Limits on amp power?

[Axtman]

Is there a theoretical or practical limit to the most amount of RMS power wattage that an amp can be built? Ampeg SVT amps are around 300 watts. Why aren't there any 400 or 500 watt tube amps?

Thanks,

[Diablo]

Ya got really strong roadies? Go lift an SVT head, does your back hurt?
The limits of tube amp power are really just a function of the amount of steel and copper in the power transformer and output transformer. Larger transformers are needed to handle the power and make the amp difficult for one person to lift. So the pros use a wall of smaller amps which are easier to handle. Also, as the power goes up, so does the heat - so you have to engineer the amp a little more carefully to handle the heat generated, using cooling fans and the like.

[Enzo]

There are. Peavey made the Classic 400 to compete with the SVT for example.


As to why: the SVT weighs about 85 pounds, and seems to me the PV weighed over 100. Tube amps have to have power and output transformers, and those are solid iron. They weigh a ton. Not only would an owner have to lift it and move it, the chassis itself would have to be strong enough to hold all that.

You not only have to have a transformer large enough to handle the power of the amp, but the tube heaters also require power. In a small amp, that can be all one power transformer. But in a high power amp - one with eight 6550s perhaps - you can easily need 100 watts of heater power. That means for just the heaters, you'd need a transformer as large as the transformers on a 100 watt head. The more power your amp puts out, the more power tubes you are going to need. And the more heater power.

High power amps are likely then to have more than one power transformer.

There is no reason you couldn;t build a 1000 watt tube amp. But you wouldn;t be able to move it.

[defaced]

I'd say for a guitar/bass amp, it's practicality. You've gotta move the thing some time, so that limits weight, and there is an expense factor - retubing a big amp is not cheap.

But outside of that, tubes can produce huge power levels which are orders of magnitude more power than the biggest transistor (from Wikipedia page on Vacuum Tubes):

Air is blown through an array of fins attached to the anode, thus cooling it. Power tubes using this cooling scheme are available up to 150 kW dissipation. Above that level, water or water-vapor cooling are used. The highest-power tube currently available is the Eimac 8974, a forced water-cooled power tetrode capable of dissipating 1.5 megawatts. (By comparison, the largest power transistor can only dissipate about 1 kilowatt.) A pair of 8974s is capable of producing 2 megawatts of audio power. The 8974 is used only in military and commercial radio-frequency installations.

[black_labb]

i've always wanted to get thousands of speaker cabinets and put them ontop of a radio station building and crank the thing (from the basement where its quieter). with all the speaker cabinets you could match the impedance pretty easily by putting them in parrallel or series. give it to a learner guitarist to play sunshine of your love badly for the whole region.

having said that it is a pretty unrealistic desire, and i dont know how well the particular circuits would handle the frequencies involver or what kind of gain there is.

[Chuck H]

There is also the issue of expense. Big watt tube amps are much more expensive than than most musicians can afford. And since musicians are the demographic for instrument amplifiers they need to be able to afford them if the manufacturers are going to sell them. I think it's fairly evident in the designs that maximum power for the lowest weight and expense has always been the primary goal of instrument amplifier manufacturers. There are designs that have challenged the limits of these limitations and susequently failrd to be marketable. It's also worth noting that some of these amps (including the Peavey 400 and SVT) are collrctible to an almost cult like market today.

Chuck

[Enzo]

I know it isn;t a tube amp, but for Black Lab's project, Crown used to make a 10,000 watt amp - MA10,000 maybe? I forget - it was rated for 10,000 watts into 1/2 ohm load. And in fact its intended purpose was for driving speakers in places like stadiums

[Steve Conner]

I heard that the MA10,000 was originally designed to drive gradient coils in MRI body scanners, but Crown rebadged it as a PA amp. Anyway here is the spec sheet:

http://www.crownaudio.com/pdf/legacy/k10684.pdf

There's no theoretical limit to the power of tube amps. Just practical limits: you don't actually need more than a few hundred watts for musical instrument use, and any more just makes it harder to carry, and hurt more when you try to crank it.

John Chambers made amps up to 1kW, as far as I know these particular ones were made to drive a reggae sound system.

Champ 1000 Watt Tube Amp

I've seen industrial amps up to 2kW RMS, but they're the size of a refrigerator and need a forklift to move.

And finally, the old AM radio stations used high-level modulators that were basically giant Class-AB audio amps. At WLW, they made up to 400kW of audio with two output transformers weighing 37,500lbs each. More roadies anyone?

http://www.ominous-valve.com/wlw.html

[R.G.]

I've always been a fan of modularity for power expansion.

Making one very high-powered anything is an exercise in juggling competing tradeoffs while keeping one eye on the practicality of construction and the other eye on the expense, and typically the other eye on several other things. As power goes up, the minor things get elevated to the point where they become major things.

Instead of making a 100W amp, it's less than half the trouble (excepting woodworking in this startup case) to make two 50W amps. It's much less than 1/4 as much work to make four identical 50W amps than one 200W; it's cheaper too, as the price on most of the parts in the 50W amps are jellybean stuff and the 200W takes more highly specialized parts and some careful tuning.

A 1KW amp? No problem, make 20 50W modules. You'll be done sooner and have a greater chance of success. A 20KW amp? A 100KW amp? No problem...

The thing that a modular approach costs you is more packaging. What it gets you is
- cheaper parts
- simpler manufacture
- economy of scale
- fault tolerance (lose a speaker? No sweat.)
- ease of transport
- flexibility - only take as many modules along as you need.

[Steve Conner]

Modularity is great, but when applied to musical instrument amps, it just amounts to the standard wall of Marshalls. Ironically, they're probably all dummies apart from one real one which is mic'd up through the PA. This PA will be modular too though. In a gig big enough to fit a wall of Marshalls on stage, the PA might be 50kW, composed of maybe 50 1kW amps, and the result is considerably louder than the wall of backline would have been if they were all real.

BTW, the original poster asked why there were no 400 or 500W tube amps. Well there is the Marshall VBA400, which as the name suggests, puts out 400W from eight 6550s.

[TD_Madden]

Also the Fender 400 PS....

The Fender 400 PS Amplifier Web Pages, by Rich Koerner

[gldtp99]

Don't forget the Hiwatt 400 watt bass head-- Hiwatt 400 Watt Bass Head. Classic Series Hand-Wired.

[Mark Black]

Gotta chime in with this one:
Mesa Boogie Bass 400+ Amplifier

"Hi - I'd like to order a matched triple-quartet of 6L6's please..."

[soundmasterg]

It would be cool if there was an easy way to use a vacuum tube in space.....you wouldn't need the bottle.....you could just stick them on the outside of the space shuttle and let 'er rip!

We could have a big amp on the moon or something....just build a building and stick some massive tube elecments on the top with no bottles, use a nuclear power source in a building right next to it, and do something useful......send radio propaganda waves at aliens or something.....

greg

[Steve Conner]

How do you know "they" haven't done that already? They could be beaming mind control frequencies onto Earth as we speak

Thanks for pointing out all the other 400W amps. Is the Mesa Bass 400+ really 400W RMS though?

And didn't Rich Koerner claim to get 530W from a 400PS once?