Double push-pull versus single push-pull

[jraraujo]

Hello!

I have buit a double push-pull (4x KT88) Williamson based.

Ecc82 Pre-amplifier and concertina fase splitter.
Ecc82 Text book differential amplifier driver.
The output impedance of the driver is about 10KOms.

When I only use 2 output valves, sounds better: More presence, better definition of sounding stage.
The bass are less grease and there is a subjective sensation leading to a bigger emotion felt when we listen to the records.

This effect is unproportional towards the expected because of the increasing Miller capacitance.

Does anyone know why this happens?

[dice45]

jrarauio,

i have observed the same, on all signal levels. And so have my buddies. I do not parallel tubes or tube sections, rather i use a juicier tube.
Be it phono input, line stage, power amp.

I do not know the reason and my tube guru does not know it either. Theoretically, a triode consists of elementary triode systems wired in parallel (due to grid spacing variations and to varaitions of the electric field at the ends of the triode system). So it should not make a difference. But it does. I do not have to understand it, i simply care not to parallel tubes.

[planet10]

I am certainly in the belief that a single pair of devices is best.

Did you compensate for the different load requirement for a pr or a quad?

dave

[jraraujo]

Thank you for the tips!

No, I did not compensate the load.
In this unconpensed conditions the slew rate increase from 6,2us to 10us.

Obviuosly the bandwith and the dumping factor should decrease in this conditions.

So, here we have all the factors for a naff sound, but the final results are totaly unexpected. It is the oposite!

I remain without knowing why this happens, and this is bad for my mind!


Regards
jraraujo

[fdegrove]

Hi,

One other possibiltiy could be the added Miller effect the driver stage can not cope with.Higher powered triodes need to be compensated for this effect if not HF roll will occur.

Paralleled tubes :

.

I do not know the reason and my tube guru does not know it either. Theoretically, a triode consists of elementary triode systems wired in parallel (due to grid spacing variations and to varaitions of the electric field at the ends of the triode system). So it should not make a difference. But it does. I do not have to understand it, i simply care not to parallel tubes.

The reason you probably won't like it is that the distortion characteristic has predominance for uneven order harmonics.
Try this without ( or partially bypassed ) cathode bypass caps and things look a lot better already.
In other words introduce some degenerative feedback.

On a general note : if you parallel tubes you should know your stuff and if you have a plotter ( or a software sim at least ) trace those curves with both sections paralleled.
You'll notice the difference in linearity.
In a nuttshell: // tubes aren't necessarily a bad thing.

Rgds,

Frank

[Denis]

Originally posted by jraraujo
Hello!

I have buit a double push-pull (4x KT88) Williamson based.

Ecc82 Pre-amplifier and concertina fase splitter.
Ecc82 Text book differential amplifier driver.
The output impedance of the driver is about 10KOms.

When I only use 2 output valves, sounds better: More presence, better definition of sounding stage.
The bass are less grease and there is a subjective sensation leading to a bigger emotion felt when we listen to the records.

This effect is unproportional towards the expected because of the increasing Miller capacitance.

Does anyone know why this happens?

Are your output KT88s triode connected as is in original Williamson?

If so, I know the answer.

Denis.

[jraraujo]

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

[Denis]

Originally posted by jraraujo
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

[tiroth]

Denis,

Very interesting! It seems like toroidal OPT generally have high bandwidth due to the topology; how (in general) is the mutual leakage inductance of toroidal vs. standard OPT? Would toroidal OPT be desirable in a paralleled design?

[Denis]

Originally posted by tiroth
Denis,

Very interesting! It seems like toroidal OPT generally have high bandwidth due to the topology; how (in general) is the mutual leakage inductance of toroidal vs. standard OPT? Would toroidal OPT be desirable in a paralleled design?

Certainly not! The toroidal cores are the worst ones for OPTs. They give lower leakage inductance in respect to conventional ones only if we speak of simple winding without interleaving. This is feasible for mains transformers reducing their stray field. Indeed, the properly interleaved IE- or, the better, double-C cored transformer is much better. I use only double-C cores of high-quality grain-oriented electric steel. The flat and uniform surface is the must for precise turn layout. This allows one to get virtualy any degree of suppression of both leakage inductance and shunt capacitance. It is a pity these design rules are seldom followed by transformer makers.

Another great drawback of the toroids is their DC succeptibility. An airgap is a must for a PP OPT too. I've found the natural airgap (25 um or so) present between the halves of a double-C core is just what Doctor has prescribed.

Keep It Simple!

Denis.