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Old 4th April 2011, 03:31 AM   #1
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Default Tube guitar amp for acoustic guitar?

Hello,
I am trying to find a good starting point to investigate a DIY guitar amp that is to be used with an acoustic guitar with pickup.
Ideally, it would put out 20-50W (for small clubs) and have a second input and mixing stage for a microphone.
Where should I start looking for circuits? Not new to electronics at all and not quite new anymore to tube amps (recently built one), but still considering myself fairly rookie-ish in this space.
I am nuts to even try myself on a guitar amp?
Thanks for your thoughts,
Stefan
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Old 4th April 2011, 03:41 AM   #2
BudP is offline BudP  United States
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Nope, not nuts at all. I would strongly suggest a basic Fender two channel design, the root mean squared of all guitar amplifiers. Something like a Bassman circuit with the interstage cap throttled back to flatten the bass out (about .1 mfd if I remeber).

Then, to blow my own horn, one of the O-Netics "Blues" output trannys. This will provide a wonderfully woody sound, with a huge presence and the ability to bend into the notes. Even the thin body / piezo guitars sound like classical guitars with this combo and the tone controls are very useful. No reason to go any larger than 50 watts either, and even that will be stage performance level. The ""Blues" comes in four wattage's by the way. PM me with your email if you are interested and I will send you a data packet.

Bud
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Old 5th April 2011, 01:07 AM   #3
jjman is offline jjman  United States
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I think there was a thread here on such a project. Or TGP. I would 1st consider if the pickup is pre-amped or not on board the guitar. If not I would be more concerned about noise and would look to a balanced setup. But you need pretty much need that anyway because of the mic input. More EQ controls than a guitar amp could be cool and an anti-rumble control of some sort is a must with acoustic and doesn't hurt as an option on mics.
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Old 5th April 2011, 01:44 AM   #4
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Quote:
Originally Posted by jjman View Post
I think there was a thread here on such a project. Or TGP. I would 1st consider if the pickup is pre-amped or not on board the guitar. If not I would be more concerned about noise and would look to a balanced setup. But you need pretty much need that anyway because of the mic input. More EQ controls than a guitar amp could be cool and an anti-rumble control of some sort is a must with acoustic and doesn't hurt as an option on mics.
Thank you for responding.

The pickup/guitar does not have a pre-amp (it's a friend's guitar and he told me the output from the pickup goes straight to the plug without any active components).

Frankly admitting to not knowing much if anything about amps for musical instruments: Is there people out there who have done similar projects or example circuits that could be used/extended (including and other than the Fender Bassman schematics I've found already)?

Also there is a valid question of whether buying a used and potentially defunct amp off of eBay and fixing/modding it may be the wiser path for me...

Anyway, thank you for your input, I appreciate it!
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Old 5th April 2011, 01:50 AM   #5
Gilgy is offline Gilgy  United Kingdom
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This may help:

http://www.diyaudio.com/forums/tubes...uitar-amp.html
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Old 5th April 2011, 02:15 AM   #6
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Originally Posted by Gilgy View Post
Sure. The amp for acoustic guitar must be very clean and transparent, unlike electric guitar amp that adds character to the instrument that does not have it.
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Old 5th April 2011, 02:25 AM   #7
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For an acoustic instrument what I would suggest is to first build one of these to place between the instrument and the amp:

Discrete FET Guitar Preamp

I can tell you from experience that it WILL give you the sparkle, edge, & tone, all without distortion. You will be very happy with the results.
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Old 5th April 2011, 02:45 AM   #8
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Since it's for acoustic guitar and wants to be uncoloured you could try a chipamp. An LM3886 will do 35W without a big design effort, you could put one together in a pretty short space of time. As for a second input, a 2-input mixer and even some tone controls only require a few components and you will find circuits either by searching here or using google.

You can get more power from this chip, a maximum of 68W into 4 ohms, but it will require more attention to detail and heatsinking.

Show a diagram of what you intend here before you build it and get some feedback.

w

I normally recommend tubes for guitars, but not for acoustic.

Last edited by wakibaki; 5th April 2011 at 02:47 AM.
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Old 5th April 2011, 04:50 PM   #9
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Quote:
Originally Posted by wakibaki View Post
I normally recommend tubes for guitars, but not for acoustic.
Interesting statement. My amateur musician friend's expectation is that a tube amp would be providing a warmer and richer sound than his current SS amp, similar to what you hear with HiFi tube amps vs. SS.
Maybe that expectation is unjustified and he really is better off overall with the SS amp he is currently using!?

I think it's time for me to reconvene with him and see what he REALLY is looking for. Tubes are cool, sure, but if sound reproduction is the priority (which it always should be imho), then maybe it's the wrong pick for his application.

Thank you again for all your input, it is really helpful.
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Old 5th April 2011, 04:56 PM   #10
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Why not make a hifi amp that doesn't use negative feedback? Then you have the fidelity for the acoustic sound without it sounding either sterile or distorted by the amplifier.
All you'd need is the power amp though it would probably be best to use a transformer-coupled design over an OTL to prevent any chance of electric shocks.

A valve preamp could be good to go with it to, check out the designs here: DIY 12AX7 Tube preamp
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