Here are the details you are missing.
First you must check all your connections to make sure they are good.
You have a 12V alternator so that will mask problems in bad connections. If you opt to go to a higher output bulb then you may need to be concerned.
I will comment that a 12V system or and alternator is not needed to change over to what I consider the best solution, the direct replacement halogen bulb. This is contrary to the advice given by those who make money selling you an alt.
As you have noticed there are several changes that will get you a halogen bulb.
There is one where you put in the ugly modern bulb and some kind of reflector. I have not played with them so I can not comment further. My assumption is they are not well focused so they just kind of throw light out inefficiently. They also cost quite a bit. They work because they are just so bright. They also use a lot of energy to work.
Then there are the large sealed beam bulb adapters. They just look wrong on the A with the bulging round lamps. They should work fine.
Then we have a simple change out of the original bulb for a halogen.
What you may not know is there are now two manufacturers on the market. There are the ones made by
Classic and Vintage Bulbs which are a high quality product.
The filaments are located in the correct position so they will focus properly.
This assumes you are using the original reflectors and headlamp sockets (very highly recommended). The repro parts will give you grief and never work like the originals.
The second direct replacment bulb I am aware of is being imported from India by
Bill Hirsch. The quality is visually less than what you get from the CVB bulbs. It is obvious the filaments are randomly placed. The Hirsch bulbs may be what you get from the supply houses.
The CVB bulbs are available in the USA from
Little British Car Co. and I will warn you that the site is not easy to navigate.
With the CVB bulbs you have the option of the 35W bulb and the higher current 50W bulb. Since you are using the 12V alt the higher watt bulb is something you could consider assuming your belt is tight enough to allow the alt to put out the current. I suggest you look up how a belt is to be tightened for an alt to work at full output on the cars that used V belts. You might be surprised at how tight it should be.
With the halogen bulbs you get something close to an original look and very good light even with less then best silvering on the original reflectors. Kurt above is my brother.
A side note that is slightly political in nature:
Keep in mind the CVB bulbs are made in a Australia where people have to be paid a real wage and follow safety rules. The India bulbs are likely being assembled by a people (kids?) working in dangerous conditions for almost no money. So the CVB bulbs have to cost more.