Wednesday, December 2, 2009

Watt’s Up With That? Part 2


The reason harp amp power ratings are so murky is that consumers often get caught up in the numbers game. I was reading a popular online harp forum last night and saw familiar discussions about “What amp should I buy?” Several people in the discussions flatly stated they went for the amp with the higher wattage rating, as if it were a no-brainer.

That is just wrong.

First of all… Is an amp rated at 50 watts twice as loud as an amp rated at 25 watts? Emphatically, NO! If these wattage ratings are accurate, the laws of physics say the 50-watt amp will be only somewhat louder than the 25-watt amp. It takes 250 watts to double the perceived volume of a 25-watt amp.

Will a 50-watt amp necessarily be any louder then a 25-watt amp? The answer is “maybe.” If the speaker in the 25-watt amp is more efficient than the speaker in the 50-watt amp, the little amp may smoke the big one.

And WHAT is all this fixation on high watts and high volume? It’s kind of pointless.

Fact: Guitar players and drummers can overwhelm us and drown us out anytime they want to. You hear stories from time to time about harp players being asked to turn down their amps, but these tales are apocryphal. A harp amp playing too loud for the venue is just as obnoxious as a guitar player doing it, and a guitar amp can do it way more easily. We cannot ever win the volume contest.

As the great Gary Smith says, when you play harp in a band the other members need to be “sympathetic to what you are trying to do.” They need to crank it down to a reasonable level and lay out even more during your solos.

The best way to be make sure you can be heard may not be to spend a zillion bucks on a high-powered amp that weighs a ton, but to use a line out from your amp to the board. I gig with an amp that puts out 10 watts on a good day, but it sounds monstrous through the PA. My advice is to find an amp whose tone you love, regardless of how many watts it generates, and do what it takes to gig that amp.

If you are a jam rat who hits the local open mic every week, the temptation to buy a louder amp is very strong. At jams you cannot always mic your amp or line it out to the PA, and there is very little, uh, volume discipline in your typical bar jam. Even with the loudest harp amp – played at the brink of feedback – you will get lost in the hash if that is the way the guitar players want it.

That is why the harp amp makers shuffle their wattage numbers like cards in a high-stakes all-in poker game. We do need a clear and consistent standard to compare the relative power of different amps. We also need to educate the consumers on the facts (and knock down all the fiction) surrounding those vague watt numbers everybody gets so agitated about.

8 comments:

Anonymous said...

Rick: Totally agree with the concept that amp wattage ratings should be standardized somehow.

Regarding big amp vs. small amp mic'ed or lined out, the line out or mic to the PA concept works great when you are the one controlling the PA; however, when you play at a venue with its own sound man, all bets are off. I think a big amp is a must if 1) you play in situations where you are at the mercy of the house sound man, or 2) you play at jams. In those cases, you have to be able to control your own volume level, and you need the horsepower (okay, wattage, speaker area, etc.) to be able to compete volume-wise with the guitarists and drummers (and don't forget bass players) - because, hey, you've worked really hard to develop a sound/technique that you're happy with, so you don't want to be buried in the mix. And I really think that some of the big harp amps that are available give you enough usable volume to compete in 95% of the jam situations out there, assuming you have good cupping technique, have the right mic, and understand amp placement. If you have all that stuff in order, it really is possible to be asked to turn down, even at loud jams. I've been asked to turn down my Meteor at multiple jams - no kidding!! :-)

-Rusty

Rick Davis said...

Hey Rusty, good to hear from you.

But I would dispute your contention that some harp amps are loud enough to be heard in 95% of jam situations. Guitar players can -- and often do -- overwhelm any harp amp at any time. But if a harp player is a jam rat, big volume is the only way to go, no doubt.

But holy crap... I don't want to be at that jam where the guitars and harp are competing for volume. Ouch!

You are making my point: When volume and watts are a buying consideration, it influences harp amp makers to inflate their wattage numbers, expecially in an market where specificity is not demanded. I'm trying to change that.

Rob said...

I can't speak for other harp amp builders, but my BH150 is measured at 25-26 watts. Don't get me wrong, I'm not trying to advertise here but doing the math is not that difficult.

Here's how and I'll use my amp as an example. One must first understand when looking at the tube data and read on what the power tube is capable of. In my case I am using a KT88(6550) tube. Both tubes have very similar data so the ballpark figures are the nealy the same.
Another point I want to make is that these values on the data are for general use. It doesn't mean that all KT88's are the same. Different brands have different construction methods and different materials used. Add this up with how the circuit is set up, voltages and output transformer primary impedance, one can calcute the wattage within a 5-10% accuracy of the tube data.

My BH150 is single ended and uses 410 volts on the plate, 35 volts on the cathode with a 500 ohm resistor.

410 - 35 = 375 volts
35 / 500 = 0.07 amps
.07 x 375 = 26.25 watts.

Easy.

If you want to measure push pull amps, remember to divide the final by two tubes or four tubes.

Rob
Buffalo Amplifiers

Bruce said...

Hi Rob.
I'm just cruising by tonight so I thought I stick my 2 cents in too...

I hope I read your response correctly and I'm not going to make an ass of myself, ha ha...

The BH150's "26.25 watts" you figured is the static DC watts with no signal applied, (you know, called idle power) not the AC output power.
And, yes, modern made KT88s are not equal.
However, a single KT88 (if it is a real one that is rated at 40 watts) running single ended Class A, biased correctly, operating at the correct plate and screen voltages, connected to the correct output impedance, etc., etc., ... will be limited to about 18-20 watts output. That's all.
Now that would be clean, linear output.
If distortion isn't an issue, you can measure more then that but eventually one side of the sine wave (as viewed on an O'scope and delivered to the speaker load) will be clipped off on the bottom as the AC signal driving it gets strong enough to negate the bias voltage on the grid.
And, smeared over as that same sine wave is driving the grid more positive, causing the power tube to go into a form of saturation.
From your description, it sounds like you are running your BH150's KT88 in an "over biased" condition (more relative bias voltage) and I wouldn't be surprised to see it being pushed in Class B with enough drive, that is, if the preceding preamp stage can muster +35vac..., actually it would need to be +70vac peak to peak.
With respect to normal operation of a single cathode biased KT88 with about 400v on the plate (operating in true Class A), it should be idling with around 90ma to 100ma of plate/cathode current and will need something like a 3000 ohm output tranny.
That setup, (with a big enough set of transformers to handle all of that idle current and power dissipation), will get you close to 20 watts of clean Class A output power through the secondary of the OT.

Rob said...

Thanks for the comment Bruce! Sounds like good info and I'll use it.

Just letting you know in light of your explanation that I stand corrected with my calculations and will make the proper adjustment in description so that my customers won't feel like they're cheated in anyway for power.

Nevertheless, what I've created sounds pretty darn good, cuts the squeal and has good volume for gigging.

Bruce Collins said...

I don't doubt at all your amps sound good!
Ronnie S. was bragging it up pretty good for quite a long while and he is no wanker.
To be honest, I have other really good players that like their amps set up in an over biased condition as it adds a certain, biting harmonic over drive tone that flat out Class A amps don't get.
"The Tone" is, after all, what we strive for and, my motto is and always will be "what sounds good, is good" no matter what road(s) leads us there.

Remember my comments are for real RMS output into a known fixed DC resistance (non reactive).
Most tests and meters will read the average output and will add the distortion products into the total for a number that is almost certainly higher then the true RMS output.
Also, most humans can not hear the difference between 20 watts and 30 watts anyhow.
When it comes to audio and volume, nuance, tonal perceptions, etc., we sometimes kid ourselves into believing we are special and a cut above the rest... ha ha.

rob said...

I can't agree enough with human hearing aspect and not being able to tell the difference in between 20 and 30 watts. I certainly cannot. And besides, isn't dynamics the most important part of music anyway?

Zack said...

I know this is old Rick, but I actually have a word on this. Even though one wouldn't need a 100 watt amp I think the reasoning for bigger amps nowadays is to be because of pedals. Ten years ago or more? I don't know, but I can tell you now that a ten inch speaker in a five watt amp would risk being blown if you use lots of effects. A bigger amp can handle it. It's just an idea, though.