Conversion to acoustic amp

[jbmdisco]

Hi, have been asked to modify a 1978 laney k100r combo to be used for an acoustic guitar. As this has 2 paralelled inputs my idea is to utilise just 1 and create an 'off the board' circuit for the intended purpose.
What is the main difference to an acoustic type amp as regards to 1 designed for electric use? And is this mod relatively easy? I am thinking that it would be to do with the initial gain and that an acoustic would require slightly more or have i left the planet at Hi,

[olddawg]

Hi, have been asked to modify a 1978 laney k100r combo to be used for an acoustic guitar. As this has 2 paralelled inputs my idea is to utilise just 1 and create an 'off the board' circuit for the intended purpose.
What is the main difference to an acoustic type amp as regards to 1 designed for electric use? And is this mod relatively easy? I am thinking that it would be to do with the initial gain and that an acoustic would require slightly more or have i left the planet at Hi,

An "acoustic" amplifier is a relatively new phenomenon. You should in general need to do nothing but set the amp up as clean as possible with no modifications. A lot depends on what type of pickup the guitar has and if it has an onboard preamp. You can purchase a acoustic guitar preamp stomp box that will match a piezo pickup. A popular popular acoustic pickup scheme today is just a special single coil or humbucking pickup that clamps in the sound hole of the acoustic guitar anyway. Most of the dedicated "acoustic guitar" amplifiers sold today are solid state amps and have onboard compression, reverb, and other features. That Laney should have plenty of overhead. With a stomp box acoustic preamp and compressor it should work just fine with no modifications whatsoever if set up correctly.

[Silvertone Jockey]

Good luck with this one. Most acoustic amps are closer to a PA than a guitar amp circuitry wise. More gain is undesirable, you don't want break up you want clean amplification that doesn't distort

[Justin Thomas]

My experience: while not everyone is going for the "modern" acoustic sound, I've had many owners of passive acoustics (soundhole pickups, microphones on the inside, etc.) say they like the sound of their guitar plugged into a classic tube Fender when I do it. Usually I have to roll the bass off severely, but I'm with olddawg - headroom, creative eqing, and not much more. Just remember - it won't sound like "today's modern sound." (Which to me sounds like buttasscrap anyway - that tinny all highs thin cuts through the mix in a really BAD way sound, but I digress. But passive acoustic into old-fangled low gain tube amp? Yeah, just might sound really good.

There is that $4000 all-tube acoustic amp out there... Dave Gilmour has one. But good luck finding a schemo!

Justin

[J M Fahey]

If the guitar has a built in preamp, it will be able to drive the existing electric guitar input without any trouble.

You will need to provide a tweeter, a piezo will do and worst case you can mount it outside in its own mini enclosure.

Most guitar amps, specially British ones are not flat by any means, and usually brutally cut lows for tightness and heavily boost highs for attack and bite, plus having way too much gain, so that amp might not be suitable.

On the other side, all blackface/silverface amps do not cut bass (that's why they can be farty sometimes) and milder tone controls can be set flat (usually Mid on 10, Bass and Treble around 6) , plus they are meant to be used clean, so they are more suitable.
An old Kustom or Acoustic (brand) or Peavey is also suitable.

Best, of course, is some SS Keyboard amp, which has almost everything needed, ... sometimes even including an XLR mic input, all they need is some extra gain because typical keyboards are very loud.

[TomCarlos]

Perfect timing and Juan is absolutely right - Don't forget the speaker!!!

Most guitar amp speakers cannot reproduce the highs that and acoustic guitar can put out. So if you look at the Peavey "Ecoustic" amp, the Fender Acoustasonic, or any other "acoustic guitar amp", you will see that they have either a piezo tweeter or they come with a coaxial speaker. And that is what you will find in the current Peavey KB amp line. The new ones have a Coax speaker.

I recently took an older Peavey KB/A 60 amp, removed the coax speaker (Pic #1 cause it was partly blown), and installed an Eminence Eminence KAPPA 12 (Pic 2, along with a Eminence APT:80, all protected by an Eminence PXB2:3K5 2-Way Crossover 3500 Hz 400W 8 Ohm. Whether a piezo or small compression horn, you drill a 3" hole for installation.

I know, it's an overkill for an acoustic guitar. You can probably get away with a piezo only (no crossover required and much cheaper option). But I also wanted the amp to be able to handle a keyboard without stressing and hey, it was a fun project.

The important take away is the SPEAKER!! If you are modding an amp, you must also plan on a speaker replacement.

Good luck...

Tom

[Chuck H]

I'm going to second Tom here and site the speaker as the main antagonist. That only applies if you need to keep one channel as an electric guitar amp. IMHE any modern speaker used for a typical electric guitar amp will be a compromise for acoustic guitar. A big compromise. AND any speaker that would be suitable for good acoustic guitar reproduction will sound like a$$ for electric guitar.

So there's that

[TomCarlos]

Let me add one thing about the coax speaker that Peavey uses- it has a "foam" ring that joins the paper cone to the frame. This is something you would find in a home or car speaker. I guess it adds "smoothness" to the sound? I'm not sure how to describe it.

The Kappa speaker that I used in my conversion project is found in PA speakers. So it's more along the lines of a "full range" plus the ability to handle highwr power.

[olddawg]

I would just use a cheap piezo horn with a 2.2uf bipolar cap or two polarized back to back. I wouldn't even take the cab apart. I would parallel it off of the speaker with a jack and set it on top. If it's to bright stick a resistor in series on the plus side.

[J M Fahey]

Agree, with a small twist: if a real Piezo (like the Motorolas or their endless cheap clones) it does not need a series cap, because internally it is a cap already.



Around 0.25uF one to be precise.

What you need is a series resistor, from 10 to 47 ohms , 1/2 W , to control its impedance (most amps do not like a capacitor in parallel with the output and that's what a Piezo looks like to it).

[TomCarlos]

And as Juan mentions, the Piezos are a high impedance device, look like a cap, so no additional cap for crossover is needed.

If you look at the older Peavey amps, for a while, they had a 51 ohm resistor in series with the Piezo. So you solder the resistor to the positive terminal. I have seen the dual Piezo horn looking gizmos (like in the KB300) with and without a resistor for each Piezo. Peavey customer support responded to me by saying the resistor helps prevent the amp from oscillating. I have the same Piezos in a few of my house amps, some with, some without the resistors- no problem at all.

Tom

[Chuck H]

Now the question is how to switch the horn in and out eloquently for the different channels.?. And there's still the limitation of an electric guitar amp speaker carrying the bass and mids for the acoustic channel. It was mentioned that the OP would like to keep one channel for electric guitar. Neat idea. Harder to do than imagine though.

[g1]

I thought the cap in series with the piezo was to limit the lower end response? Some of them supposedly go down to 1K, you might not want it reproducing that.
Chuck, I think you would have to live with a manual switch.

[olddawg]

I used a lot of those Motorola piezo tweeters back in the day to make cheap monitors. They always said that you didn't need to use a high pass capacitor. But I found that if you didn't that the damn things wouldn't last. This is purely anecdotal and my experience. But a cap is cheap insurance.