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Re: BX690A/BX695A

Geschreven door John Hupse op 05. Jul 2006 14:10:53:

In reactie op: BX690A/BX695A geschreven door PANDU RAJAN op 05. Jul 2006 09:59:20:

Hello Pandu,

Never a dull moment when repairing Philips reveivers!

The first output tube (B4) works "normal", i.e. it get its input from the AF pre-amp B3. The second output tube (B7) has its grid at earth. The cathode, however is fed by the output transformer (cathode injection). The phase of the injected G-C signal is 180 degrees shifted compared to the input of B4. So we can say that B4 is not only one of the PP output tubes, but is also the phase inverter (in cooperation with the output transformer).

The amplification of B4 plus the ratio of the output transformer is chosen to be 1x, so the input signal of B7 equals the input signal of B4, only the phase differs.

Advantage is reduction of the number of parts used, disadvantage is the inherently more complex working of this design. Notice that the ECC40 tube was not yet available in 1949.

Even more interesting is the way the tone control works!





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