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Old 12th April 2009, 05:49 PM   #1
athos56 is offline athos56  United States
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Default Tube amp designe for Mic/Acoustic guitar

So I'm thinking about building a tube amp for a friend, he sings and plays acoustic guitar in small venues, homes/coffee shops/small gig type places. I have built small electric guitar amps (Champ style amps), and kt-88 high-fi amps, hybrid SS/tube amps etc... I'm thinking of something around 10 to 20 watts (6L6, EL34, something else maybe??) Ideally, it would have 2 channels, 1 mic, 1 guitar, master volume, tone control. I'm thinking of making it just a head. So I've never made a mic section and am not exactly sure about the amp section for the acoustic guitar. In pictures of him playing there is a 1/4 wire coming out the back of his guitar, but I don't think it is a plezioelectric pickup.

Does anybody know of a schematic for an amp I can work from? Thanks
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Old 12th April 2009, 10:21 PM   #2
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can't really tell you much without knowing whether that's a piezo or not. what type of pickup or microphone determines what input impedance and gain you need for the first stage of the preamp. if it's a piezo, there can't be a whole lot of input capacitance either.
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Old 13th April 2009, 12:07 PM   #3
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As it's for acoustic guitar and vocals, I would suggest you should be looking at the higher quality of a transistor or IC amplifier, presumably he's not wanting the distortion, noise, and poor frequency response a valve amp gives.

As for the mike in his guitar, most use electret mikes with their own preamp and battery in the guitar body.
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Old 23rd February 2011, 12:29 AM   #4
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I wouldn't recommend IC/transistor. There are dozens of those type amps. Most of them are built and sound like crap. As for true tube acoustic amps, there is only one out there. That's the Rivera Sedona. Great sounding amp, but could be much better if Rivera cared to do that. I own one and frankly haven't been thrilled with their customer service.

People think that it's the tubes that are uncomplimentary to an acoustic guitar. But it's really more the cabinet design and the hi-gain/overdrive of the preamp. Otherwise, why would so many people use tubes for hi-fi's, or mic preamps, or even mics? A high quality old fender will often do a respectable job on an acoustic (as many have over the years).

I really think there's a hole in the market here. If you're trying to build a tube based two-channel, one piece, combo acoustic guitar/vocal amp I am ALL EARS. A few things I would consider:

1) Caution about the "beaminess" of the cabinet. You want wide, smooth dispertion on something like this. You don't often need a lot of tweeter (Fishman over does it), but you need a smooth tweeter with a LOT of horizontal dispertion to fill a coffee shop, etc... This is the primary weakness of the Rivera. It will take your head off directly in line. But move 15 degrees right or left and the top end disappears. The Rivera is essentially a Twin with a tweeter thrown in and some added tone control. I'm not too nuts about the choice of speaker in the Rivera either. It's a neodym and a little "barky". But as mentioned, that is all there is in this arena.

2) A 15" or 2 x 10" will probably work better than a 12". Just my observations over the years in terms of response curve. A simple 30 watt tube amp through a 15" can put out a LOT of low end (witness the Peavey Delta Blues). Most acoustic/vocal guys will pump the bass in their eq to make up for the lack of a true bass instrument.

3) You want flexible tone, but not "pre-defined" tone that gets in the way. Especially in the all important mids. I would much rather prefer a simple tilt-type or even passive rolloff tone, or a high/low shelf in combination with a adjustable notch (with depth/freq.). I would nix the traditional "tone stack" that builds the low/mid/high as overlapping bandpass filters. Tone stacks are peaky. Not what you want on an acoustic. Amplify what is already there, flat, then add a notch for feedback emergencies. Overall, if the guitar sounds balanced, the amp should. It shouldn't color the tone dramatically other than adding that tube "sweetness". Same for the vocal mic channel. Straight line with tube gain is best. If people want to get picky on tone, an effects loop to add an outboard graphic could be nice.

4) When you use a K&K pickup on the acoustic like myself there is an advantage. K&Ks are mid-impedance and designed for a 1Mohm input impedance which precisely matches a 12AX7. Other pickups not as much, so perhaps an impedance switch or extra gain stage is in order for your Baggs/Fishmans. But the K&Ks are inexpensive, fairly neutral, and gaining popularity. Straight line into the tube for the K&Ks.

5) Finally, a tube preamp through a solid state power amp (like the new Vox) is not a tube amp, regardless of what the marketing execs say. I'm sure you know this and not meaning to be insulting. But anyone decent who's played electric guitar for more than a few years knows that so much of the "tube" tone is that power tube compression at higher levels. Having said this, you don't need a lot of watts for something like this. I would say minimum 20 (ok...a loud 20 to maximum 50. A couple wattage selections (pentode/triode switch?) would be a bonus.

Let me know if you're still at it, athos, and hope this helped.
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Old 23rd February 2011, 10:29 AM   #5
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Quote:
Originally Posted by jtharris View Post
Otherwise, why would so many people use tubes for hi-fi's, or mic preamps, or even mics?
You need rethink your premise!

The number of people using tube/valve amps for HiFi is so small as to be completely insignificant - and for those that do it's mainly a fashion statement and to show how rich they are.

Likewise mike preamps, basically it's just an advertising con, charge people lot's more money for something that seems to be fashionable at the moment.

Quote:
) When you use a K&K pickup on the acoustic like myself there is an advantage. K&Ks are mid-impedance and designed for a 1Mohm input impedance which precisely matches a 12AX7.
Not at all - the input impedance is set by the grid resistor, and can be pretty well any value you want, a 12AX7 makes no difference.

Quote:

5) Finally, a tube preamp through a solid state power amp (like the new Vox) is not a tube amp, regardless of what the marketing execs say. I'm sure you know this and not meaning to be insulting. But anyone decent who's played electric guitar for more than a few years knows that so much of the "tube" tone is that power tube compression at higher levels. Having said this, you don't need a lot of watts for something like this. I would say minimum 20 (ok...a loud 20 to maximum 50. A couple wattage selections (pentode/triode switch?) would be a bonus.
I agree, a valve preamp isn't a valve amp, much of the 'heavy metal' sound from valve amps is because they are clipping the entire amp, preamps and power amps, not just a single preamp stage.

Valve amps sound louder than transistor ones because they are distorted, and distorted sound appears louder than clean sound - even transistor amps driven 'slightly' into distortion appear louder, but valve amps can be driven MUCH further in to distortion and still sound acceptable.
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Old 23rd February 2011, 05:07 PM   #6
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Well Nigel, we'll have to agree to disagree on the premise. We could debate endlessly on valve/solid state. But I don't think all those recording engineers seeking out old Telefunkens and U67s are doing it just to prove how rich they are. And I'd wager that if you put 10 random listeners in a room and ran an identical signal through equivalent quality solid state and tube amplifiers, just to the point where the tubes began to squash a little, 8 out of 10 would choose the tube path as sounding better. But that's speculation.

I'm simply trying to let the OP know that I think there's a market for what he's considering if done correctly. There's a local amp builder that I've considered hiring for this exact task, if he'd take it on.

Thanks for the info on the grid resistor. I guess 1MOhm just seems to be a pretty common choice for that value in guitar amps cause it works well with pickup impedance.
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Old 24th February 2011, 09:04 AM   #7
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Originally Posted by jtharris View Post
Well Nigel, we'll have to agree to disagree on the premise. We could debate endlessly on valve/solid state. But I don't think all those recording engineers seeking out old Telefunkens and U67s are doing it just to prove how rich they are. And I'd wager that if you put 10 random listeners in a room and ran an identical signal through equivalent quality solid state and tube amplifiers, just to the point where the tubes began to squash a little, 8 out of 10 would choose the tube path as sounding better. But that's speculation.
Yes, pure speculation - recording engineers seeking out antique gear are doing it solely because there's a small number of people (usually young) who have the strange idea it's supposed to be 'better'.

They even rebuild the old Abbey Road Studio gear, not for quality (which was poor) but because the Beatles used it.

Almost universally the people from back in the valve era aren't conned by any such sillyness

Quote:

I'm simply trying to let the OP know that I think there's a market for what he's considering if done correctly. There's a local amp builder that I've considered hiring for this exact task, if he'd take it on.
It all really depends on how good a value it is - valve amps are EXPENSIVE, and for the small market potential they would be very expensive.

Quote:

Thanks for the info on the grid resistor. I guess 1MOhm just seems to be a pretty common choice for that value in guitar amps cause it works well with pickup impedance.
1M or 2.2M are the common values used, and because it's solely that resistor that sets the input impedance you can easily make it variable (say a 1M pot and a 10K resistor in series - which would allow you to alter the input impedance between 10K and 1M), some commercial amps do this.

I'd quite like to build a valve amp again, but I wouldn't spend much money on it - but if I can 'come across' suitable output and mains transformers I'll probably make one. But I'm certainly not spending the money require to buy the transformers.

Makes you regret all the ones you chucked away over the years
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Old 24th February 2011, 12:32 PM   #8
teemuk is offline teemuk  Finland
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I guess it's a matter of preference. The sole purpose of tube amps today - especially those for musical instruements - seems to be that the add distortion and all other sorts of non-linearities (such as non-flat frequency response). If you want your acoustic instrument to sound distorted and coloured then I guess they're godsend for you. In other cases you're better off with a solid-state design that can be made to sound like that as well, with the exception that you can actually also turn off the effect. Naturally it's certainly possible to build a very linear and high power tube amp with plenty of headroom but I don't quite see the point since achieving that goal is so much easier and cheaper with solid-state technology.

Schematics..? Can't point off any at quick notice but you could do a few minutes of google searching for acoustic amp schematics (from e.g. Fender and Roland) and take a bit of inspiration how they're designed and what features serve the user. They usually incorporate a mic channel too and it's often a design aiming for low noise ratio (typically achieved with well-selected discrete components in the front end). A phantom power is a nice feature too. Naturally researching a bit of how such things are generally designed gets you far too.

Last edited by teemuk; 24th February 2011 at 12:38 PM.
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Old 28th February 2011, 12:35 PM   #9
Garyepe is offline Garyepe  China
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Default Broadcasting Microphone

If you need Electret Condenser microphone,please check LXMIC

They have the Studio MIC capsule. And in the guitar the capsule always use the 14mm diamater

They also have the Big diaghragm capsule.




Quote:
Originally Posted by athos56 View Post
So I'm thinking about building a tube amp for a friend, he sings and plays acoustic guitar in small venues, homes/coffee shops/small gig type places. I have built small electric guitar amps (Champ style amps), and kt-88 high-fi amps, hybrid SS/tube amps etc... I'm thinking of something around 10 to 20 watts (6L6, EL34, something else maybe??) Ideally, it would have 2 channels, 1 mic, 1 guitar, master volume, tone control. I'm thinking of making it just a head. So I've never made a mic section and am not exactly sure about the amp section for the acoustic guitar. In pictures of him playing there is a 1/4 wire coming out the back of his guitar, but I don't think it is a plezioelectric pickup.

Does anybody know of a schematic for an amp I can work from? Thanks
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Old 14th March 2011, 10:19 AM   #10
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Actually, the loudspeakers are more problematic than the amp. Small, easily transportable, wide directivity but directional enough they can be aimed away from the mic for minimum feedback…

In my far off youth I used to lug a Vortexion 30/50 around folk clubs and small jazz venues, and will definitively state that a tube setup like this is not there for its distortion. On the other hand, as well as the microphony from its input EF86s one difference you will notice with solid state is the weight.

The power amp section is relatively conventional, but I'd put a bit more than two mixer inputs. Not only does this give you backup if ever an input goes noisy, or stops; if you're in a very dead environment you've got somewhere to plug in a reverb, or something to make the performance a bit less dry.

Sophisticated equalisation is much easier in solid state, too; you might have the mic that sounds just right for your voice, and a well balanced pickup system, different rooms can sound drastically different, and with the vocal mic running through it what you have is basically a minimalist PA system rather than an instrument amp. At the very least put an inset jack between the mixing preamp stage and the power output stage so you can plug in a graphic or something for problem gigs. Have you considered a hybrid rig?

Lots of ventilation, I'd build it 19 inch (plenty of standard flightcases available), minimum complexity front panel (gain, treble/bass and aux send for each channel, signal present LED, master volume, master treble mid (switched frequency) and bass. Input connectors duplicate front and back, power, insert and speaker connecors behind. Sounds like a nice little project.

Nothing very special in the circuitry if it's an all tube design; more or less a Mullard 5/20 for the power amp stage (depending on the transformers you can lay your hands on), two tube stages per input and transformer for loZ mics, another double triode for output mix and tone controls, rectify a heater winding to make LED power supply for indicators, probably play with tone control values a bit to make it sound right.

Really good potentiometers everywhere is probably going to be one of the most expensive points.

Last edited by chrispenycate; 14th March 2011 at 10:32 AM.
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