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Old 30th Aug 2019, 10:47 pm   #42
kalee20
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Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Interstage Audio coupling transformer

Quote:
Originally Posted by ChristianFletcher View Post
... I have retained the -4.5 volt grid bias to the pentode output via a 500k resistor and also capacitive coupled to the detector output. The radio tunes and it’s as loud as I would ever want a radio to be. Sound quality is about as good as any 1930s radio I have owned. Now I’m still going to rewind the transformer, but cannot help thinking why did they bother to use a transformer when capacitor coupling works well and would have been cheaper.
Hmmm... Not only cheaper, but better fidelity.

However, transformer coupling does offer the possibility of extra gain. There's not only the 3x which is possible with a decent transformer (Ferranti even made a 1:5 step-up transformer), but the previous valve can be loaded with the transformer primary and thereby operate at near full HT on its anode, which would give greater signal handling and greater gain from the valve itself (although that won't apply with your shunt-fed circuit). And it will work down to a lower battery voltage. Also, the transformer secondary gives a very much lower DC resistance in the grid circuit of the following valve, which can be important if it is operating into grid-current.

In your case probably none of these apply, and if RC coupling works well enough then the arguments for the transformer are not very strong. All the same, it would be great to read your reports on the rewind, and the performance attained!
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