Has anyone tried building a tube acoustic amp?

[5thumbs]

I've been toying around with the idea of building an acoustic tube amp. I've built 6-12 guitar amps, so I'm not an experienced builder but can follow a schematic. Would an audio amp make a good acoustic amp or would it be overkill for an instrument that has a more limited range? I have seen a guy named Humphrey is making an acoustic 15W guitar amp he is selling for around $2500! It looks like it is 2-EL84 (push-pull?) and 2 12AU7 preamp tubes with a 5y3 rectifier. This sounds more like a standard electric guitar amp with a lower gain preamp section.Humphrey Amp - Handcrafted tube amplifiers for acoustic guitars

I would love any ideas or suggestions before I attempt this project.

[mooreamps]

I did once.. It was for a friend of mine at work... Actually, it was just the preamp for an electric cello... I used a cathode follower in the front end, to interface to the piezoelectric pickup, and then a pair of gain stages in parallel for warmth... Yes, it was fixed bias on all of it.. Then, he plugged that into a Princeton...

-g

[Steve Conner]

12 guitar amps sounds like a fair amount of experience.

The thing with acoustic players is, they're usually happy with the tone of their instrument. They don't want it modified, just louder. So an acoustic amp ends up like a PA amp, and solid-state is preferred because it makes lots of clean power cheaply.

IMO, a hi-fi amp might not be such a good starting point. A tube acoustic amp will be underpowered, so it's going to get overdriven, and the hi-fi amps have so much feedback (10x more than a guitar amp) it could react badly.

I'd take one of those unpopular silverface UL Fenders, and put Eminence Beta speakers in it. If I were building from scratch, I'd make something with a regular Fender preamp and a power amp similar to the Leslie speaker one. But that's JM$.02.

[5thumbs]

Hi Steve,
Well, I should state that the amps I have built for myself my sons and some of their friends have been very "follow the recipe". So, I can cook, but I am by no means a chef!

I know most acoustic guys hate the sound of an amplified guitar, but most of that is from those crappy piezo pickups and their "quack". So are you saying that the hi-fi amps use a large amount of negative feedback in the circuit? Is that the feedback you are talking about? I guess maybe I should try to find a schematic for a nice jazz amp design or maybe go with a Music Man type approach? I'm just wanting a new challenge and was thinking this might be it, inspired by the Humphrey that I saw.

[Bruce / Mission Amps]

The pickup issue is one that will need real attention.
Some of those pickups can be weak (compared to full blown electric guitar pickups) so you'll need an extremely clean and flat, first preamp stage that is not too gainy. High gain will be your enemy, higher clean power will be your friend.
But, I'd try building a version of an old Ampeg amp at around 45-65 watts using a pair of 6L6GCs or maybe a pair of 6550s, with standard Hi-Fi Hammond type transformers.
I'd pick an Ampeg model that uses the modified Baxandall type tone controls.
With the controls they have, seems like they can be set pretty flat ... and with some minor tweaking, I bet you could come up with a very good sounding HI-Fi'ish amp for acoustic guitar.
I think you could do the whole thing with a good set of trannys, nice choke, large uF value filter caps, two over the counter power tubes, two 12AX7 preamp tubes, and a solid state rectifier.

[pdf64]

Speakers are another big thing; whether to go the hifi / PA route with tweeters and a flat response / low efficiency, or more regular guitar amp speakers which lack true high end and whose 'peaky' frequency response may cause feedback but will be more efficient. Maybe try some acoustic amps out and see which speaker type seems to work best for you.
The low output impedance of UL type amps may also help to damp speaker resonance, without resorting to a lot of global negative feedback (which as Steve points out, may not cope with clipping very well). Pete.

[Ronsonic]

If I were to do it, I'd look at ultra-fi audio practice. Full range hifi speakers, maybe Fostex or whatever else is out there. Looking for high efficiency, maybe a back loaded horn and no crossover in the guitar's range. That can be done.

For the amp, how about big push pull triodes or triode strapped pentodes. A pair of 300B would be awesome. Expensive but awesome. With efficient speakers you wouldn't need a lot of power. How about a SE 6550 for about 15 watts.

Pre would have to accommodate the high Z and crazy peaks of a piezo with a mic in option. Decent EQ to adapt to the room and maybe, if it couldn't be avoided a notcher to stop feedback. I'd avoid the last thing if possible and just tell the guy to move the amp.