Unorthodox hacks: 12AX7 and EL84 car stereo amp

[bluefoxicy]

So let's say you have something like this...

http://www.crutchfield.com/S-eS1L88EXzLF/App/Product/Item/Main.aspx?I=236704X

And let's say you play guitar or sing or whatever and have something like this...

http://www.musiciansfriend.com/product/Epiphone-Valve-Jr-HalfStack

And your friend just showed you something like this...

http://www.tnt-audio.com/sorgenti/jolida-jd100_e.html

And your guitar amp has some lovely recircuiting done to it, all high-quality parts like this...

http://turretboards.com/valve_junior_page.htm

Hmmmmmmmmm.....

I bet I could find an old flat case car amp that nobody wants, from 5-10 years ago, pick it off for $20 maybe. Two channels, I can do two sets of 12AX7 and (EL34|6V6|6L6) easy on a car battery/alternator. If needed, I could do four sets, running the power amps in parallel; probably want this, though a nice high-gain 6L6 might give me enough clean headroom to pull this off (don't want distortion here).

With the right transformers and a little circuit modification, I think I could definitely build a tube amp for my car in as little as $300, plus tubes. Put the tubes opposite the input transformer to avoid heat and noise.

Any thoughts on the whole idea?

[cminor9]

Why not just get a power inverter? That way you can plug your favorite amp into your car. Would cost you a lot less, too.

[bluefoxicy]

What? A power inverter to plug in two tube amps? And how would I wire that?

It'd be more fun mostly to build my own tube-based two channel amp, whether from scratch or somewhat modified kits.

[Steve Conner]

Three more words: "It's been done"
http://www.milbert.com/

The difficult part, for a newb at least, is making the DC-DC converter to get your 400V B+ from the car battery. You'll have to get your hands dirty with all sorts of horrid MOSFETs, inductors, ferrite cores, fast diodes, PWM controller ICs, etc. :eek:

The other difficult part would probably be getting output transformers in the right "form factor" to fit inside a flat case. The ones from the Marshall EL84 20/20 might suit.

[bluefoxicy]

Ah yes. Getting the OT down isn't a problem if you reshape the case a little.

That's a nice looking amp, I wonder how much it runs. $2995 is the only price quote I can find...

... you know I could build something like that within a few hundred.

[Steve Conner]

Don't forget to cost your time getting the DC-DC converter working reliably :(

Hint: Those cheap power inverters usually begin with a DC-DC converter stage that converts from 12V to 300V, before the inverter stage turns the 300V DC to 220V AC. Well, they do here in Europe, Americans only get 160V. You could get a 220V model on Ebay and steal the DC-DC out of it.

[bluefoxicy]

Yeah but you can probably have someone fab one out for you. There's plenty of companies in america that'll make custom prototype hardware cheap, fab chips for you, etc. Hobbyists here do have their own custom microchip designs from time to time.

In the worst case, this is a bit lossy but I can get a silicon commutator (hey, my computer is plugged into one!) and just use an AC transformer, they're small. About the size of my fist. Rectifiers are tiny too.

[cminor9]

What? A power inverter to plug in two tube amps? And how would I wire that?

It'd be more fun mostly to build my own tube-based two channel amp, whether from scratch or somewhat modified kits.

A power inverter just plugs into the cigarette lighter of your car and provides line voltage (110vac). You can plug any amp into that. Just google power inverter and you'll see what I mean. There is no wiring to do, unless of course you count plugging things in as wiring :)

The point I was trying to make is this: Why spend the time and money building a very difficult amp that can only run off 12vdc? Might be interesting from an academic point of view, but you'd have to invest your time and money on what amounts to an experiment.

If you want to build an amp, then I'd recommend a 5E3. I just built my first one, and it was really enjoyable. Mission amps is great according to most folks, but since I wanted a head instead of a combo I built one from a combination of parts I sourced myself and a 5E3 kit from the triode store. It wasn't that difficult after I had done my homework, though if you read some of my other posts here I am not done yet because I have had some problems.

[bluefoxicy]

The point I was trying to make is this: Why spend the time and money building a very difficult amp that can only run off 12vdc? Might be interesting from an academic point of view, but you'd have to invest your time and money on what amounts to an experiment.

If you want to build an amp, then I'd recommend a 5E3. I just built my first one, and it was really enjoyable. Mission amps is great according to most folks, but since I wanted a head instead of a combo I built one from a combination of parts I sourced myself and a 5E3 kit from the triode store.

Why would I want a combo? :p

i'm talking about the big box in the back of a car that you run your stereo to, to drive speakers. Like the 600 watt thing people use to drive their bass speakers so you can hear their lousy rap non-music from 4 blocks away, except for normal volume levels.

[black_labb]

i think what people are struggling with here is what you hope to achieve here that is better than any other car radio available for 40$. i think we all love working on things, but this concept doesnt seem to have a very useful outcome. if you explained the percieved advantage in this concept im sure people would probably be able to help some more.

[bluefoxicy]

i think what people are struggling with here is what you hope to achieve here that is better than any other car radio available for 40$.

I build $400 computers that 5 years later still have better specs than $600 entry level models o_O

The only thing I hope to achieve here is building a hifi amplifier from scratch, and using it for a particular purpose. I've seen people do this for in-home stereo systems, where the units cost like $6000 and they built something out of $200 of components.

"achieving something better than any other" whatever is just a matter of circumstance; will a tube amplifier sound better than a $600 solid state 1000 watt amp? Maybe, maybe not. If it does, have you done something incredibly amazing? Hey, these things used to go for $40 before (Fender tube amps for guitar were cheap like that 40 years ago), but now the same stuff goes for thousands.

The hard part is just figuring out the electronics. I don't have an EE :p

[Steve Conner]

OK, time for some Economics 101.

The $600 entry level computer probably only has $200 worth of parts in it, if you bought it from a major retailer. The rest is markup and the Windows license fee. So it's not surprising that you could make something better with $400 worth of parts.

Again, the 6 grand stereo is the same, but probably with an even bigger markup. Audiophile manufacturers are smaller operations than computer makers, they shift less product, so they need a bigger profit margin to stay in business. With high-end stereo gear, you're paying for image too. A high-end stereo is more like a sculpture or artwork. A Van Gogh is just $50 worth of wood and canvas, and a few bucks worth of oil paint, right?

If you had an EE, your time would be worth more, maybe so much more that you could do 5 days consulting work on some industrial IT nightmare at $600/day and just buy the Milbert amp quicker than you could have made it. :eek:

[bluefoxicy]

Wonderful explanation of margin and volume there.


If you had an EE, your time would be worth more, maybe so much more that you could do 5 days consulting work on some industrial IT nightmare at $600/day and just buy the Milbert amp quicker than you could have made it. :eek:

Yes but which would be more fun and enriching? I've modded up my guitar amp and in the process noticed it's easy to do, and also if anything blows (resistors caps tubes) I can replace it myself for cheap. I learned what the caps do, now I can build in some interesting circuits in my guitar (a bright bypass on volume + resistor on neck pickup to make my les paul act like a telecaster when I yank the volume knob, for example). Or I could have paid an amp tech to apply the mods to my amp right?

[Steve Conner]

Well, that's my point exactly. Do it because it's fun and enriching. Don't do it because you think you might save money.