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Old 19th June 2009, 02:04 AM   #1
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Default El84 push pull

I've always looked schematics with a pair of El84's and always found there were fixed bias... why aren't many of them with grid bias? Is it the sound that's better with fixed bias? It would have better perfomance with adjustable bias, right?

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Old 19th June 2009, 02:10 AM   #2
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I believe that Roger Modjeski's RM 10 amplifier has the bias voltage applied to the grid, rather than supplied via a fixed cathode resistance. I've listened to Eric Hayes' TAL 1773 amplifier which also runs pairs of EL84 in push/pull, grid biased configuration. It is a wonderful amplifier, which sounds more powerful than an EL84 amp should.

I think that EL84 perform reasonably well with cathode bias, and most manufacturers would rather not incur even the slight increase in cost and complexity required by a grid bias design.
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Old 19th June 2009, 02:11 AM   #3
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Default huh?

"fixed" bias vs cathode bias. Both bias the grid. Majority I have seen are cathode bias either using a simple resistor or a piece of sand - are we talking the same terminology?
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Old 19th June 2009, 02:21 AM   #4
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I suspect the original poster is using the term "fixed bias" for situations where the bias is set by a fixed value cathode resistor. The other popular bias strategy is to use a variable pot to apply a negative bias voltage directly to the grid.

I believe the confusion arises since the grid applied bias strategy is frequently referred to as "fixed bias". It is fixed in the sense that the bias voltage is always constant, regardless of the music signal applied to the circuit. It is necessary that this kind of fixed bias circuit be adjustable to accommodate differences in output tubes.

The cathode biased circuits are non-adjustable, but the bias voltage (in terms of potential between grid and cathode) is not "fixed" in the sense that it will vary depending on the amplitude of the input signal. As ever larger input signals are applied, the current through the tube will increase. The curious thing is that this increased current through the cathode resistor will result in a larger voltage drop across it, causing a greater bias to be applied to the tube.
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Old 19th June 2009, 03:40 PM   #5
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Well, I was talking about bias the output with a large resistor from grid to ground (isn't it self bias?) and the other method, using a negative voltage with a pot to adjust the quiescent current through the tubes (if it's fixed by a non adjustable voltage divider it's the same, but not what I was refering with fixed bias). I always found PP EL84 with the self bias method.
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