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Topic: AB763 Frequency Response |
Rich Hlaves
From:
Wildomar, California, USA
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Posted 21 Apr 2010 8:55 am |
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The older I get the less I like the sound of piezo electronics plugged into solid state amps. A tube mic channel or preamp is ok but often impractical live.
Have any of you actually measured the frequency response of a Fender black face amp? Not the speaker output but the actual amp output before the speaker? I'm looking for numbers in this type of format:
80-15Khz @ (measured output) +/- 3db
Just wondering what the things are capable of without the roll off of the speaker itself, about 6-7Khz I would think.
I'm thinking of massaging the AB763 circuit and scratch building an all tube acoustic amp. I'm looking for a little more warmth than I am getting with what is on the market today. All the pieces are there in the original design and with a little work on tone stacks and a two or three way cabinet with crossover I think I can get it right.
I'm thinking a 40 watt power section, solid state rectifier, reverb and an effects loop. I might even add a low Z mic/guitar input for vocs or Taylor expression system.
I'm just thinking out loud right now but what do you guys think?
RH
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On man....let the smoke out of another one. |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 21 Apr 2010 9:52 am
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What's interesting as you experiment with piezos and tubes is that it's not about rolling off or removing the highs, but instead has to do with harmonic content. Piezo's naturally create quite a bit of distortion when they "see" a transistor or even an FET. The wildly varying and complex dynamic impedance characteristics of piezos simply can't cleanly be tracked by transistors. If a piezo "sees" a tube directly, it sounds much cleaner, much less crispy, much less distorted, and the midrange needs far less EQ. It's dramatic. So instead of looking at bandwidth and frequency response of an amp, I'd suggest trying a setup where a piezo sees a tube directly, or at the very least sees a tube immediately after it's incorporated preamp stage. Tubes naturally restructure the harmonic content from a piezo signal in a very musical way that allows you to retain the clear, airy high frequencies. It's that the highs become much more pleasing to the ear, and also the midrange becomes much smoother and less needing of "fixing" with EQ.
Most people haven't had the pleasure of hearing piezos connected to tubes, and that's led to the popular belief that piezos naturally have that harsh, gritty, glassy treble with a barky midrange. Those characteristics are actually the result of distortion from transistors no being able to handle the piezo signal well. Tubes actually handle piezos in a less distorted manner. That combined with the subtle, musical tube overtones creates the ideal piezo amplification scenario.
Brad |
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Brad Sarno
From:
St. Louis, MO USA
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Posted 21 Apr 2010 9:57 am
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And more directly to your question about the Fender's circuit. I find they're not ideal for acoustic instrument voicing. They work ok, but not ideal. The Fender EQ voice generally has a big midrange dip, a big bass bump, and a treble rise that begins to drop again above 8kHz or so. Great for electric guitars, but not ideal for acoustics.
You could, however, do some simple mods to a Fender amp to make the tone section more flat and hi-fi. But you then also have the issue of the speaker and lack of tweeter or horn.
Rivera makes the Sedona acoustic tube amp. It's pretty sweet.
Brad |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 21 Apr 2010 10:43 am
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Brad,
Thanks for providing the "why" to what my ears are hearing.
To me, the piezo sounds very quacky run through a solid state circuit. I'm amazed at how many live and even studio recordings of acoustic guitars sound this way. Yuk. One artist I happen to really like that brings this to mind is Dave Matthews. Some of his live stuff leaves me thinking "Who engineered this?".
One thing I want to incorporate in the preamp stage of the amp is a notch filter for feedback control. These, at least to me, are not the greatest sounding circuits but can help in a live application to find the pesky resonant frequency that is causing feedback. I'm thinking a low Z preout is also a must so the signal can be sent to a mixing console.
All this came about and got my wheels spinning in my pea brain because I installed a Martin Thinline in a Taylor 110 for a customer last week. When he came to pick up the guitar we demoed it through my Fishman Loud Box 100. He loved it, I thought it was less than stellar. Icepicky highs that I could not EQ out dominated the tone. I plugged it into my '67 Pro Reverb and viola! Pretty damn nice with the bright switch engaged and a little knob fiddling.
It will be interesting to see where this project takes me. I want to investigate some onboard electronic designs that are already in the field to see what effect whose designs might have as well. My Gibson Chet Atkins SST, quackless, Guild Songbird thinks it's a duck and Taylor 814ce is good most of the time.
Best,
RH
_________________
On man....let the smoke out of another one. |
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Rich Hlaves
From:
Wildomar, California, USA
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Posted 21 Apr 2010 11:50 am
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| Brad Sarno wrote: |
You could, however, do some simple mods to a Fender amp to make the tone section more flat and hi-fi. But you then also have the issue of the speaker and lack of tweeter or horn.
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Yes! Exactly what I'm thinking of with the addition of the goodies I memtioned above.
The Super Reverb circuit & chassis size looks about right for a base, delete the trem, re-work the tone stacks and add the notch filter, FX loop(w/pre & post gain control) and pre-out. The cab would have to be custom size(shorter)with a baffle built for say a single 12" woofer, 6" midrange and a tweeter.
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On man....let the smoke out of another one. |
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Danny Hall
From:
Nevada, USA
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Posted 23 Apr 2010 4:42 pm
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Use a HIWATT D-103 tone stack, with Dynaco transformers from Triode Electronics. Then as mentioned use a speaker that gets more highs. Make a head out of it so you can run electric with one speaker cabinet and acoustic with another. I think Peavey might even make a PA type speaker with a horn tweeter that would be just the ticket. Oh and ditto with ditching the belt pre or the onboard pre. You'll have an impedance mismatch though. I'd have to research that one. A lundal or Jensen mic transformer for one input on the amp and the standard 68K/34K or the other. And by the way, you will have to play with preamp voltages (HIWATT circuit) in order to drive (say) 6V6 or EL-84. It's less of a problem with the 6V6 though; they require more drive than 6BQ5. Both will work fine into about 8K P to P impedance for what you are fixin' to do. Finally, I'd go HiWATT up to the tone stack then Princeton thereafter for cleeeeeeen.
My .02
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The Last of the World's Great Human Beings. Ok, well maybe one of the last. Oh alright then, a perfectly ordinary slacker. |
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Dave Grafe
From:
Hudson River Valley NY
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Posted 23 Apr 2010 6:16 pm
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Have you played with Duncan's "Tone Stack" software? If you want to know why your amp does not sound "flat" check it out:
http://www.duncanamps.com/tsc/
Great question, too, Rich, certainly most great sounding guitar amps don't actually sound all that "great" as MUSIC amplifiers, although often a great amp for playing Pedal Steel through or reinforcing fine acoustic instruments needs very much to have this quality. |
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John Billings
From:
Ohio, USA
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Posted 24 Apr 2010 8:05 am
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| There are devices available that automatically identify feedback, and kill it. That would probably eliminate the need for a notch filter. |
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Danny Hall
From:
Nevada, USA
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Posted 24 Apr 2010 9:36 am
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| Dave Grafe wrote: |
Have you played with Duncan's "Tone Stack" software? If you want to know why your amp does not sound "flat" check it out:
http://www.duncanamps.com/tsc/
Great question, too, Rich, certainly most great sounding guitar amps don't actually sound all that "great" as MUSIC amplifiers, although often a great amp for playing Pedal Steel through or reinforcing fine acoustic instruments needs very much to have this quality. |
I've used Duncan's software since it came out. The Power Supply modeler is great too.
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The Last of the World's Great Human Beings. Ok, well maybe one of the last. Oh alright then, a perfectly ordinary slacker. |
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