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541 Posts
Discussion Starter #1
Over at the VHTusers site the man himself (Steve Fryette) chimed in to answer a question I had been wondering about for a while now. How is it that my 60 watt can keep up to some 100+ watt amps.

Check it out:
For a good margin of safety, peak power can be considered 2 times RMS. That's a very crude generalization, but accepted as typical by the industry.

Speaker peak handling capacity must also be generalized at 2 times RMS because of the many factors used to determine hard figures.

The typically accepted rule of thumb for safety is 2 X Speaker RMS to 1 X amp RMS, or in this case, 120 speaker watts to 60 amp watts.

VHT amplifier and speaker power ratings are conservative (what we publish is equal to or greater than actual based on generally accepted test parameters - see below).

So normally you'll be OK with a 2X12 P50 cab and a D60. However, speaker rating goes down in an open back cab. But as long as you are not slamming the amp, you should be OK.

Using the amplifiers published model power category as a power rating can be misleading. Same goes for speakers.

Many amp companies do not rate their amps based on actual power performance. Instead they just let the consumer assume that 2 tubes = 50W, 4 = 100W, etc. If pressed some would have to admit their power ratings are based on distortion figures as high as 20% or more, which would translate to ratings that are 20 - 40% greater than RMS at onset of clipping. Hey, you want distortion anyway, right?

Some examples:

Published - 60 Watts
Actual - 62 Watts RMS

Published - 120 Watts
Actual - 118 Watts RMS

Published - 50 Watts
Actual - 50 Watts RMS

Published - 90 Watts/Channel
Actual - 105 Watts RMS/Channel

Published - 50 Watts/Channel
Actual - 62 Watts RMS/Channel

Published - 120 Watts
Actual - 72 Watts RMS ----------->:eek:

Dual Rec
Published - 100 Watts
Actual - 72 Watts RMS

Triple Rec
Published - 150 Watts
Actual - 108 Watts RMS

Published - 90 Watts/Channel
Actual - 65 Watts RMS/Channel

Published - 100 Watts
Actual - 78 Watts RMS

P50 - 50 Watts RMS in a sealed enclosure. Closer to 35 open back.
V30 - 75 Watts in a sealed enclosure. Closer to 50 watts open back.
G25M 50 Watts RMS in a sealed enclosure. Closer to 35 open back.

All amplifier power figures above are based on identical test procedures:
AC Output RMS at onset of clipping (assumes amp under test will actually produce a symetrical wave form at clipping. If not, we use whichever half-cycle clips first).
8 ohm/200W non-inductive test load.
Regulated 120V AC source.
Bias set to manufacturers spec and stock tubes.
Signal input directly to power amp stage for accuracy.
All power amp controls set for maximum clean power (typically Presence - 0, Depth -0, SS Rect Mode, Class AB, etc).

1,909 Posts
Yes watt ratings don't tell the whole story. There are many other factors involved. But they do give you a general range of power for the amp.

Some manufacturers rate RMS, some peak, some conservative RMS.
Older Marshalls are an example of conservative RMS. My 84(?) 50 wt JCM800 1-12 will blow the doors off my 96(?) 60 wt JTM60 2-12.

And remember, twice the wattage doesn't equal twice the volume.
Here's some numbers I pulled off the net (assuming they are correct):
Watts Decibels
10..... 10
50..... 16.9897
100.... 20
200.... 23.0103

2,665 Posts
Ratings sometimes come from suits instead of techs...

Output power ratings are usually part of the advertising to get the uneducated to choose one amp over another.

First off, your ear can't tell the difference between a 40 watt amp and a 50. You have to go up in BIG jumps! From a 20 watt Deluxe Reverb to a 50 watt Plexi to a 100 watt amp.

Second, a tech would rate the amp quite differently than a "suit". Also, you need to specify the application. For a hifi amp you want to know the maximum power out before you start to get even a trace of distortion. With a guitar amp you take a fair bit of distortion as normal!

Third, you can't forget about the speakers! Especially with a combo amp instead of just a head. Speakers vary big time in efficiency and tone. I put new Eminence 10" speakers in old 15 watt Gibsons all the time for customers. They sound a LOT louder!

Meanwhile, I've got a bluesman running 50 watts into a 4-10 cab stuffed with early 60's Oxford speakers. They aren't very loud for that power compared to the new Eminence series but they are loud enough and have TONS of tone!

Finally, bright amps sound louder to the human ear. That's just the way our ears work! It's the reason why an AC30 Vox cuts over the drums so easily at what seems to be a modest power rating.

It all comes down to being loud enough while having the tone you personally like!
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