Amp Gain Block

You see that the feedback is DC coupled, not through a capacitor, from the plate of the second triode to the cathode of the first. An earlier design used AC coupled feedback. This version was unstable at low frequencies. Bode analysis showed that a 3 microfarad coupling capacitor would have been needed to make it stable. The 220 k ohm resistor makes
Amp Gain Block - schematic

up most of the load for the second triode which is where most of the distortion occurs. I don`t recommend changing the value of Rf because it effects the operating point of the second triode. If this resistor is changed the cathode resistor in the second triode will have to be readjusted for minimum distortion. Someone emailed me with the information that when this circuit was run on a spice simulator there was a 1 dB peak in the frequency response below 10 cycles. This really isn`t very important but it still might bother some people. It was caused by the fact that the feedback which goes to the cathode of the first triode and is negative also is fed in a small amount to the grid which is positive feedback. I downloaded a spice simulator and checked it out and confirmed the peak. The trick is to adjust the relative sizes of the inter-stage coupling capacitor and the input coupling capacitor so the positive part of the feedback is rolled off before it can effect the gain. I also tested it with the volume control set at half resistance which is the maximum impedance condition. The roll-off is now smooth at both ends. I trust everyone, perfectionists included, can now sleep. Am I a perfectionists Well, grudgingly, yes. You may be wondering why I have gone for such low distortion while on other pages of this section I didn`t indicate it was that important. This amplifier is NOT meant to be inside the global feedback loop. As...

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