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Old 12th May 2006, 06:21 PM   #1
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Question Power amp harmonics ?

Ok, I am trying to understand what I am hearing, and how to replicate it, adjust it, enhance it, play with it etc. When I crank up a large tube guitar amp i.e Marshall, Fender etc, something is happening in the power amp side that I love the sound of but cannot seem to achive by any other method. Is it rectifier sag? Is there some weird electromagnetic interaction between the transformer and the tubes? Does anyone one know what I am talking about? It is a large dark sound, very smooth but not the creamy heavy compressed sound of cranked pre-amp tubes. It just has a body and depth and a rich thick spanky kinda quality. No it is not speaker cone break up, that I understand ! Those lil amps that use pre-amp tubes don't have it, EL84's seem unable to produce it. 5881's seem prone to it as do some 6550's. Is it the large beam in some tubes? Anyway what I want to know is.....How do I get it at low volumes? No I dont want to use a power soak or thd hot plate or even a marshall power brake, cause all of them seem to reduce this strange thing I hear and love so much ! Can't I just build something that would be rich in that sound? What is "that" sound ?
Anyone ?
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Old 12th May 2006, 07:43 PM   #2
Merlinb is offline Merlinb  United Kingdom
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I think you'll need to provide more info- is the amp Class A or AB? What power valves? (They all sound different when overdriven). EL84's will do it, they simple have a different quality of distortion to the octals.
The phenomenon of power valve distortion versus the fuzzy pre-valve distortion is discussed all over the web- even a cursory Google should turn up lots of info. Here's what wikipedia offers for example: http://en.wikipedia.org/wiki/Valve_sound
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Old 12th May 2006, 08:34 PM   #3
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wow you have good hearing, must be the clear Dakota air. Interesting you find 'that sound' from 5881 and 6550 and not EL84. Perhaps because the former has higher plate resistance so forms a more ideal filter together with the output tranny? Or do higher impedance types compress in a manner to make the transition from clean to crunch better sounding? Or is it the higher power that makes the acoustics in the air compress in a manner to make 'that sound'. Perhaps a power soak doesn't do it for you because the sound waves need a certain energy to make that feeling?
Anyways, I'm sure there are tons of theories, but most amps that have 'that sound' simply have them by chance. I dont think they were designed to have 'that sound' but just have a lucky combination to make it. If I knew I'd be making them....
Then there's the fact that some guitarist prefer one sound over the other and perhaps dont hear 'that sound' in the same amps you do, and some again simply dont care, they just play...
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Old 13th May 2006, 01:03 AM   #4
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Well thanks for the input but.......so far I'm still lost (but makin real good time) ! Let me try this. I have not as yet heard "that sound" from many amps. So the idea that it just sorta happens is a possibility. But..... My SovteK mig 60 does it, as do a couple of my Marshalls, my 100watt superlead (a 1972 )I think? But my marshall EL84 20/20 does not, but my Marshall 9005 50/50 only kinda does it, and it really seems not to matter much if it is on full or 1/2 power setting. I got some "darker" EL84 from the tubedepot and set biasing "just so" according to their suggestion and it did make my 20/20 a lil darker but........"that sound" still isnt there at all. I certainly understand the subjective nature of this, but my buddy's fender twin will do it (a blackface) if it is turned up all the way with both channels patched together (I run the SLP with the inputs patched) I had a univox that did it, and a way old relic heathkit from way back that did it, (it was a unit for mono hi-fi and I wish I still had it) and have heard it in some other units too. It's driving me crazy! I have been searching and searching the web, listening to amps all over for years and still do not know WHAT "that sound" is ! That is my major issue these days (and I have more issues than a magazine rack) so I am still seeking the answer to this epic quest what causes "that sound" ? Any other thoughts, ideas, leads, websites, builders, experiences ?
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Old 13th May 2006, 01:03 AM   #5
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I would second the notion that it has to do with the the speakers just compressing a massive amount of air. Maybe your ears distorting too. My Fender Twin does what you describe - at least to my ears. The closest I can get at lower power is my SF Princeton (pp 6v6) which packs a JBL E110. The magnet is about the size of my head. It is still pretty loud, and probably still much louder than what you are looking for.

If you record that sound, is it preserved on the recording? Freddy King lugged Twins and Quads to get his sound, but I know a guy who can nail the CD sound on a Champ. When he plays it in person on a Twin, it is like something you have never heard before.

Sometimes it just doesn't scale down. The ax84 project has been working on that for years. Kinda sorta successful. The closest I came is with a borrowed P1 into the speakers on my Twin. Close, but no cigar, and still pretty loud.

Will a tele into a dimed Twin, you really are not getting much distortion from the circuit per-se. A Bassman is a basic Fender circuit with a transformer designed to give you another octave or so lower. A Marshall was just a Bassman with EL34s. They are all very similar. Designed to my eyes as PAs.

And remember that your guitar is interacting with the amp output too. Without volume, that goes away.
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Old 13th May 2006, 01:24 AM   #6
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Hi Dakotadave,
I'd bet you'll get many opinions on this. Most agree that three things happen-

(a) The Power Supply Sags
(b) The Tubes themselves distort (how depends how they've been wired)
(c) Transformer Saturation
(d) The tubes have a high output impedence, which leads to a high damping factor (in most of these guitar amps anyway)
(e) The speaker cone itself breaking up


If you're talking about the spanky tone, I would suggest that the transformer saturates (or comes close) for the initial attack, and releases its energy gradually, prolonging the note's tone.

Transformer cores saturate more quickly with lower frequency, which would explain some of the fatness lent to the note.


I think in reality all of the factors are what creates the sound. A guitar amp's output is an extremely complicated non-linear system. Just look at all the well researched, unsuccessful attempts by commercial companies to get the sound without using the real deal.

Most don't sound even similar, some sound close, but none do the job as well- which is why we still use the same stuff these days
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Old 13th May 2006, 12:23 PM   #7
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Alrighty then, how do I "play" with transformer saturation levels? Can anyone tell me how to build something that will allow me to adust the above mentioned variables on some sort of test unit ? I will buy a variac or build an adjustable tube rectifier (with some of you good peoples help of course!) How do I twist all three of these at the same time to discover the magic ?

1) Power supply sag -Ok maybe I will just buy a variac
2) Transformer saturation-clueless here <----
3) Power tube distortion -perhaps just overdrive the input ?

Sounds like a great project huh? Oh and although some have resonable sound and all---- NONE of those lil pre-amp tube for power amps have ever made "that sound" in my world!

I am getting more and more inclined to think it is a tranny thing? Combined with the sag in the power supply ? Ill order the variac just as soon as someone can tell me how to adjust tranny saturation levels ? Yes ? No ? Get a life Dave ? Oh and yes it can be recorded, and from my super low tech listening experiments "that sound" can be enhanced or softened with speakers but........it is not produced by the speakers themselves
that much I do know and understand !
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Old 13th May 2006, 12:32 PM   #8
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Get a life Dave!

Got to PA a gig for my daughters band next week (20th May), will be using a Behringer BX1200 (120W transistor bass combo), a Fender KXR-60 60W transistor keyboard combo (fed via a DI box from a guitar), and a Behringer 400W+400W mixer/amp - not a valve in sight, and don't want any!.
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Old 13th May 2006, 12:43 PM   #9
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Get Thee Behind Me Satan ! NO TUBES !

Hey I do pa hire, have a recording studio, and do a ton of installs, churches mostly, and my studio is the only place tubes can share their happy glowing warmth these days !
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Old 13th May 2006, 01:27 PM   #10
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One thing that hasn't been mentioned is the fact that the power section has negative feedback wrapped around it. That MUST affect the overdriven sound as that's when the NFB is failing to do its 'intended' job. When clipping it adds a slew of harmonics instead of reducing them.

The other (obvious) thing is the choice of tube. I'll agree that the 6V6 does what you talk about well, maybe the best of all, assuming I know what it is you're hearing. If you want to get it at low levels you might look for a small beam tube in the 6x6 family. The 6F6 is actually a really nice tube, though not a whole lot less power than a 6V6. With 300V on plate & screen it'll put out about 5W. There are also a bunch of small signal beamers and pentodes, probably way to many to choose from!

A variac is an absolute necessity when experimenting, but it won't do a thing to contribute to so-called sag. For that you need a loose supply; in my experience too much capacitance in a guitar amp kills the tone. 20uF on plate and 10 or 15 on the screen and earlier stages is a good place to start. You can try adding more to taste.

When playing with the variac, don't be surprised if higher voltage gives you a brown sound you're looking for. It's counter intuitive....

As for transformer saturation, I think most people grossly over estimate its influence. The flux level in the core is inversely proportional to frequency. In other words the OT is sized for the output power and the lowest frequency that needs to be amplified. (Low E on a guitar is about 80Hz; 4x the 20Hz that most hi-fi amps are designed for. That's the real reason guitar amps have smaller output transformer, not because the (vintage) manufacturers were cheapskates or designing for distortion.) Anyway, you might get some saturation when playing power cords way down low, but when playing leads higher up you're NOT going to be working the core hard.

Having said that, I do believe that the OT has a lot to do with the overall sound, but not because it's saturating. Unfortunately, that power amp sound you're looking for might require some iron even if you're not trying to drive a speaker.

Good luck. Your quest is an old one that has been attempted by many, many people before you. But that doesn't mean you can't find success. Worst case you'll learn a ton.

-- Dave
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