TubeCAD article - Tube CAD Journal

So when the driver tube sees an input swingof +62/-62 volts, its plate will swing -221/+195volts, which in turn this imbalance will counterthe imbalance that the output tubes will forceonto the its output voltage swing, as it is theinverse of the output stage's imbalance.The unfortunate feature of this amplifier isthat because the 300B mu is low, the outputstage never realizes it full potential output, as thecathode follower output stage's low gain requiresa much greater voltage swing at its grid to bringout the full output power that the 300B can give.Thus a better choice for an output tube might betriode connected 8417 or EL34. In the schematicbelow, we see an EL34-based output stage. Thisamplifier would provide about 15W of relativelyinexpensive, clean, pure single-ended power.The power amplifier's input stage also usescomplementary inverse pre-distortion to lowerthis stage's contribution to the amplifier's finaldistortion figure by not adding excessivedistortion to the mix. Thus the same principle ofinverse-complementary-distortion cancellationthat lowers the output stage's distortion is usedto in the input stage. The same triode, the samecathode-to-plate voltage, the same load resistorvalue--all are used to inverse symmetricallycancel the distortion from the input stage.The end result of all these techniques is anamplifier with a low output impedance and lowdistortion and wide bandwidth, an amplifier thatdoes not use feedback (at least not a globalfeedback loop). The cathode follower outputstage deserves some comments. This circuitemploys 100% cathode degeneration of theoutput signal to keep the output in inline with itsinput. If the cathode fails to move as positive asthe grid moves, then the grid effectivelybecomes more positive and the tube's conductionincreases, which will force the cathode to agreater positive voltage. Conversely, if thecathode falls less negatively as the grid falls,then the grid effectively becomes more negativeand the tube's conduction decreases, whichresults in the cathode's voltage to collapse. Inother words, the cathode follower output stageuses all of the output tube's transconductance toforce the output to follow the input signal. Thisshort, quick feedback mechanism also brings theoutput impedance down and extends thebandwidth. It also serves to lower thetransformer's distortion contribution, as outputtransformers distort least with low impedanceinput sources. This is an import advantage ofthis output stage, as the transformer is notenclosed in a distortion lowering feedback loop.10mA35k+650vdc5k2.5k100 100 100+800vdc+400vdc30k+300vdc50mA.5µF2k100kEL34 EL34 EL34+37vdc+150vdc30059653005965+153vdc+2.8vdc.5µF+400vdc1001kEL34+37vdc150mAOne Electron UBT-11684GND1M56030k470k74185< PREVIOUSPg. 5www.glass-ware.comwww.tubecad.com Copyright © 2001 GlassWare All Rights Reserved NEXT >