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Nederlands Forum over Oude Radio's |
Re: BX690A/BX695A
Geschreven door John Hupse op 07. Jul 2006 14:18:55:
In reactie op: Re: BX690A/BX695A geschreven door PANDU RAJAN op 07. Jul 2006 12:13:54:
>>>As you have explained that the input voltage between G-C in B4 will be 2U and the input voltage between G-C in B7 will be U. So will the amplification not be different for the two EL41's as the input voltage is not same? Kindly also let me know the working of the tone control circuit in detail.
Hallo Pandu,
What I said was "If the G-C signal for B4 has the value U, the injected cathode signal is also U. So the grid signal of B4 has the value U + U = 2U."
This means that the grid voltage is 2U, but the voltage between the grid and the cathode (G-C) is U. As the G-C voltage of B7 equals -U, the PP amplifier is perfectly in balance. Both EL41 tubes get the same input signal (G-C), only the phase differs.
The Volume control is combined with a correction for lower and higher tones (e.g. loudness control). The RC combination R16-C33 increases the lower tones at a lower loudness using negative feedback. The RC combination R10-C32 increases the higher tones at a lower loudness using positive feedback. C34 has the same function, but operates in forward mode.The Bandwidth can be controlled by pushing/pulling the Volume Control knob, this will reduce the higher tones (above 10 kHz) in the "Low Bandwith" position.
The Tone Control RC combination R17/18-C37 decreases the higher tones using negative feedback, but only when the potmeter slider is above the tap position. In the position beneath the tap the RC combination R20-C36 will decrease the lower tones, using negative feedback. When the potmeter slider is exactly at the tap position a mechanical resistance can be felt. At this point the tone control knob is in the "Quality" position.