Cossor 222 Output stage.

[Cobaltblue]

I am working on one of my 222's at the moment.

It's had wires cut to the speaker so I am having to make sure I understand the original wiring. There is a plug and socket on the 222A not so on the 222.

I would compare it to my other 222 but that's in storage.

I don't have the circuit for the 222 only the 222A which is a different set.

Fortunately much of the circuit is the same except the front end, although the layout is totally different.

This set in common with a few other early Cossor manufactured as opposed to kit sets has a slightly different arrangement on the output stage which I am sure is not unique but different enough to the norm for later sets be worth discussion.

The Output Valve Anode is connected to HT via the loudspeaker field coil winding.

The anode is then capacitor coupled to the output transformer.

There must have been a benefit to this arrangement including no current through the transformer primary.

This set also has an Early Electrolytic smoothing capacitor in a big tubular copper can.

Cheers

Mike T

[Boater Sam]

Interesting, presumably the phase of the secondary has to be correct in relation to the field as they are both carrying audio?

[Cobaltblue]

Hi Sam

We will find out in a week or so when I re-connect it all

I will be taking best guess based on the position of the snipped original wiring.

pictures to follow later.

Cheers

Mike T

[Nanozeugma]

There is currently a 222 on offer on Ebay which carries a couple of fairly high resolution images of the loudspeaker and attached transformer, I don't know if these would be of any assistance?
https://www.ebay.co.uk/itm/COSSOR-MO...-/192705879516

[Cobaltblue]

Hi Nanozeugma

Yes I spotted that one, the bit I need to see is behind the middle (Output valve). It was what encouraged me to start on one of mine

The same seller also has the cabinet only for the 233

Also you can see that wiring has been disconnected so may not be as original.

I am pretty sure I have correctly identified which of the 4 wires go where. At worst the phase will be wrong but I doubt it would make much difference in reality.

Cheers

Mike T

[kalee20]

Originally Posted by Boater Sam

Interesting, presumably the phase of the secondary has to be correct in relation to the field as they are both carrying audio?

I'd say not, actually.

The field will have a large inductance, so current through it will hardly alter, at least at an AF rate. So the AF component of the output valve's anode current will be diverted through the output transformer, and drive the speech coil.

It does avoid DC through the output transformer, so it can be smaller and cheaper, but it does add an extra capacitor.

If there actually IS some AF in the field, there'll be a bit of distortion caused, just think of the extreme case where the same AF flows through both field and speech coil. Force on the coil is then proportional to I² and not to I.

[PJL]

Around 1930, an inductor for the anode load and capacitor coupling was a common method of keeping the DC off high-impedance headphones and horns/speakers. Not ever seen this before though.

[Cobaltblue]

Thanks for the responses.

Here are the pictures of where the speaker connections are made.

4 Cap terminals 4 connections.

And that great big copper cap.

No real problem to work out which is which I have already marked it on my notes.

Cheers

Mike T

[Nanozeugma]

Love the copper cap, never seen one before

[Silicon]

The Copper capacitor - Is it a wet electrolytic?

[Cobaltblue]

Yes it was a wet type long since dried out of course.

I will restuff it to keep the look of the chassis.

All the green metal paper caps are leaky but I need to check how badly at full HT.

There are several oc resistors to deal with but fortunately all the wound components seem ok.

Next week before I can play more

Cheers Mike T