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Tube preamp stage

Hi all, Me and a friend at work argue about the behaviour of a simple tube preamp stage found in most guitar amp front ends. I know I'm right but he says he's right. Well, here we go: Imagine a standard circuit with 100kohms anode resistor and 1.5kohms cathode resistor. The tube is a standard ECC83 (12AX7). Across the

1.5k resistor a 1u cap is in parallell. In my eyes this will introduce a slope at around 100hz (1/2piRC) simply limiting low frequencies. My friend (former...) says that changing the 1U cap to a 50u cap will increase gain. This is true, but this also affects the low frequency gain of the stage. Instead of 100Hz, it will go down to around 2Hz. But my opponent says this is not true. I'm not used to tube circuits, just transistors stages, but I know the above applies to transistor stages. So, would anyone just tell me I'm right in my point? Thanks, Regards, PAF
Reply to
powerampfreak
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You be right.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
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Reply to
Jim Thompson

Why don't you build it and measure it?

martin

Reply to
martin griffith

You are right, sort of. But, in the bass region below 106Hz, the break point of the 1u, 1.5k resistor, the gain is falling. Using a larger cap will increase the gain in this region by moving the break point lower. It won't increase the gain at high frequencies. However, the kicker is what happens to the gain in the critical region from 100 to 300 Hz where the fundamental frequencies of notes around middle C are. Here the gain will increase one to three dB depending on the exact frequency. This increase coupled with the increase in the bass will give the sound added umph or the perception of more "body" and power. Depending on the speakers and other factors, you'll probably like the sound of the added cap better than without it. If there is feedback around this tube stage, all bets are off.

Reply to
Bob Eld

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