SG Brown H4 Loudspeaker Adjustment.

[a_strong]

I have been given an SG Brown H4 Horn type loudspeaker which appears to be complete but is in pretty poor condition. I intend to use it with a replica "All World Two" receiver that I am in the process of building. The coils would appear to be intact (approximately 2000Ω resistance).

By any chance does anyone have instructions or can offer information/advice on adjusting the H4 reed assembly for optimum sound quality and volume?

The magnet will doubtless benefit from remagnetising, which I plan to achieve by placing a neodimium bar magnet across the pole pieces for a while.

Thanks in anticipation

Alan S.

[Mike. Watterson]

It depends on bias current and on loudness required.
Optimum sound quality for a loud station is NOT the same setting (large gap) as for optimum sensitivity. A smaller gap.
Essentially the same idea as the adjustable gap on some headphones. Some horn drivers are almost identical to an earpiece.
So it would have been adjusted as batteries aged and for good sensitivity on weak stations (but then a stronger station or more volume would make it clatter).
There is no real best "quality" setting, just sensitivity vs clatter.

[Argus25]

Originally Posted by a_strong

By any chance does anyone have instructions or can offer information/advice on adjusting the H4 reed assembly for optimum sound quality and volume?

The magnet will doubtless benefit from remagnetising, which I plan to achieve by placing a neodimium bar magnet across the pole pieces for a while.

Thanks in anticipation

Alan S.

Yes I can help.

I restored my H4 as a young man back in the mid 1970's. I have attached a photo. The coils were O/C due to green spot corrosion. I manufactured new bobbins and rewound them and studied the speaker design.

Actually, if you have any need to do that it turns out the bobbins can be replaced by a number of types used in vintage headphones.

I put a ruler in the photo to show others who have not seen the H4, just what a compact horn speaker it is.

Mine had severe corrosion of the aluminium horn section (right through its thickness) and the cast body. After much preparation and repair, it was re sprayed and baked in an industrial oven.

In any case, it is extremely unlikely you will need to attempt to re-magnetize the existing magnet (The one in mine is fine). If you attempt it be very careful with any coils you use to do it (its easy to make an error with the direction of the current flow) and I would suggest not putting any neodymium magnets near it.

The central pin on the aluminium diaphragm is soldered to the armature arm and the adjustment mechanism. The round nut has 4 optional holes where the arm can screw in. In any case you can get at least 180 deg rotation of the nut with the arm fitted.This biases the armature toward or away from the pole pieces.

The adjustment allows the armature to approach the magnetic gap, closer and closer. As this happens the sensitivity of the speaker increases, until the armature jumps toward and sticks to the gap with a louad click, let's just call that a "jump".

At the point just before this happens, ideally the aluminium "cone" has a negligible mechanical bias and the soldering of the cone pin to the armature should be re-heated to correctly set that position.

If DC applied to the coil (depending if the speaker is in fact in a valve plate load scenario) it will affect the point where it jumps but that can be offset with the mechanical bias, but if the circuit driving it is the secondary of a transformer, there will be no DC bias.

When the mechanics are set up correctly, swinging the arm over its range will produce a click and a pop as the armature engages and disengages the pole pieces.

I did my magnet like this... https://www.vintage-radio.net/forum/...ad.php?t=65735

[a_strong]

Many thanks for your advice, I am much wiser now about how the speaker works. Fortunately the coils in my H4 show no sign of green rot and the resistance of the coils measures a few tens of Ohms less than the 2000 punched into th ealuminium cover on the base.

The cast section of the horn is broken off at the base but it is still a good mechnical fit so I plan to use metal working epoxy adhesive to glue it back together and leave it to cure for a few days before attending to the paint-work.

The flared section of the horn has a few minor dents which need cosmetic attention but otherwise I have high hopes of getting it working satifactorily.

Regards - Alan S.