Introduction

[mashaffer]

Just stumbled across this forum. I am starting the planning process for a guitar amp for my grandson and this looks like the right place to get some serious input.

I am not a guitar player but I do play keyboard and sax. I would like to play bass some day when I have more time and money to invest.

Wes likes to play mostly Christian punk (odd thought there) and metal but is looking at learning some other styles as well including acoustic. I will be striving to incorpoate as much versatility as possible since his interests and style are not fully formed as of yet. The only firm points are tube amplification and full spectrum speaker.

mike

[Arthur B.]

A general purpose amplifier could be made by cloning an old PA/bass/HIFI circuit. What sort will depend on how loud you want it to be and what features you want it to have.

[Don Symes]

To echo Arthur B, these circuits are sort of modular.

Pick a power level - if he's going to be playing in a band, 50 to 100W will let him get a good, loud clean tone - and clone the power amp that gets there - give that a set of 4 to 6 buffered inputs (maybe as a small PA mixer).

For preamps, he'll probably want to pick from a Marshall/Fender 5F6-A Bassman, a Fender Blackface Deluxe, some modern OD amp like a Boogie MkII or the legendary Dumble ODS, a Vox/Matchless and, for acoustic, a JFET input preamp. These can each be in a separate chassis, or integrated with the power amp.

Each preamp needs only a few mA of B+ supply, but you need to watch the heater currents as they add up.

Of course, I'm a feature creep and keep adding ideas until it gets impossible. Simple is always better - especially at first.

Hope this helps!

[Enzo]

ANd "full spectrum speaker" scares me. Keyboards and sax when amplified want basically a small PA system in a combo amp. Meaning the idea there is to REPRODUCE the sound faithfully. But guitar amps are not there to REproduce anything, they are a PART of the instrument. That is why guys select a MArshall, or a Fender, or a Peavey, or a whatever - for its characteristic sound. If guitar amps faithfully reproduced the sound, then they all would sound alike and it wouldn't matter which you used.

Guitar amps have little high end response, they tend to roll off at somewhere in the 3000-5000Hz region. You will never see a tweeter on a guitar amp. In fact, if you played your guitar amp through a hifi speaker or a PA speaker it will sound thin and flat. Guitar speakers are designed with their own tone, they are not made to yield flat response across the audio spectrum.

In other words, a great amp for keyboard or sax would make a lousy guitar amp and vice versa.

Or course if that is not what you meant... never mind...

If his tastes are not settled, think of a design that can be fairly clean. Most designs can make distortion and overdrive pretty easily, but some won't also make clean so well. He can always add a distortion pedal to his setup, but there are no clean-up pedals.

One thing we should ask is how technical you are. Well, how?

Have you built amps before? Have you worked on them? Advice for a novice will be different from advice for an experienced builder.

And why are you building it? If you intend to save money by building, you probably won't. Buy him a nice used amp. But if you enjoy doing this stuff and want to learn or just have the pride of completion, hey, we're here.

[mashaffer]

Thanks to all who replied. I too have to be careful of feature creep.

I have put together a few SS circuits over the years and have some understanding of tubes but have not yet built anything (though I am chomping at the bit to do so). I have confidence in my ability to do it with enough learnin' and some time.

The speaker I am considering is the Hawthorne Audio Silver Iris. This is a 15" coaxial which is essentially a domesticated (slightly) Eminence coax using a horn tweeter. The fans have been experimenting with it rather successfully in PA applications. The interesting thing is that when the woofer unit is used alone it's response resembles a 12" guitar speaker rather closely. It has more extended bass but the 3-5K bump is there just like their guitar drivers. The Qts is appropriate for open back use since it was designed with OB as its main application.

What I was thinking is that for normal electric guitar use he could switch out the crossover and tweeter and drive the woofer directly. If he wanted to play bass or acoustic (or even keyboard) through it he could switch in the tweeter and crossover. Oh, just to be clear I would not be using the audiophile HF crossover at all in this application. The crossover would be a standard pro unit to handle the possible abuse and to reduce the "hi-fi"ishness.

Another issue that would have to be addressed in the preamp stage(s) is that the standard mid cut in the tone controls that addresses the uneven response of guitar pickups would have to be switchable for when other sources were used. Feature creep.

I am building it because I want to build something fun and I think he would really appreciate something that grampa mike made for him.

For a start I am considering building the speaker system separately initially for him to play with using his existing practice amp. The SI is quite efficient so I think that he could get some use out of it in that way and we can learn about how the driver does and weather it is going to work out in the long run. If it doesn't I can always build him something else and use the SI for my keyboard (or get another for my stereo).

He is not to the point of playing big venues right now. He pretty much plays for fun and might play some small get togethers.

[Arthur B.]

The speaker's 150 watt rating sets the maximum wattage. You could go for 150 watt output stage, but I think 100 watts should be plenty. As a counterpoint to Enzo, I have plugged a guitar into a HiFi. It doesn't sound thin due to the extended bass response, but it sounds "flat", in the same way as a Jazz Chorus. Not a bad sound, but not one that has a lot of appeal to someone who plays blues or blues based music.

[Enzo]

All in the words I guess. To me those speakers sound thin because of all the high end that would not be there otherwise. There is plenty of bottom, but in terms of overall balance the hifi is skewed towards the high end. I describe that as thin, perhaps a poor choice, but...