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1950's Vehicle Toasts Electronics - How to Condition Power?

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New Member
I drive a 1955 Chevrolet Bel-Air, which has a 12vDC Generator, yes Generator. I recently installed a piece of audio equipment, which the system quickly destroyed. System, when sampled by a multi-meter is delivering 14.2v (which is spot on according to the "55 Chev shop manual).
When viewed on a scope however, spikes of 30v showed up. Obviously these did not affect any of the original equipment (such as the vacuum tube radio), lights etc. I am sure that this vehicle is thus devoid of any power conditioning apparatus, and this is why it will likely continue to destroy modern electronics installed.
Does anyone have good advice on taming this beast, at a reasonable cost (Power conditioning circuitry or apparatus that can handle the spikes over extended use)??? The Audio Amplifier that I destroyed needs 12v, up to 30amp peak maximum. Thanks.


Active Member
Hi Bel-Air,

Welcome to the forums.
I'm thinking along the lines of a heavy duty 17 or18 volt Zener.
See what others say,
theres plenty of pretty smart people come onto these pages.

John :)

Nigel Goodwin

Super Moderator
Most Helpful Member
Presumably it uses a dynamo, rather than the more modern alternator?. Does it have a mechanical control box? - a horrible collection of relay type devices. These are extremely crude, you can replace them with electronics to make things a lot better.

I would also expect the battery to absorb most spikes, have you checked on the battery? - perhaps rewiring the stereo direct to the battery might help?.

Also dynamo's are fairly low power devices, adding a high current demand device like your stereo is probably going to overload it.


New Member
The best filter has an inductor in series with the zener or capacitor. The transients from the starter can exceed 60 volts, so you need at least a low pass filter. 30 amps is a lot of current, so finding an inductor higher than 10uH or 20uH will be difficult. The transients are relatively low energy, however, so a 15 volt, 1 watt zener should suffice.


Active Member

People who have these classic-type vehicles will very often
want to keep the working parts as original as reasonable,
sometimes at shows points are deducted if modifications are
such that are not 'in keeping' with the perception of the
vehicle. Better quality brushes and suppression would most
likely be acceptable, changing the unit for an alternator
would probably not be. There has been an issue about polarity,
a lot of earlier motor cars had positive ground (chassis),
but later it was thought by many that having a negative body
could actually reduce corrosion of the metalwork, most cars
are now produced with negative ground and have been for years.
The thing is, those who altered their early vehicles to this
standard at one time had points deducted at shows, but now i
think that alteration has become accepted, and now considered
ok as it does not alter any of the working parts of the car.
Yes, i realise the EMFs and the magnetic fields are altered
but they aren't physical parts.

A lot of the spikes could be coming from the field regulator,
which is an electro-magnetic device using contacts opening and
closing to give a 'Mark-to-Space' ratio on the field windings
thereby aiming at the 14 volt output from the dynamo.

If this unit is in its own case, it may be acceptable to use
another case of the same type to house an electronic version
of field regulator, the original could be kept with the cars
box of spares, so that if any purists should want to, it can
be replaced at some future time.

It might be possible to use the regulator from a modern
alternator to do this job, but the circuit would have to be
checked out, from what i recall the ignition lamp is involved
somewhere in this. I think that it is possible to make a direct
replacement with an electronic unit, such that the original
wiring is unchanged.

I suppose it depends on how far 'Belair' wants to preserve
the original appearance.

Changing the radio for a later type is unlikely to detract
from the originality, although many like to see the old car
'tube' radios, but the newer ones do sound much better.

Personally i feel that using a zener is probably the easiest
way, it could be placed anywhere tidily, but i think that 15
volts is a bit low, a weak battery could reach that on charge
with no load, i would suggest at least 17 volts, and make it
a large one, the sort of thing that motorcycles use.

Best of luck with it, John :)


Active Member
I am passing along what I've learned from people who are familiar with the fine points of automotive electrical systems. One can expect to find noise and voltage spikes well above the nominal system voltage in a modern auto. I read that "good design practice" suggests the ability to withstand short term transients as high as 60 volts. No information was offered to define "short term." I put a scope on the cigarette lighter in my '95 Chev and saw incredible spikes. A friend of mine who repairs commercial mobile communications systems said that modern vehicle electrical systems are full of junk - transients, noise, etc - and that well made equipment is designed to operate reliably with this in mind.

I have no way to compare vintage auto electrical systems with modern systems however I would not assume that the noise/transients in your Bel Air were the cause of failure. That's not to say that a voltage regulator failure or battery disconnect couldn't allow system voltage to go high long enough to do damage. Often when this happens light bulbs are failing, batteries get cooked, etc although if intermittent you might not see the other symptoms.
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