• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Is this a C core OPT?

You can "C" the gap.

Yep, they're C cores alright. At the bottom of the picture on the left you can see the small gap where the two halves of the core butt together. Two C shaped halves that face each other. They are made in a similar way to a toroid by winding a long strip of metal in a sort of square shape around a square thing. Then they are varnished and cooked. Then the core is cut into two halves and the abutting surfaces are polished to a smooth finish so they will fit together with minimum airgap (assuming an airgap is not wanted as in a conventional tranny).



diyAudio Senior Member
2002-08-21 1:20 am


Sorry but I somehow lost track of this.
in case you're still wondering:

These are definitely C-cores probably pull-outs from a Philips OTL.
These amps allowed for either low-z OPT couplig or C coupling to a 800R speaker.
I modded a few of those (in my less lazy period) and you can actually get pretty good sound from them even on more conventional speakers.
The circuitdiagram shows how they were operated with a pair of
I'm going to mail you that diagram.


Frank's picture is attached. I put these up on eBay and an astute bidder has informed me that these are double chokes for a Philips OTL. The amp, which had unquestionably had the smoke let out, is from a Philips organ and drove a 2-way 800 ohm speaker (12" + 6x8" whizzer cone mid-tweeter).



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