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Lineair

Only a few devices were built with the giant F443 pentode. Thanks to it, power up to 25W was obtained. I am presenting an unknown amplifier from the early 1930s, containing two F443 tubes. The device was manufactured in Denmark by Bofa - Struer, cooperating with Philips. In my free time, I will try to recreate the electrical diagram (I devote much more time to old TV sets). Apart from two F443, there is one REN904 and 3x barretter 0.72A.38-114V model C21. I haven't found the latter in any catalogue. Maybe someone has information about C21? Philips was probably the manufacturer. Amplifier circuits with the F443 tube or its successor, the F443N, are welcome. Thanks,
Adam

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These C 21 's are current stabilisers, with iron filaments
and gas- filled with some hydrogen.
Does the amplifier contain a power transformer?
Regards,
Nico

--
een kinderhand
is ook weer gauw leeg

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The power supply system was an external unit. Unfortunately, this module is missing. Cut off lead wires are visible.

Adam

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Dear Adam,

Would it be posible to post some additional pictures?
As a B&O collector I'm very interested to see more pictures of this amplifier.
Also my friends in Struer will be very interested to see some additional pictures.

The amplifier is a very rare one made by BOFA, Bang & Olufsen Forstaerker Anlag, the department for professional equipment of B&O in Struer.
The transformers and chokecoils (drosselspoler) are all made by B&O.

Thanks in advance,

Rudy

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I am creating my own website with old tube devices and I intend to publish more photos there. It will be in a few months. You are right, the transformers are made by B&O - see photo. The amplifier is very massive and different from others from this period. It looks like an industrial device.

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Yes, the output transformer is one of the type P20.
Outut transformer class B for penthodes in push pull for perma dynamic loudspeakers.
I found two diagrams in a 1932 B&O catalog.

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Excellent. Can you send photos in a better resolution via private message?

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It's very an amplifier for a cinema.

In the early 1930's BOFA started to build cinema equipmnt like prjectors, amplifiers and speaker systems.

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More info about BOFA can be found on the following website.

B&O's professionelle afdeling Bofa
https://www.biografmuseet.dk/nyheder/2010/bofahistorie/index.htm

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Two pictures of the first BOFA amplifier , on display at the Struer museum.

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Test setup at B&O.

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I see many analogies in construction to my amplifier. I think the version in the museum is more modernist, i.e. younger. The F443 pentode appeared in 1930, and in 1933 its successor, the F443N, was produced. So I assume my amplifier is from 1930-1933.
If we are dealing with very unique devices, the question always arises about the sense of renovating such a piece. This is a difficult topic to discuss. I collect old TVs every day. I like it when their screen lights up, but I decided to leave some models without repairs. The Danish amplifier will probably remain in pristine condition. I will only try to prepare an electrical diagram.

Adam

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Beauty that is.

Looks like a total DC current amplifier with al those big resistors and ballast lamps.
Would be great if you get thing going.

Beware could be used in a cinema or big Stadion in that case could be wired 100V for
multiple loudspeakers.

--
"I haven't even turned this on to see if it works because as long as I don't it might"

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Looks like a total DC current amplifier with al those big resistors and ballast lamps.
Would be great if you get thing going.

It is very likely a DC-supplied amplifier: up until 1945 the Danish grid was 220 VDC fed by many local powerstations.

Kind regards, EJ

--
Dingen moeilijk maken is makkelijk, dingen makkelijk maken is daarentegen moeilijk.

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All right. The amplifier will be renovated while maintaining its original structure. I will inform you about the progress of work on this forum. Unfortunately, it will take some time. I have something else lined up. I'll start by testing the valves. I'm glad that there are other enthusiasts of such old rusty junk. Thank you all for all the information so far.

Regards, Adam

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Indeed could take some time figuring out some details.
Like;
-think extra knop right from Tone-Reg, see next to "Mark"sign.
-center square hole, if that mean something.
- 3x left side indicators , could be original or changed, right one seems most original.
-2 missing knops, now with picture of first build amp, you know the type.
-Separate powersupply to get it all going again.

Overal seems to be in good condition and not too much done lately to the amplifier.

Just have fun with it, bear in mind to keep as original look as possible.
A dead peace of iron isn't what supposed to be.
It needs power, some incoming signal and a big loudspeaker :-D

Could be amassed of the 1930's sound, who knows.

--
"I haven't even turned this on to see if it works because as long as I don't it might"

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Adam,

At the bottom of the Biografmuseet.dk homepage you'll find a link to contact Mikael Christensen.
He's the BOFA specialist in Denmark and he has also restored many BOFA amplifiers.

I'm sure he'll be very happy to assist you.

Rudy

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Thank you for your tips. Indeed, this iron monster may pleasantly surprise you with its sound from the 1930s. It will be a good heating stove on cold winter days

Adam

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