What makes an Acoustic guitar amp an acoustic guitar amp....

[playonit]

I started building acoustic guitars and thought what the heck... Let's build an amp that's specifically built for acoustic guitars.... Low power, compact, unique voicing?? Will all tube work well as an acoustic amp... curious to hear your thought's...

[xtian]

Whether you're plugging in with a piezo pickup or just using a microphone, acoustiic guitars benefit from a much flatter frequency response than electric guitar amps provide.

I really like running my Laraveé thru a mixer into a pair of Mackie SRM 450 monitors. Add reverb and EQ to taste.

Many purpose-built acoustic combo amps have a horn or tweeter element in addition to a 10 or 12" cone. And some players (especially finger-style soloists) rely on subwoofers to really extend the guitar's voice.

Recently saw Willy Porter. Fully, fully awesome man. Really uses the full range of his instruments.

http://www.youtube.com/watch?v=oo_19YWwEW4#t=07m35s

[SoundPerf]

IMO, you're wanting lots of clean headroom for an acoustic. That being the case it's more practical to go with quality solid state power. If expense, size, weight is not a consideration, I'm sure a very nice sounding all tube "acoustic" amp would be excellent sounding, just not practical. An acoustic tube preamp with a purpose built EQ could have some merit, though.

I also use a set of Mackie SRM 350's (not the newer ver.2) I use a Jumbo Koa cutaway and need to use an EQ to tame the bottom end.

[gingertube]

What I would do - just one mans opinion.

Clean Preamp - fender BF but using a 6SL7,
Clean Power amp - Cathode biased 6V6 for around 10-12 Watts (320V rail) or cathode biased EL34 (350V rail) for approx 35 watts.
Slightly higher feedback levels than normal for a little tighter sound.

For passive crystal pickup - use a JFet buffer ahead of the preamp - don't compromise noise performance by using a 10M grid leak on the first tube input.

Cheers,
Ian

[xtian]

Revisiting this, because I was just commissioned to make an all-tube (well, except for the proposed FET input buffer/amp) amp for acoustic guitar.

Anyone have recent experience with this?

I'm tempted to get a good acoustic guitar preamp, Yamaha or Radial or something, and build the power amp as a fairly flat-eq deal.

Will probably want 1x12 plus a tweeter in the cabinet, which leads to the need for a passive crossover, right? Don't really want to bi-amp.

[Bob-I]

I built am amp for Dave wendler a few years ago. He builds electroCoustic guitars. http://www.electrocoustic.com/gallerydisplay.html the amp is the tweed shown behind the sunburst.

I had great success getting an amplified acoustic tone using a very simple preamp into a 2xEL34 power section with lots of filtering, SS rectifier and a very clean LTP. The preamp is a 12AT7 with a single knob tone control. I spent a lot of time getting the gain structure so that nothing in the preamp overdrives before the output tubes. I don't remember all the component values but it was really straightforward. Just tweak Rp and Rk for a 12at7 and keep the gain low.

Another key was the speaker. We used an emi full range speaker with a whizzer cone. Again I don't remember the details, but its very flat response.

It was a fun project.

[xtian]

Great suggestions, Bob. I see EV SP12B full range speakers on eBay for reasonable prices. Ever try those?

[xtian]

JFET preamp!

http://buildyourownclone.com/fetpreamp.html

[bcmatt]

I had an LR Baggs M1A magnetic pickup in my old Gibson J50, and I was never really happy straight into the PA system (Just recently went to the Anthem (true-mic) system though and not really tried it much yet).

Anyways, what it turned out that I liked was using a big clean tube amp with a quad of KT-88s. (Traynor YGL-3a), into an Ampeg 4x10 bass cab with a horn tweeter. For that magnetic pickup it was a beautiful combination. I could mix in tube reverb and tremolo to taste as well. I should point out that I have that same traynor stock with Mullard 6Ca7s and it did not sound nearly as good with the acoustic as that one with the KT88s.
I think it's just that KT88s are a great HiFi sort of power tube that might work well for this sort of project.
As far as Piezo style acoustic pickups, I can't comment because I don't use them. They always sound like a stinking pile of buzzy quacky crap to my ears.

[Cantplay]

Just because its for acoustic doesn't mean they don't want to play into distortion sometimes.

I would let this customer plug into a regular guitar amp, play it and see what their reactions are at low gain, and as you turn it up into the slight distortion that most of us love.

How much power do they need? If they really want clean I'd be looking at hifi designs, and probably a high effeciency singledriver speaker in open baffle.

John

[cesout]

What I would do - just one mans opinion.

Clean Preamp - fender BF but using a 6SL7,
Clean Power amp - Cathode biased 6V6 for around 10-12 Watts (320V rail) or cathode biased EL34 (350V rail) for approx 35 watts.
Slightly higher feedback levels than normal for a little tighter sound.

For passive crystal pickup - use a JFet buffer ahead of the preamp - don't compromise noise performance by using a 10M grid leak on the first tube input.

Cheers,
Ian

Right you are on the JFET.

[Firestorm]

What I would do - just one mans opinion.

Clean Preamp - fender BF but using a 6SL7,
Clean Power amp - Cathode biased 6V6 for around 10-12 Watts (320V rail) or cathode biased EL34 (350V rail) for approx 35 watts.
Slightly higher feedback levels than normal for a little tighter sound.

For passive crystal pickup - use a JFet buffer ahead of the preamp - don't compromise noise performance by using a 10M grid leak on the first tube input.

Cheers,
Ian

Right you are on the JFET.

JFET works fine, but you can also get very high input impedance from a tube stage (like you need for piezo) by bootstrapping the input -- basically you split your grid-to-ground input resistor and capacitively couple that midpoint to the cathode. Can't use a bypass cap, so the first stage won't have as much gain (and you have to put a seriously large cap on the grid to pass all frequencies, like 3n3 or larger), but it does work. Built that recently for an acoustic bass player who had a piezo pickup.

As previously mentioned, you want a flat tonal response, so you should look at a Baxandall (actually James) tonestack.

[surfsup]

I've been thinking about this too and I would at this point in my knowledge of ampbuilding use a cascoded input stage to get an expansive range, into a mosfet SF then tonestack into a PI with a mosfet driver or 12AT7 current driver on both sides into a clean poweramp.

Would one need more than a single input stage or cascoded stage? I don't know how much Vpp an acoustic outputs currently so another stage could be needed.

Interested in following this thread...

[xtian]

Just sent client a detailed proposal. Keep ur fingers crossed for me, gents.

[echuta13]

An interesting acoustic amp: http://www.humphreyamps.com/