Hi,
I am thinking of building a tube amp for my nylon electric guitar (LR Baggs iMix pickup system with Element undersaddle pickup and iBeam soundboard pickup). The guitar is custom built but resembles Godin Multiac instruments.
I already have an Acus transistor amp that is very natural and clean in sound, so I came across the idea to build myself (or better, ask my father in law who has experience with building HiFi speakers and amps) a tube combo amp. I read some stuff to find out that tubes are generally not recommended for acoustic as they cause some undesired sound coloration, however as this is just another sound option for me and also a DIY project to do, I don't think it would be impossible. Also, there are High-end HiFi tube amps where sound coloration is something very undesired and still working. I therefore think that there should be an option to build an acoustic-ready tube combo.
And now the question. I would very much appreciate any recommendations as to schematics, design, speakers, what to do and what to avoid etc. Is it a good idea to start from the design of high-end amp such as Fender Vibro King etc. or is this the wrong way to go? There are probably no acoustic tube amps to start with. An important note also is that I need to amp to play well on low volume as I need to use it at home without cracking the plaster of the walls ;-).
Thanks for any comments.
I am thinking of building a tube amp for my nylon electric guitar (LR Baggs iMix pickup system with Element undersaddle pickup and iBeam soundboard pickup). The guitar is custom built but resembles Godin Multiac instruments.
I already have an Acus transistor amp that is very natural and clean in sound, so I came across the idea to build myself (or better, ask my father in law who has experience with building HiFi speakers and amps) a tube combo amp. I read some stuff to find out that tubes are generally not recommended for acoustic as they cause some undesired sound coloration, however as this is just another sound option for me and also a DIY project to do, I don't think it would be impossible. Also, there are High-end HiFi tube amps where sound coloration is something very undesired and still working. I therefore think that there should be an option to build an acoustic-ready tube combo.
And now the question. I would very much appreciate any recommendations as to schematics, design, speakers, what to do and what to avoid etc. Is it a good idea to start from the design of high-end amp such as Fender Vibro King etc. or is this the wrong way to go? There are probably no acoustic tube amps to start with. An important note also is that I need to amp to play well on low volume as I need to use it at home without cracking the plaster of the walls ;-).
Thanks for any comments.
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Start Simple!, get a good soldering station or set up, a good DMM like a fluke and build a test lead with a clip and a resistor to bleed capacitors. Start with a fender Champ like a 5F1 or 5F2, easy to build and sound great with anything. There are a lot of good layouts to follow, but from experience, the best advise I can give a first time builder is to plan carefully, verify the correct parts and type of parts ( carbon vs metal resistors etc. ) and keep ALL of the wiring on the front of the board, I seldom run anything hidden, too hard to inspect or troubleshoot. Have Fun
There are two kinds of guitar amps. Ther are GUITAR amps, meant to use with electric guitars. Those amps are part of the instrument, and they contribute susbstantially to the overall sound. Thus the amps are not remotely hifi in nature. Then there are "Acoustic guitar" amps. These are meant to more or less faithfully reproduce the sound of an acoustic guitar. In essence they are small PA systems. They are not meant to color the sound to any extent.
SO I could be wrong, but it looks to me like you are more of the latter - someone wanting their electrified acoustic guitar to sound pretty much like what it is. Plastic strings never make me think a guy wants overdrive. And you mention you already have a "clean and natural" sounding amp.
So I think you need to ask yourself WHY you want a tube amp. I like them myself. They tend to break up in predictable ways. Is that what you want?
I would take my guitar with me and try it through as many amps as possible. You want to build an amp? Then pick one you like the sound of to copy.
Tubes themselves are neutral, none are "good" or "bad" for guitar. SO you can design a hifi or a guitar amp with tubes. Tube amps are designed to be overdrivien when needed, and for overall tone. Hifi amps are designed for clean. So there is no chart like 12AX7 is good for guitar and bad for hifi.
SO I could be wrong, but it looks to me like you are more of the latter - someone wanting their electrified acoustic guitar to sound pretty much like what it is. Plastic strings never make me think a guy wants overdrive. And you mention you already have a "clean and natural" sounding amp.
So I think you need to ask yourself WHY you want a tube amp. I like them myself. They tend to break up in predictable ways. Is that what you want?
I would take my guitar with me and try it through as many amps as possible. You want to build an amp? Then pick one you like the sound of to copy.
Tubes themselves are neutral, none are "good" or "bad" for guitar. SO you can design a hifi or a guitar amp with tubes. Tube amps are designed to be overdrivien when needed, and for overall tone. Hifi amps are designed for clean. So there is no chart like 12AX7 is good for guitar and bad for hifi.
^^^^ agree.
One other option is to buy one of the inexpensive tube pre-amps that are available for guitars / microphones. But my guess is that you might find that as you increase the tube distortion you like the sound less.
Reminds me of when a salesman demonstrated a multi-effect unit to me in a guitar shop. After pressing all sorts of buttons and twisting all the knobs, he eventually came up with a great sound. I said 'Ah that's it - much better'. At which point he had to admit he had hit the bypass switch which was just connecting the guitar straight through to the amp! 🙂
One other option is to buy one of the inexpensive tube pre-amps that are available for guitars / microphones. But my guess is that you might find that as you increase the tube distortion you like the sound less.
Reminds me of when a salesman demonstrated a multi-effect unit to me in a guitar shop. After pressing all sorts of buttons and twisting all the knobs, he eventually came up with a great sound. I said 'Ah that's it - much better'. At which point he had to admit he had hit the bypass switch which was just connecting the guitar straight through to the amp! 🙂
A couple of questions. How loud do you need - what kind of room/venue? Does the amp need to have tone controls, or do you already have those on your guitar (assuming it has a built-in preamp)?
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+1
You could also check the amps that musicians you like are using with their nylon string instruments (lots of 'rig' pages on the web), and get some ideas from that.
But, unless your father-in-law has requested a project to build, why not just buy the amp you prefer (since it sounds like you won't be getting the satisfaction of playing something you built yourself...)?
It won't be any cheaper to build something 'from scratch', especially if you need an amp that is 'giggable' - i.e. robust enough to be transported and still work reliably.
Thank you guys for the suggestions.
As for controls - tone adjustment would be fine (EQ or some tone control). I don't have any on my guitar except for volume and pickup mixing. I usually run the guitar through a multieffect stompbox that has lots of controls but I would like to be able to plug the guitar directly in this tube amp.
As for volume, I don't need to be too loud. In fact, I need to be quite quiet. I just play in small venues that can usually be played just acoustically (though not on this guitar). I particularly need to play at home, so good sound on low volumes is crucial.
And yes, I could buy an amp but as I said, this is not a need for me. Building it, not having it, is the main reason. And playing something I built (or at least participated in building) is the main reason and will bring a lot of satisfaction. It'all about experiments with sound. I don't need a specific sound, I am more interested in having something special to play with.
And concerning the equipment, I have all I need. My father-in-law (who will be doing it mostly) is an experienced electrician with all the necessary equipment and, most importantly, experience. Although he has no experience with guitar amps, he has built several HiFi speaker sets (I have one from him that is on par with 2,5 USD speakers if not better) and some time ago, he also built some HiFi amps and even a record player.
As for controls - tone adjustment would be fine (EQ or some tone control). I don't have any on my guitar except for volume and pickup mixing. I usually run the guitar through a multieffect stompbox that has lots of controls but I would like to be able to plug the guitar directly in this tube amp.
As for volume, I don't need to be too loud. In fact, I need to be quite quiet. I just play in small venues that can usually be played just acoustically (though not on this guitar). I particularly need to play at home, so good sound on low volumes is crucial.
And yes, I could buy an amp but as I said, this is not a need for me. Building it, not having it, is the main reason. And playing something I built (or at least participated in building) is the main reason and will bring a lot of satisfaction. It'all about experiments with sound. I don't need a specific sound, I am more interested in having something special to play with.
And concerning the equipment, I have all I need. My father-in-law (who will be doing it mostly) is an experienced electrician with all the necessary equipment and, most importantly, experience. Although he has no experience with guitar amps, he has built several HiFi speaker sets (I have one from him that is on par with 2,5 USD speakers if not better) and some time ago, he also built some HiFi amps and even a record player.
Nothing wrong with tube for acoustics. It is easier to get good sound from simple tube ckts than transistor. Just dont go for an AC30 or Trainwreck. Build something more HiFi-ish. Baxandal style tone stack might be preferable. Old ampeg b15 uses that type tone stack. Theres a very nice 6V6 based Hifi amp under the tube section. Think its called musicmaster or something like that. As for speaker I would try a fostex 8inch fullrange in a proper cabinet. 10watts should be fine and if tubed it will handle overdrive well so it will appear to be more powerful than similar transistor amp.
Given that explanation, I have to agree with Semper, those old AMpegs have a super sound about them, the B15 would be a fine amp to emulate.
Experience or no, speakers are mostly a wood box with a little bit of electronics inside.
You might look into kit amps. You get all the satisfaction of building something you can use, but someone else has done all the work of collecting ALL the parts and providing some sort of circuit drawings and instructions. Doing it on your own often results in a project stalled while you order some resistor you overlooked. And most kits include a nice looking chassis. I can solder with the best of them, but making nice even rows of control holes and putting lettering on a panel is not my strong suit.
Kits are not cheap, but here is one similar to an old AMpeg:
https://www.trinityamps.com/product/trinity-trip-top-kit/
But that is a substantial amp, you can find kits for small amps like a Champ for a lot less.
You mention playing quietly. Please be aware that watts are not loudness. A 60 watt amp is not twice as loud as a 30 watt amp. Don't delude yourself that a 20 watt amp will be lots quieter than a 50 watt amp. 5 watts from a Champ is plenty loud. Whatever you use, it will be turned way down.
Experience or no, speakers are mostly a wood box with a little bit of electronics inside.
You might look into kit amps. You get all the satisfaction of building something you can use, but someone else has done all the work of collecting ALL the parts and providing some sort of circuit drawings and instructions. Doing it on your own often results in a project stalled while you order some resistor you overlooked. And most kits include a nice looking chassis. I can solder with the best of them, but making nice even rows of control holes and putting lettering on a panel is not my strong suit.
Kits are not cheap, but here is one similar to an old AMpeg:
https://www.trinityamps.com/product/trinity-trip-top-kit/
But that is a substantial amp, you can find kits for small amps like a Champ for a lot less.
You mention playing quietly. Please be aware that watts are not loudness. A 60 watt amp is not twice as loud as a 30 watt amp. Don't delude yourself that a 20 watt amp will be lots quieter than a 50 watt amp. 5 watts from a Champ is plenty loud. Whatever you use, it will be turned way down.
I totally understand that watts are not loudness. Although I may not have much experience with tube amps, I am a musician that has come across quite some music-related electronics.
A production kit is no fun for me. Purchasing everything in a kit and just putting it together could be done by anyone who knows what a soldering iron is. The fun in this is the design and the hunt for information, getting in dead ends and finding a way out. That's why I am interested in building something myself. There is no need to do it fast. Buying a kit is the same as buying a production amp, there's just more manual work but not much more. Thanks anyway.
A production kit is no fun for me. Purchasing everything in a kit and just putting it together could be done by anyone who knows what a soldering iron is. The fun in this is the design and the hunt for information, getting in dead ends and finding a way out. That's why I am interested in building something myself. There is no need to do it fast. Buying a kit is the same as buying a production amp, there's just more manual work but not much more. Thanks anyway.
IMO a portaflex bass amp would be somewhat of an overkill as well as a rather heavy beast to lug around. I would also suggest around 5W being about right for low volume playing with adequate clean head room. Something like Trinity's Tramp comes to mind as being very suitable, with the 6V6 power tube option, and a more hifi- ish speaker.
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Absolutely agree with this - B15 is heavy, and bass-y, and powerful.
If using a hifi amp circuit, be aware that most are looking for a 2v input, so a preamp (or additional gain stage) would probably be needed.
I think there might be impedance matching issues with the pickups and amp to be addressed as well. (??)
For a home (not gigging) amp, the construction could be simpler - exposed tubes, etc might be OK if there aren't concerns about childrens' safety.
Another possibility would be to get a tube guitar amp (head or combo) and rebuild or modify the circuit for cleaner output. That makes it easier to concentrate on the electronics side vs sourcing transformers and doing metalwork and carpentry. Cutting the gain and swapping speakers could get you a long way toward your goal, perhaps.
I didnt mean build a portaflex just that the b15 is a better mould than say an ac15 for acoustics. I was mainly refering to the tonestack and more HiFi-ish ckt of the ampeg. Yeah I know it is meant for bass and the other guitar but u get the pic.
Regarding a kit I think u look at it bass ackword. If u look at hundreds of tube ckts u will notice they are very similar in terms of biasing and choice of components. That is bc there is mainly one right way of using the tubes and then have some variations around that for uniqueness and personality. Mainly most designs are 2/3-rule based or variations of early rca-book ckts. So that means with a kit u get all parts and then u change some values to make the build your own. As you learn from that the next build will be of your own design. So a kit is not a bad idea.
That said I know what u mean. I am hopelessly modifying everything I own be it kitchen stuff to the lawn mower. I cannot leave well enough alone. And I too have to build stuff myself even tho it is more expencive and time consuming than buying factory fresh. Its an illness a lot of us diy folks have I think.
Regarding a kit I think u look at it bass ackword. If u look at hundreds of tube ckts u will notice they are very similar in terms of biasing and choice of components. That is bc there is mainly one right way of using the tubes and then have some variations around that for uniqueness and personality. Mainly most designs are 2/3-rule based or variations of early rca-book ckts. So that means with a kit u get all parts and then u change some values to make the build your own. As you learn from that the next build will be of your own design. So a kit is not a bad idea.
That said I know what u mean. I am hopelessly modifying everything I own be it kitchen stuff to the lawn mower. I cannot leave well enough alone. And I too have to build stuff myself even tho it is more expencive and time consuming than buying factory fresh. Its an illness a lot of us diy folks have I think.
Agree^^^ I think the B15 sounds great as a chassis on my bench into my bench speakers. The portaflex cab is great, but the circuit is no slouch by itself. The whole Ampeg design philosophy was CLEAN.
I have no experience with Ampeg valve amps, but Leonidas Fender had the same philosophy (once he got past early mistakes like the 5E3).
For instance, the power section of a Princeton (a 12AX7 driving a push-pull 6V6 pair) is very clean, complete with global negative feedback, and would probably make a good starting point for a valve nylon-string guitar amp.
Fender preamps are a different story. Specifically, Leonidas' awful "tone stack" has no place in an acoustic guitar amp. Avoid it at all costs.
I concur with the suggestion of using an active Baxandall tone control. It became the standard in Hi-Fi for one reason in particular: it produces a perfectly flat frequency response when the controls are centred. This means the sound of your guitar will come through unaltered, unless you deliberately and intentionally alter it.
By the way, I am a guitarist myself, and have also had a life-long interest in audio electronics, DIY and otherwise. I vastly prefer valve guitar amps for use with my electric guitars. I also play acoustic guitar, both steel-string and nylon-string.
And I still don't understand why anyone would want a valve amp to amplify a nylon-string guitar!
IMO the only reason to use valves in a guitar amp is to make a bad-sounding electric guitar sound better by modifying its sound (adding harmonic distortion in the amp). But the only reason to play a nylon-string guitar is because you like the unmodified sound of the guitar!
-Gnobuddy
Because you have a choice between:
1) a amp which will clip
or
2) enough wattage to not clip, allowing a conventional (high-feedback) design
With a decent (that is, semi-hifi) speaker you're struggling to get in the 95db/w in something luggable. Indeed, most hifi speakers are barely 90db/w
I.E. case 2 above needs in excess of 500W or 1KW to avoid clipping.
Amps for case 1) that sound good when they clip are mostly low GFB amps; which are mostly valve amps.
(The n'est plus ultra would be Lynn Olson's Karna, which is worth reading for a whole lot of background into the why)
Clever wits will note that there is a third way: put a "soft limiter" before your op-amp-on-steroids to ensure it never clips. The challenge there is to try and get that to produce even order harmonics - which might be nicely done with a valve😀
What a bizarre comment! I suppose then you also don't get why anyone would want valve hifi amps. The reasons are exactly the same (and BTW I really love playing nylon string guitar myself).
Anyway, let's try and help the OP converge on a solution, applying the KISS principle:
- Yes, the James/"Bax" stack makes sense, but you could use that in any number of amp designs, in addition to an Ampeg Portaflex bass amp.
- Since the OP says he only wants moderate volume levels, IMHO the 60W B15/B18 amps would be complete overkill here, as well as very unwieldy to lug around. Just think of what the transformers weigh.
- I am still inclined to think a single-ended amplifier of 5W to 10W will be all that's needed. Keep in mind that 5W is approximately half as loud as 50W, and 10W half as loud as 100W. So the power stage from AX84's P1 eXtreme or Trinity's Tramp would be right on the money, with the ability to use a variety of different octal power tubes.
- Gnoboddy is absolutely right that post-Tweed area Fender amps are very clean. But there are the clean amps out there. In this case we should keep in mind that the typical James/Bax tone stack has high insertion losses, which will help cleanup many a guitar preamp. So I would suggest a two-stage 12AX7 preamp with the tone stack sitting between the two, so that neither stage will get overdriven.
- The other big question is what the loudspeaker should be, as a typical guitar speaker driver unit's frequency response falls off at the high end. Maybe a guitar loudspeaker, plus a robust tweeter? Or maybe a pair of coaxial 2-way speakers used in car audio (JVC, Pioneer, Kenwood, etc)?
A word about amp kits.
Kits solve many issues you will encounter when building a amp. Kits also force you to use the parts they want to sell you. Sometimes that is a good thing and sometimes not. A kit is in general all the stuff you need to build a amp of a certain design. If you build a Fender Bassman for example, you will need the same parts whether it comes in a kit or from many different places.
Not to long ago I decided to build a Fender Deluxe Reverb style amp from scratch and make everything myself.
Here is a list of stuff that was easy for me and stuff that gave me considerable grief when I built everything myself.
1. The woodworking was easy. I have all the power tools I need and know how to use them.
2. The Tolex covering was much more difficult than I assumed it would be.
3. The chassis was a true PITB to bend up and drill everything correctly. I have a CNC milling machine and skills to use it. You can buy a blank chassis but you still have to cut the holes for the tubes by some method. That can be done on a cheap drill press but here is the problem with that. You will need two fairly large diameter drill bits with shanks small enough to fit the drill press. There exist punch out tools that will work....not cheep. Also the rectangular hole for the AC plug is not easy to cut by hand. Easy for me because I have a CNC mill.
NOTE: Part of the issues I had was not taking the time to produce precise drawings of the finished product. Fail to plan=Plan to fail...lol
4. The turret board was easy for me because of the CNC mill. Inserting the turrets required building a metal fitting that I had to turn on a machine lathe.
There are several styles of turrets which require one to know what is really needed.
5. The transformers I first used I found I did not like and had to buy a second set....a $200 mistake.
6. I bought $400 worth of speakers before I found one I liked.
7. The electronic parts were no issue for me because I have in stock everything I need to build most any tube amp. I did change types of capacitors to hear for myself the change in sound. No one can tell you what you will like.
8. I DID NOT DESIGN THIS AMP...Fender did...I just more or less copied their design of an AB763. The point I am making here is if you copy a pre existing design, a kit will be not much different than collecting up all the parts yourself.
9. The face plate...there is no easy solution to this issue. Make a software drawing of what is needed and send the file to a company to have it produced. Around $50...Figure out how to do silk screening, good luck with that...lol My future solution will be to invest another $2000 for an engraving system for my CNC mill.
10. Home Depot or Lowes will not have all the screws of the proper size and you will have to know what you need and where to get them.
11. Test equipment...I own most everything I need and I am still in a pretty steep learning curve with my scope.
This was certainly not the first amp I have built. I have built about 20 amps to date. That is not a lot of experience, but enough to give me a good idea of how to build amps.
Building amps is fun, frustrating, rewarding, and pretty expensive. In my case I did not care what the amp cost but they can get pretty expensive if you use top quality parts and do first class work.
The issues you will encounter are not self evident. Even building a kit is not the easiest thing in the world to do. People come to me on a fairly regular basis asking me to finish kits they started.
If you decide to build a amp from scratch, there are some people here on the site that will be kind enough to give you a hand.
Have fun and good luck with the project.
Cheers,
Billy
Kits solve many issues you will encounter when building a amp. Kits also force you to use the parts they want to sell you. Sometimes that is a good thing and sometimes not. A kit is in general all the stuff you need to build a amp of a certain design. If you build a Fender Bassman for example, you will need the same parts whether it comes in a kit or from many different places.
Not to long ago I decided to build a Fender Deluxe Reverb style amp from scratch and make everything myself.
Here is a list of stuff that was easy for me and stuff that gave me considerable grief when I built everything myself.
1. The woodworking was easy. I have all the power tools I need and know how to use them.
2. The Tolex covering was much more difficult than I assumed it would be.
3. The chassis was a true PITB to bend up and drill everything correctly. I have a CNC milling machine and skills to use it. You can buy a blank chassis but you still have to cut the holes for the tubes by some method. That can be done on a cheap drill press but here is the problem with that. You will need two fairly large diameter drill bits with shanks small enough to fit the drill press. There exist punch out tools that will work....not cheep. Also the rectangular hole for the AC plug is not easy to cut by hand. Easy for me because I have a CNC mill.
NOTE: Part of the issues I had was not taking the time to produce precise drawings of the finished product. Fail to plan=Plan to fail...lol
4. The turret board was easy for me because of the CNC mill. Inserting the turrets required building a metal fitting that I had to turn on a machine lathe.
There are several styles of turrets which require one to know what is really needed.
5. The transformers I first used I found I did not like and had to buy a second set....a $200 mistake.
6. I bought $400 worth of speakers before I found one I liked.
7. The electronic parts were no issue for me because I have in stock everything I need to build most any tube amp. I did change types of capacitors to hear for myself the change in sound. No one can tell you what you will like.
8. I DID NOT DESIGN THIS AMP...Fender did...I just more or less copied their design of an AB763. The point I am making here is if you copy a pre existing design, a kit will be not much different than collecting up all the parts yourself.
9. The face plate...there is no easy solution to this issue. Make a software drawing of what is needed and send the file to a company to have it produced. Around $50...Figure out how to do silk screening, good luck with that...lol My future solution will be to invest another $2000 for an engraving system for my CNC mill.
10. Home Depot or Lowes will not have all the screws of the proper size and you will have to know what you need and where to get them.
11. Test equipment...I own most everything I need and I am still in a pretty steep learning curve with my scope.
This was certainly not the first amp I have built. I have built about 20 amps to date. That is not a lot of experience, but enough to give me a good idea of how to build amps.
Building amps is fun, frustrating, rewarding, and pretty expensive. In my case I did not care what the amp cost but they can get pretty expensive if you use top quality parts and do first class work.
The issues you will encounter are not self evident. Even building a kit is not the easiest thing in the world to do. People come to me on a fairly regular basis asking me to finish kits they started.
If you decide to build a amp from scratch, there are some people here on the site that will be kind enough to give you a hand.
Have fun and good luck with the project.
Cheers,
Billy
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Yikes! 😱 😱
I use a solid-state 30-watt Acoustic AG 30 amp with my acoustic guitars. The amp has an 8" speaker in a wedge-shaped enclosure, and those thirty watts are more than enough to provoke uncontrollable acoustic feedback with the guitar at the far end of a reasonable length of guitar cable.
The AG 30 has a coaxial soft-dome tweeter, a rubber roll surround on the woofer, and is generally designed and constructed along the lines of a Hi-Fi (or, more realistically, mid-fi) powered wedge monitor / small P.A. system. It is designed to handle vocals and acoustic guitar, but also works well as a keyboard speaker, monitor, or small P.A. system.
I've never used my AG 30 anywhere near clipping, and it has served as the sole P.A. system for up to seventy people. Not drunk people, admittedly, but they weren't library-quiet, either!
I agree entirely with you on what valves do best. And I love what valve distortion does for my otherwise unpleasant-sounding solid-body electric guitars.
But I prefer Hi-Fi reproduction for the nylon-string ones, which sound beautiful on their own, and are, IMO, only tarnished by the addition of distortion. Andres Segovia must be rolling over in his grave at the mere thought of electronically distorting the delicate tone-colours he worked so hard to coax out of his instrument!
And AquaTarkis was right, I think Hi-Fi is a job that solid-state electronics does far, far better than valves, and that's been the case for many decades now. Probably since the late '70s at least.
But this thread isn't about my preferences, so if the O.P. wants the functional equivalent of a Sopwith Camel rather than a Cessna 350 Corvalis, that is entirely his prerogative. 😀 As you can see, I've tried to help him in his quest, even though I can't see a good reason for it.
But to remain in good conscience I had to at least mention my doubts about the intended "mission statement". Johnylon can always ignore my opinions if he chooses to!
-Gnobuddy
Exactly. My suggestion was to combine a Baxandall active tone control with a power section taken from a small Blackface Fender, such as a Princeton Reverb.
Agreed, and one of the many reasons why I think valves are a poor choice for Hi-Fi (or Mid-Fi) amplification.
My LTSpice simulations of passive James or Baxandall tone control circuits have revealed rather poor characteristics, including extremely asymmetric cut and boost, and lack of a flat-response setting.
My suggestion was to use an active Baxandall, as was done in a number of valve Hi-Fi amps from a certain era. These have the near-ideal characteristics mentioned earlier.
Active Baxandall tone controls are never used in guitar amps designed to be overdriven, and for good reason. You never want to overdrive a circuit which has lots of negative feedback!
But the situation here is different, this is a near Hi-Fi amp for nylon string guitar, and hopefully nobody is planning to destroy the sound of a good nylon-string guitar by overdriving the amp it's plugged into. 😱
The optimal location for the active Baxandall is probably somewhere after the master volume, with some sort of buffer stage in between (it must be fed from a low impedance). Located there, it will never be overdriven until long after the power section of the amp itself is horribly overdriven, which will hopefully never happen in the case of this amp.
The 8" coaxial speaker used in the Acoustic AG 30 has some similarity to those car speakers, in particular, it uses a coaxial post-mounted Mylar tweeter. I suspect this driver may actually be one that was intended for high quality in-wall home audio, but that's only a hunch. No data to back it up.
Two tweeters in close proximity emitting the same high-frequency signal opens up a huge can of worms (nasty notches in the frequency response that vary wildly with the listening position), and this approach is best avoided IMO.
Of course we all know that many classic electric guitar cabs used multiple drivers - but those were guitar speakers with limited high frequency response, not tweeters. And even so, they had nasty notches in the frequency response!
-Gnobuddy
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