
3) Get a smaller amp; one that breaks up at a lower volume.


8 years ago

You're actually describing Weber's line of MASS attenuators (they also make resistive attenuators.)

While it's gotta be more complicated than an resistive attenuator, the approach does use an actual inductive load (which is a much better idea musically than a resistive load.) However, I'd expect you'd need some power resistors too, to make it adjustable.

Ideally, the input impedance of your attenuator needs to match the output impedance of your output transformer, or you'll fry the transformer and/or output tubes. You could approach that the same way you would when you're wiring a speaker cabinet. So a single speaker motor, wired together with your 4x12 cab (in series or parallel, depending on the impedance required) probably isn't going to attenuate the signal much. Maybe some power resistors with the inductive load will help.

FYI: your output transformer should be able to handle an impedance mis-match that's about 50 to 100% out of spec (like a 4ohm load when the OT is 8 ohm.) The worse thing for an output transformer is no load at all (open circuit.) A dead short circuit (<1 ohm) is somewhat preferable for the trannie... although the power tubes won't survive, and the OT might not either.

The resistive attenuators are simpler, such as the L-pad design. Some references: one, two.  L-pads are nice, because you can buy ganged rheostat L-pads commercially.

It might be difficult to find a 200 watt L-pad, so you'd probably have to wire it together with some fixed resistors (for the 120W amp.) See the link labeled "two" above--that includes a calculator for your own L-pad designs...

Of course, people's opinions of attenuators vary. Some like 'em, some don't... A 4X12 cab is going to be really LOUD, even with a smaller amp (I have two 30 watt tube amps, and they are too loud for home use...)

Ah, 200W L-Pads aren't that hard to find. Search, and you'll find some more...

I don't know whether the amp would tolerate it, but yes, putting a second speaker in parallel with the existing one (minus code so it's semi-silenced, and preferably mechanically isolated as well so it doesn't couple low frequencies through) would double the load on the amp. Assuming both speakers are 8-ohm units (most common), you'd now be driving itno 4 ohms.

Parallelling the existing speaker with an eight-ohm high-wattage resistor would do the same thing, of course.

Again: Remember that the amp and speaker were selected as a matched pair, and there's no particular reason for the amp to have been designed to tolerate greater load. I don't remember enough about tube electronics to know how it's likely to react. The answer to your question is "yes", but the answer to the question behind it is "I don't know."

I'd just use the resistor. The speaker without its cone is no resistive load.

You want to achieve saturation by cranking the volume right-up, but without actually cranking the output up?
Could you not saturate with a pre-amp or something instead?


Why do you think so many of the greats used 18 watt amps?