Tube amp for acoustic guitar

[cfreeborn]

I may be getting a little ahead of myself here, but here goes anyway...
I'm gearing up to build a tube amp for acoustic guitar amplification. So far the recommendation(s) I have is to use a Marshall Plexi platform (from K. O'Conners TUT5 book) and tweak the preamps to better suit the pickups in the acoustic. My pickup system is 2 channel, which I prefer to process individually. One is an undersaddle or soundboard piezo transducer and the other is a soundhole magnetic. I lean towards a Johnny Smith type jazz pickup which I fabricate the soundhole mount for, rather than the usual "body sensing" soundhole pu's that are commonly sold in the acoustic guitar realm.
Current thought is to have a 2/10"s cabinet with 2 horns for high freq's. It's been suggested that I look into a PA type tone stack rather than conventional electric guitar design.
Would appreciate any thoughts or comments.
Many thanks in advance,
-Charles
freebornguitars.com

[jjasilli]

Need more info: 

*  Output impedance of piezo PU & of Bartolini humbucker  -- Impedance matching of PU impedance > amp's input impedance is needed.  There are simple and complex ways to do this.  And, the piezo - humbucker signals need to be balanced to ea other for vol & maybe tone too. 

*  The piezo may benefit from its own preamp buffer, very close to the piezo

*  given 2 PU's, a phase reversal SW may be needed; and a variable, or switchable, notch filter to squelch feed back

Arguably, all this calls for an outboard preamp, or "pre-Preamp". 

[cfreeborn]

Ah yes, key piece of info missing. Apologies. I use a pre-amp buffer either inside the instrument or first thing in the chain. I like the K&K unit, which has volume, low,mid,hi and gain controls for each channel. Stereo or mono (blended) out. Quiet, compact and reasonably priced.

Specs:
Technical Information:
Frequency range 10 - 30,000 Hz
Input Impedance: 1 Mega Ohm
Output Impedance: 3 K Ohm
Average Current: 3.5 mA
Battery hours: about 60
Dimensions: 110 x 85 x 38 mm / 4+3/8" x 3+3/8 x 1+ 1/2 "
Input and Output Jacks: 1/4" instrument jacks
Bass, mid, treble EQ, gain and volume controls per channel

http://kksound.com/dualchannel.html

A notch filter could be useful for isolating the "quack" that piezo elements sometimes produce.

Many thanks,
-C

[jjasilli]

OK the "stereo" K&K seems to a preamp/mixer which will take the piezo PU in one channel, the mag PU n the other; and mix them with a single 3K output impedance.  This is a friendly output impedance for the input of any vacuum tube guitar amp.  Focusing on the guitar amp, for acoustic guitar amplification, IMHO, the Power Amp section is largely irrelevant; i.e., any configuration will do. The preamp will shape the tone.  Marshal's aren't known for good clean tone, but this is a subjective thing.  In my experience, acoustic guitars sound great through Fender amps with their FMV tonestack.  That tonestack can be modified with a RAW control to lessen the mid frequency cut.  Or, instead, a Baxandall or James tonestack can be used for a default flat EQ.  Maybe the simplest thing is to play your guitar > K&K preamp > various guitar amps to see what you like.

Note that when 2 or more sound sources (here, PU's) are used, issues might develop:  1.  The PU's may pickup the same signal from the guitar strings at slightly different times - a phasing issue -- causing signal cancellation when blended.  2.  The PU's frequency response may overlap over part of the guitar's frequency range, thereby "boosting" those frequencies especially for certain resonant frequency peaks, unique to your particular setup (and possibly the ambiance of the particular venue).  This can skew your EQ; and cause feedback.  Notch filters can target the offending resonance frequencies to eliminate this cause of feedback.  Ironically, phase cancellation may also help with this specific issue, but might also thin-out all the tone of the guitar's signal.  It is not clear that the K&K product deals with these specific issues.  There are no hard and fast solutions here.  The point is to build-in the solutions for issues that may foresee-ably arise in the field. 

[cfreeborn]

Excellent. Many thanks for your thorough reply.I am planning on having the pre-amp(s) separate - maybe even on their own boards so I can swap or modify them until I get it all worked out. Sort of a Frankenamp...
I'll go looking for the suggested pre-amp designs and go from there.
-C

[tubenit]

The Dumblish amps use an FET input for acoustic guitars (as I understand it).  Larry Carlton is an example of this, I believe?

My favorite acoustic amp is my Tweed BluezMeister clean channel with a digital delay in the FX loop.

With respect, Tubenit

[jjasilli]

Setup preferences are everything.  For mukltui-PU's, blending is needed.  For live performance, many acoustic guitar players prefer to have a preamp handy: on the guitar, the guitar strap; their belt, or  the mic stand. If so, then they can use a preamp with a vacuum tube friendly output.  This eliminates the need for a special, or switchable, input for the vacuum tube guitar amp to accommodate high (piezo) or low (mic) source impedances.  It also allows for the handy mixing/blending of multi-PU's, with EQ and Notch filtering to balance the set-up to the venue.

I've been mentally wrestling with this for my regular acoustic gig at a local bar.  I use 2 acoustic guitars, regular & dobro.  I decided to go old school:  one Shure 58 mic directly into the house PA. 

[cfreeborn]

For strictly acoustic, a mic is, in my opinion the best route, especially if you've got the coin for a good condensor.

This project is to devise a system that my customers who are primarily electric players can use for the occasional ballad or "slow number" mixed in with their electric set. In a perfect world this amp could have 2 channels, one well suited to the acoustic input, which when downline from the onboard  (such as the K&K) or pedal board pre-amp will receive a blended signal of the transducer and magnetic. This leaves the 2nd channel available for a more straight forward magnetic pickup input from the electric guitar source. When there is no electric guitar present both channels can be used for the acoustic signal as previously described. This way they won't need to schlep 2 amps. I completely realize that multiple functions means compromise, but from my limited perspective it seems this is do-able.

Many thanks again for all your input.

-C

[balcorn80]

I recently built an amp for a customer who uses a twin mic/pu setup on his acoustics with different effects chaisn for each pickup. He wanted a 2 channel amp that was basically 2 amps in one so he could grab one and go. He brought me an old Knight integrated stereo amp that i gutted and built 2 identical cathode-biased BF Princeton-type circuits in, and removed all the preamp bypass caps. I don't know that I would call in a true acoustic amp per se, but he seems to enjoy it quite a bit, and still sounds great with electric plugged in. My thoughts were to take a good-sounding amp and make it as clean/distortion-free as possible.

I also built for another customer a small single-ended amp based on a tweed champ running a 6l6 that he plays acoustic through. Again, I removed all the bypass caps to keep it clean and gave it a nice stuff power supply so the bass didn't get too farty. And again, I wouldn't neccesarilly call it an acoustic amp. But it sounds good, and with his onboard preamp he's able to tailor it reasonably well. If your customer is planning on playing electric through the same amp, you could put all the bypass caps on a switch and have an 'Acoustic Mode'.

Both of these amps were more standard guitar amp circuits that I worked over to be as clean as possible, vs a true engineered specifically for acoustic guitar. But they do work well with the acoustics.

[jjasilli]

For strictly acoustic, a mic is, in my opinion the best route, especially if you've got the coin for a good condensor.

But what is the application? Condensor mics sound better, but have drawbacks for live operformance.  Not only are they expensive but also delicate; require power and are omni-directional.  OK in the studio, but can present challenges in a noisy venue -- where their superior sound quality may be lost anyway.  So I've opted for one cardioid (unidirectional) dynamic mic.

For an interesting take on this surprisingly complex topic, checkout this:  http://www.john-renbourn.com/ > My guitars

[cfreeborn]

Yes, all true about condensors. There are a couple of small mixer-di's out there (the D-Tar Solstice comes to mind) that provide phantom power, but yes again it's a bigger commitment that's probably lost on most live audiences.

I had thought of putting the caps on on/off switches, or a selector switch to switch between different sets of caps. My fear is this will steamroll into an overly complicated and expensive amp.

I heard (or read) an interview with Gillian Welch where she described traveling with a selection of SM 57's for micing instruments. The variations between even the same model gave them enough variation to dial in their sound from venue to venue during sound check. They're cheap enough to have a half dozen or so for each player.

-C

[Willabe]

Man I LOVE Guillian Welch's music! Her boyfriend/husband? is a great player.


                      Brad