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In the Forum: Melquiades Amplifier
In the Thread: Bias help
Post Subject: Bias helpPosted by cb on: 10/9/2007

Hi Romy

I have a mono completed full range Melquiades and another only with the bias circuit hooked-up.

The bias voltage I get on the 6e5p is around minus 4.4v on both the circuits. This is with the inputs shorted. With the inputs connected to adjust voltage to 0.0mvdc, I get a reading of plus 0.4vdc. This is with VR1 set at maximum (20kohm). This is the “lowest” reading I can get without increasing R9, VR1 or R4. I can’t figure out why i am getting these readings.

To get around minus 3.4v on the 6e5p bias and 0.0mVdc on the input, I tried 510kohm resistors in R4 and R6 and increased R5 to 10kohm. This seems to work.

On the melquiades circuit revision 3.6 the C5 (10uF 250v) is wired up +ve to ground and in revision 4.0 C5 is wired –ve to ground. Revision 3.6 works for me, with revision 4.0, I get around -130v on OA2 and -0.75v on the 6e5p bias. Which is the right way?

Everything else measures ok. Any help appreciated.

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