Quote:
Originally Posted by VT FUSE
Great stuff Tim
I have been thinking of a hybrid build as I have several of both pre TO3 types of OC16 stud mount Transistors of the type used by Argus25 and even predating the type he used.
Had thought of having the transformers specially wound but your iteration sounds very good.
Mike
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On the transformers in mine as noted the driver was pre made for the task, according to the Mullard/Philips specs.
I wound my own butt stacked output choke. I had an 8R speaker, and the choke (autotransformer) was tapped to give an impedance ratio of 22:8. Also looking back at the circuit, the OC16's emitter current was 420mA, and I use a 3.3R emitter resistor dropping 1.38v. As I recall it could easily deliver 1.5 to 2W audio out which was plenty for a desk top radio. In a car they push it a bit harder to about 2.5W.
The OC16 is mounted on a 1/8" copper flange which is also thermally coupled to the steel chassis (The paint was removed under it, and heat conducting compound used). I did thermal stability tests on it at the time and the heat sinking was perfectly adequate for our sometimes 40 Deg C ambient temps, if not over killed. I also incorporated NFB from the speaker connection to the cathode of the EF98 driver valve, simply by adding a 75R resistor in the cathode and a 560R resistor to the speaker terminal. Also as I recall, the input sensitivity is such that about 1V or 2V rms at the EF98 grid, attains full output, but I would have to check that. I could attach the schematic of the radio if it is any help.