Re: The KT88 are conected without degenerative kathode
jraraujo said:
Thank you for your repply
No they aren't.
The KT88 have an invdividual kathode c.c.s., 87mA bias, bypassed with a 100uF capacitor.
The ccs is tunabled type for a fine DC cancelation on OPT.
The rest is more or less like williamson or better like the Leak 50.
But I've some friends that have found this same phenomenon on other topologies.
Please inform me if you know the reason why sounds better with only two valves.
Regards
But I suppose the screen grids of KT88s are connected to their plates as in original Williamson so we see four parallel push-pull triodes.
Any push-pull triode stage with a center-tapped primary of the OPT (not like a circlotron) is prone to a specific kind of odd order distortion, which is caused by a finite mutual leakage inductance between the halves of a primary. Naturally, this mutual leakage inductance cannot be zero in any real output transformer, although it can be reduced by a proper design.
The physics behind this distortion mechanism is quite complex, so I cannot describe it just by few words, but the result is following:
The EMF induced at this mentioned mutual leakage inductance (its waveform looks like sharp spikes) simultaneously modulates the instant plate voltages of both triodes of a PP pair in such a way that the amount of odd harmonics generated by the power stage is increased. As is known, the PP circuit cancels even harmonics, while the odd ones are expected to be not influenced.
Indeed, the mentioned effect results in more odd harmonics production in comparison to parallel SE operation of the same two tubes and same plate current swing. Nothing to say this smears fine structure of the audio signal and results in an appreciable loss of resolution. What is most intriguing, this effect increases with lowering of the internal resistance of the triodes, i.e., when you put two PP pairs instead of one to the same OPT, this will result in greater resolution loss!
Historically this effect was first observed in large class AB2 PP triode modulators of AM broadcast transmitters. In those stages this effect was so pronounced that an easely observable sharp crossover step appeared in the output waveform when the modulator was fed by reasonably high frequency. Of course, nothing like is expected in the case of a class A triode PP stage, by the increased amount of odd order nonlinearity is still there. This may be a reason why SE triode amplifiers achieved so high praise.
A really good PP OPT for a triode amplifier is very difficult to design and make. Indeed, the abovedescribed distortions are unlikelyu to appear if the -3dB point of full power frequency response is somewhere at 200kHz and both the amplitude and phase responses are essentially the same from the both halves of a primary to a secondary. Such PP topologies as the Circlotron requiring no central tap are free of such distortion mechanism, but also there can be other sources of distortion in the circuits with multiple power tubes.
The better or maybe even ultimate way to introduce a local NFB to the pentode output stage is a cathode feedback (CFB) circuit as that first employed in QUAD II amplifier. This circuit is free of such distortions and possess many other great advantages.
The UL topology is somewhat less prone to these distortions than a triode connection, but has its own drawback.
The above distortion can appear in PP output stages with any bias circuit.
Regards,
Denis
