Fun circuit with a 12ax7 (ecc83)

[ebijma]

Hello everybody,

Found this amp circuit that uses a automotive ignition coil as a output transformer and a single ecc83 (for stereo use two).

Could be nice for a small mp3 man (ipod) tube amp.

http://www.allaboutcircuits.com/vol_6/chpt_5/19.html

Good for almost 1 watt.

[kathodyne]

Hey, that's really nice!!

Is there a way to get a changing current from a battery???
using cristal or other mehod....without too much losses???
Then a possibility for a portable amp comes close!
Just use a 6.0v 14000maH battery

give 6.0v dc to the ecc83 heaters! they will withstand .3v less heater voltage
But what's the impedance of the ignition coil, important for calculating rp...or can you ust use the 8ohm impedance for calculations????

i'd really like to try this!

[ebijma]

Originally posted by kathodyne
Hey, that's really nice!!

Is there a way to get a changing current from a battery???
using cristal or other mehod....without too much losses???
Then a possibility for a portable amp comes close!
Just use a 6.0v 14000maH battery

give 6.0v dc to the ecc83 heaters! they will withstand .3v less heater voltage
But what's the impedance of the ignition coil, important for calculating rp...or can you ust use the 8ohm impedance for calculations????

i'd really like to try this!

I don't know the answer to your questions, but i to think it would be fun to build this circuit.
On a battery seams nice to, don't know about the current drawn.
The transformer used in the circuit seams to be a bit large to me.
120 va is not needed here.
All ready am looking for small ignition coils, maybe one from a scooter.
They are small.

[planet10]

Quite interseting that... and a site well worth further perusal if the 12AX7 article is any indication. Thanx.

dave

[alexistheo]

don't really see how one could get ~1Watt (or even close) out of a 12AX7, and i don't really like the idea of 170V on the speakers. also, the lack of any bias for the tube makes me wonder how well this will work. interesting, maybe, but i don't think it would sound that good and i think there are far better single-tube amps if that's what you're looking for. just my thoughts... by all means prove me wrong.

[SY]

Probably all true, but this is more of an educational/science fair/do it for the hell of it kind of project than anything to use in a serious hifi system. And in those roles, it's an excruciatingly clever idea.

[PRR]

> what's the impedance of the ignition coil

Of course a coil/transformer does not have an impedance, except in context of a frequency(range) and usually a load impedance.

The Ford coil used ~1950-1970 is about 1:100 turn-ratio (100:1 the way it is used in this plan). If you actually BUY one, ask for "1966 Ford Galaxie 289CID ignition coil". (It will be the same coil for any Ford of that vintage, but the parts-dude has to start from a specific car to find the generic coil number. And I know the '66 Galaxie all too well.) (If you need to be "cool", say it is for a 1967 or 1970 Mustang with 289/302 engine.) (If the parts-dude asks "transistor ignition?", say "No".)

There were two types of coils in this era: 12V Ford coils were really 6V coils, plus an external resistor (often a long skinny wire inside the harness), switched-out for starting. There were also coils with the resistor built in. The latter may not work so well. I think the resistor was OTOO 6 ohms, which is a lot of loss in series with 8 ohms. If the parts-dude cross-references to two universal-replacement ignition coils, you want "External Resistor".

Taking 100:1 turn-ratio loaded with 8 ohms, the hi-Z side is about 80,000 ohms. That's a superb coupling for dual 12AX7.

The lo-Z side saturates at around 1 Amp. It is designed to live with DC: it is not a closed-core. Reflected over to the hi-Z side, 10mA is the limit, and dual 12AX7 will not soak the iron that much.

FWIW: "scooter coils" are generally magneto, and magnetos do not give the well-known 12 volt primary voltage. It has to be a conveniently low voltage for the ignition points to break, but it might be higher than 12V. Combined with smaller combustion zone and lower demand for efficiency, a scooter-coil might be a significantly lower ratio than a car-coil. Also the narrower range of good-working RPM and the magneto action does not demand as much "bandwidth" as a battery-spark system. Sure, try any ignition coil you stumble over, but bigger may be better.

> i don't really like the idea of 170V on the speakers.

Shudder!

> the lack of any bias for the tube makes me wonder how well this will work.

Ah, zero-bias hi-Mu amplifiers are common in radio. You need a low driving impedance, well under 1K, so the 0.1uFd input cap isn't the hi-fi idea. The 12AX7 probably won't die under these conditions.

> i don't think it would sound that good

It is stunningly strange. Finally a use for all those Ford coils in my garage!

But... it might not be that bad. There is a LOT of inductance in those coils. They bite! And while you would not think so, the "high frequency response" has to be fairly good. The spark voltage must rise quickly even against leakage and compression pressure. I've scoped ignitions with a wide-band scope: you MUST use a special-made attenuator, and I had to experiment to avoid roll-off in the attenuator.

OH: If you are wondering how 12V into a 1:100 tranny gives 15,000-40,000 volts spark: it charges at 12V but when the points open the primary kicks-up to 400V. This reflects to 40,000V at the secondary, though once the spark starts it sucks current and the arc voltage may only be 10,000V.

[planet10]

Originally posted by alexistheo
i don't really like the idea of 170V on the speakers

Should be 0 VDC across the speaker's coil, but you wouldn't want to touch a terminal & ground at the same time... ouch

dave