Tube amp for acoustic electrics

[Ron Worley]

Anyone have ideas on an existing or suggested design for this application? A friend is a typical bar solo guitar player / singer who wants something better than aa Fender Acoustasonic, etc. Many thanks in advance!

[Phil_S]

While a bit pricey, look at Bose L1 compact system. IIRC, these get great reviews. I'm pretty sure there isn't anything to be gained by using a tube amp. The acoustic needs EQ and the right speaker more than anything.

[xtian]

And yet, there's a market. I played a friend's Humphrey:

http://www.premierguitar.com/articles/H ... Amp_Review

A classy build, to be sure, expensive, sounded...ok. It had a sound. For acoustic with built-in piezo or similar pickup, I prefer PA style (flat) amplification, usually with some minor, but surgical EQ tweaks.

[Ron Worley]

He specifically wants a hand-built tube amp.

[xtian]

[quote="Ron Worley"]He specifically wants a hand-built tube amp.[/quote

Awesome! Don't argue!

I'd build him a fixed bias Fenderey thing with variable mid lift and plenty of gNFB.

[martin manning]

How much power does he need? Is he thinking a combo, or does he use separate speakers? How much weight will he tolerate? What kind of FX?

I think it should be something sparkling clean, so a Fender circuit most likely. For a small to medium room a pair of 6L6's would be plenty, and no worries about running it at low volume settings if necessary. A blonde Bassman with a MV perhaps?

Maybe a clean channel-only low-plate Dumble in 50W? (I like this idea)

I got a relatively inexpensive Mackie 12 channel mixer with FX for my lad, and he does very well with the guitar and an SM58 into the mixer and then into a small combo bass amp.

[labb]

Waiting to see how this discussion ends. I have tried several times to build a tube amp that was good with an acoustic guitar and have never had much luck. Something to do with the harmonics that one gets from an acoustic. Also always ran into the problem of even order harmonics from a SE amp vrs odd order harmonics from a PP. There are a lot of dynamics going on with an acoustic that don't happen with an electric. But then a semi hollow body electric sounds really good. Mind you, there are a lot of things I don't know about amps.

[strelok]

The best advice I can give for making a tube acoustic amp is to look at hifi circuits. My recommendation, without knowing how much power you need would be a pair of KT88's running in ultra-linear mode. You'll have to drop some coin on the output transformer. Do some research and find one that's well liked for hifi applications. Additionally you'll want to draw out some load lines for the plate voltage you will use (probably between 400-500v). The general consensus it seems with hifi is you want the load line to pass through the knee or just above the knee of the plate curves. This will dictate the primary impedance of your OT. You'll also want a decent amount of global NFB.

For the driver/preamp stages, I recommend 12AT7's or 6SN7's. You shouldn't need a huge amount of gain. One or two stages driving a PI would probably suffice. Certainly a cascode driving a cathode follower into a tone stack to the PI would be good. You could also direct couple most of the stages if you're up for learning a bit about that.

As for the tone stack, something like a james/baxandall might not be a bad place to start.

I've never designed an acoustic amp before, so this may all be overkill, take it for what its worth. As I understand it though, acoustic amps are more of a hifi thing and these are all common practices of hifi, so its at least in the direction that you want to go in. Best of luck!

[10thTx]

I made this for my son-in-law who is a great acoustic guitarist. He is very happy with it and has gotten positive comments from his bandmates.

I don't remember which direct box he used with this? It wasn't the H&K Red Box DI but was some other brand. You can see it velcroed on the ceiling of the head cab in the corner in the back of head photo.

This used the Weber WRVBPT power transformer.

There is a mediocre sound clip of this tube acoustic preamp here:
http://www.soundclick.com/player/single ... i&newref=1

With respect, 10thtx

[10thTx]

I originally tried building this with an Edcor trannie to make a "direct box" that was part of the amp. That didn't work.

So, we simply removed that and attached a direct box to the ceiling of the amp head.

Compare the schematic with layout to make sure there are no errors. If there is a discrepancy, then go with the schematic.

As an FYI, I liked the FX loop in this and thought a delay or reverb pedal sounded very nice in that loop.

The photo of the chassis interior still had the "on board" direct input using the Edcor trannie. This was removed and the mute spdt went to a output jack instead which then went to the D.I. box.

With respect, 10thtx

[martin manning]

Some recommendations and advice from legendary acoustic player Tommy Emmanuel: https://www.youtube.com/watch?v=0vkwZmOqGg0

He gives a very generous interview including quite a bit of his fantastic playing, and he does a bit of a clinic too.

[M Fowler]

I built Tony King from Brooks & Dunn band a 50w 6L6GC Rocket preamp tube amp for acoustics.

Since then he asked for more tone stack interaction so we decided to go with changing coupling caps and tone stack pots to Fender Twin Reverb specs and it is where he wants it.

A single channel Twin Reverb either 50w or 100w version. They want clean head room plus ability to dial in those mids and bass. Getting treble is the easy part.

I know that Tony King ran Tony Albany's VVT amps on tour before the band broke up, mostly acoustic guitar.

Mark

[10thTx]

I built Tony King from Brooks & Dunn band a 50w 6L6GC Rocket preamp tube amp for acoustics

.

Just curious, does this mean you built just the preamp section of a 50w Rocket (prior to the LTPI and power amp) ?

And if so, then did you run it straight into the PA system?

With respect, 10thtx

[JazzGuitarGimp]

Some recommendations and advice from legendary acoustic player Tommy Emmanuel: https://www.youtube.com/watch?v=0vkwZmOqGg0

He gives a very generous interview including quite a bit of his fantastic playing, and he does a bit of a clinic too.

Thanks for that, Martin. I discovered Tommy about 8 years ago. He is such an amazing musician and he is such a joy to listen to - at least, once I get past the part about him reminding me my ears have a lot of training and learning to do And he is so damn funny to boot.

[martin manning]

His rig is pretty simple: tuner, AER EQ box, and a 60W AER solid state amp that is basically a two channel PA with an 8" speaker. Line out to the house PA (For those who didn't watch the video). When I last saw him a couple of years ago he was using a rack-mounted amp of some kind that looked like it had been around a while.