Hi all, here is the first part of this saga. I'm making a guitar and a matching amplifier, all at the same time.
Naturally this is a 'bit at a time' project and part 1 shows the rough outline of where the project is heading.
The guitar is a 'Pitbull' Bass kit and the amplifier scratch built from junk.
The amplifier is mainly a valve unit powering a single 12" woofer in a folded port cabinet.
I want to make it as light and small as possible consistent with using valves and having a 100 watt or so rating.
But not light and made out of crap like so many amps on the market from you know where!
I will use actual timber in the construction, not cardboard, and glue and screw all together.
Part 1 will be my usual PDF that I will send to Brad.
Any comments on the project are welcome like "why are doing such and such for?".
I am still learning!
Fred.

Making a guitar and amplifier - Part 1

A 100W Bass Amp with valves.

Hmmm. I can already see two massive transformers using up your "as light and small as possible" budget.

I bought an old 1975 Fender Bassman 100 two years ago. Head only, no speakers, and had it mailed here from Melbourne.

I live in the bush, and our mail contracor, a small lady, could not hide her contempt for having to deliver a 22kg parcel to me.

Can't wait to see your design. Are you going for two output valves or four?

If you consider using 12AU7's for drivers or preamps, give me a yell, I have a few new originals (no good to me, all my gear uses 12AX7 in the front end)

Some new original 12AT7's floating around here as well.

Good luck, can't wait to see it.

Hi BBTV, no no no, nil by output valves!!!
Been there, done that, now for something different.
Just a 6BM8 "doing" 100 watt.
The scheme worked a treat in my previous 300 watt bass amp shown on this site ( go back in special projects) but that unit was a two roadie monster relegated to the studio!
Wait till Brad gets the PDF up tomorrow and check the test circuit out.
Simples!
Fred.

Sounds like the laws of physics are being violated Fred!

Not to mention the 6BM8!

Or, is that just the driver.......

A Hybrid!

You traitor!

Good luck anyway.

Yes Ian and BBTV I have passed over to the DARK SIDE..............the 6BM8 is the voltage driver, those cold cathode diode things just current drive the impedance down to 4 ohms......the voices in my head made me do it!
At least I have transformers for the power supplies I could have used switch mode supplies with more of those cold cathode things to make it really light weight and recycled cardboard for the cabinet but even I cant go that far!
Fred.

Real transformers for the power supplies, not a switching thingy?

You are redeemed.

Well if you are going that way, you want high power and light weight, TI have a neat chip that will give you 350 watts mono or 2 x 175 watts stereo. I've evaluated it for a design I'm working on right now and it's pretty amazing. And I've seen some amazing s**t.

It's around 99% efficient, has internal soft clipping - ideal for a musical instrument amp because you can overdrive it all day and it just sustains for ever. You can get a cheap evaluation module - just add your own power supply. They are quite cheap because TI are trying to sell chips.

The amount of power you get is determined by the speaker impedance and what the power supply can give you.

The whole amp C/W heatsink will fit in your pocket.

http://www.ti.com/sitesearch/docs/universalsearch.tsp?searchTerm=tpa3251#linkId=1&src=top

Quite honestly I'd never design with discrete mosfet or bipolar power devices ever again.

99% efficiency!

Class D is here to stay!

It's not shabby this thing. I've had the EVM running 300 watts RMS 1kHz sinewave into a 2.5 ohm dummy load all day and the heatsink just gets warm.

The 25 amp bridge in the power supply gets hotter!

It looks like it has a good claim to being HiFi too. There is negative feedback around it, something unusual for a Class D amp. Amazing that they can get that to work!

Datasheet specs are impressive and the amp sounds like it would meet them too.

I had a good read of the data sheets when you posted the link.

Definitely alien technology, without a doubt.

If it can drive a 2.5 Ohm load without getting hot a 4 Ohm 12" woofer should not bother it----having said that, we now have impedance.

Amazing stuff, what is their retail cost? I didn't want to click the "Buy now" tag.

Holy smoke Batman!
Then I could build the whole thing into the guitar!

Thinks............maybe the speaker would have to be in a back pack...um front pack.......
Yes, back pack carries the power supplys, front pack has speaker and amplifier in guitar body.
Next project?
NO.
I think I will stay in 1980 for the moment and not listen to the voices........................yet....
Bugger now the idea is implanted....curses.......
Fred

The trick with this device is the fact that it has a really low source impedance. It's an almost perfect voltage source.

Impedance matching with amplifiers is necessary when the amplifier has a finite source impedance. Max power transfer occurs when the source impedance matches the load. But that automatically means that half the power is dissipated in the amplifier.

Technology moves on.

I don't remember how much it costs but if you click on the Buy button you get redirected to a distributor. You don't have to buy it but you will find out the cost. The EVM comes with other useful stuff like PCB layouts, gerber files etc.It can work as 1, 2 or 4 amplifiers, you just have to set some jumpers.

Sorry about that Fred.....

Fred,

Why not fit the device into the guitar?

Many years ago I played a cheap Tele copy in a "garage band". I built a single stage preamp using a BC108 on a tiny veroboard and along with a 9V battery fitted the assembly in a cavity I chiselled out behind the pick-ups.

A toggle switch stuck out about level with my guts, and all I had to do to activate my "overdrive" was to rub the guitar across my guts and bingo! Insane screeching feedback. The entire assembly was about the size of a matchbox.

Your bass guitar will certainly provide more space than my Tele.


Ian,

A thought wave regarding Class D.

You will remember loudspeakers with transformers spot welded onto them. What about speakers fitted with LC networks for Class D operation?

Are the inductor/capacitor values generic? I notice the LC network on the TI sheets specifies 470mF.

Is this value fixed? Standard? Your thoughts please---(keep in mind Fred is driving the amp with bass notes)

How exciting is this? Imagine if Faraday, Tesla, Marconi had the internet!

Imagine if Faraday, Tesla, Marconi had the internet!

I'm picturing the Twitter feuds.