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Jan 21 2015

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GU80 Push Pull (updated)

P1130126 gb

{{ updated 2015-01-21

It turned out that the asymmetry of the setup, that I observed, is a result of a blown silver mica capacitor, which was hidden / integrated with the tube socket.  It was rated for 500V, between S3 and Cathode, which would be a reasonable value if the S3 was reasonably used. 
Not the case in my case. I used the S3 as an additional “mini anode” – or as a place to connect the high voltage Ultra-Linear signals.  Meaning the full anode voltage, DC and AC on top of that, easily peaking over 2kV.  The darned Russian 500V rated silver mica decided to fail after 3 hours of listening to music.  A lousy cap, that cannot withstand continuous 2000+ V when rated for 500 … :)   Anyways, it blew up with a big bang, almost pulling an 450V rated elko with it. 

The Elko survived, which is really bizarre as it also received the 2000V plus on its taps.  It did heat up to something like 100 degrees though. But it did not blow up in my face. The fuses  blew first. Funny thing. After cooling off – the elko is STILL operational, although it does not have much resemblance of a geometrical “cylinder” any more.  Very intriguing. Re-formated. It works. Unbelieveable.  A Fat elko.  Elko with curves. Almost sexy. 
Anyways, all is now symmetrical and under control. 

The Centered Differential Ultra-Path works just simply Super. But what’s more, it does not get upset by the electrolytic bypass, the one that is a necessary prerequisite as a bypass to the nonlinear elements within the cathode circuits of this configuration.  The two “High Voltage” single ended ultra-path caps, that are an extra addition to the centered differential ones, also seem to amicably work together here within this setup. Of little capacitance but of a high voltage rating, their “ultra-pathing” capability is limited, but then – every little bit helps. All in all, the ultra-path setup being used is somewhat of a very hybrid contraption, and what’s more, one that is supposed to provide the additional benefit of PSRR noise cancellation, once the driver stages come into play. 

As for the “Symmetrical Current Feedback” thing, not all behaves as expected. I am experiencing some instability here, so will need to further investigate and/or ponder about this.  Maybe it is simply the case of a slightly too big a transformer, with some excessive parasitic capacitance, leakage inductance or other. Whatever. Will need to shorten those wires, and try to convert this spaghetti contraption into something somewhat more decent and orderly looking. 

As opposed to the Current Feedback, the traditional “Symmetrical Voltage Feedback” works just fine, even in a very crude and brutal force type of application, via resistor divider networks, where the shunt resistors bypass the secondary windings of the symmetrical step-up interstage transformer. I use such an interstage as a fast and dirty “substitute” for an input/driver stage. Crude, but it works, serves its intended purpose and performs well in terms of workbench testing needs. 

The “High Voltage, Suppressor Grid Ultra-Linear” idea, a rather sick idea that I came up with and decided to test – also works just fine. The idea is based on the notion to apply the ultra-linear signal behind the screen grid, and not before it.  The influence of the S3 on the electron flow is very limited, due to the sparse wire spacing. But mind you, there are some significant AC voltages running up over there, so what we are missing in mu_G3 and on the big distance from cathode and the sparse wire spacing, we are heavily catching up on with the shear amplitude of the AC running upon it.  I suppose that the 43% of the UL is not optimal in the case of the post-screen_grid location of the S3, but hey, it works.  Maybe it does not work optimally, but it works better than without it. 

One problem though.  Is it normal that the cathode current rises like crazy to a much higher level, that the gain of the tube at least doubles, and that the thin wires of the suppressor grid start to glow a dull orange? Basically – all answers to those questions are very logical. Three times a definite YES.
Speaking of which, I am a bit worried with those glowing suppressor grids. Poor things.
So maybe, just maybe, I will put in a high power resistor in this ultra-linear path, so as to limit the current being forced through this S3.  Would not like to melt it prematurely.  As for the sound of such a setup – sounds good. The “ultra-linear” thing seems to be working properly. So now it comes down to the value of that current limiting resistor and if and to what extent shall such resistors prove to be detrimental to the quality of the sound. This resistor will sweat. Quite a lot actually. 10 Watts seems to be good point for starters, but it will be a’smoking, man, once we crank up the beat … I hope that the value that I have in mind will not be with adverse effects upon the sound. 


The configuration is gradually expanding.

Recently added the “second leg” of the push pull setup.

Ultra-Path is implemented in a hybrid manner.  Each tube has a small valued, but high-voltage capacitor (10uF/1500V) between cathode and B+.   At the same time, it has a much bigger electrolytic capacitor between Cathode and the bottom of the “Cathode Resistor”.

Each tube is running in a special kind of auto-bias. It is based on a power thermistor. Specifically, on an array of G9 230V/42W filament based light bulbs.  These behave as barriers, i.e. a device which could be described as a semi-, or “lousy” current source.

Indeed, the filament of the bulb possesses properties very similar to a Positive Temperature Coefficient thermistor device.

(dR/R) / (dV/V) = 0.5 over a wide range of applicable voltages.

This provides added DC operating point security, and indeed, has rescued my back more than one time now. A TURBO Auto-Bias, if you will.

As for the AC signal, it is bypassed around the PTC by means of an electrolythic capacitor (to be further bypassed by foil, MKP, MKT, MKS, … or something, as the project evolves).

The Ultra-Path could also be implemented as a differential setup, from cathode-to-cathode, but I need to try this out first.

Another concept, being currently under test, is a negative current feedback mechanism, but for some bizarre reason, it seems to misbehave and starts to oscillate, irrespective of phase orientation. Need to research this some more. Intriguing.

The current feedback loop basically is a topology where the cathode current of the output tubes is routed via respective half of the symmetrical secondary winding to ground. The idea being that the current flowing through the speaker creates back EM Force which modulates the voltage of the cathode, hence providing a negative feedback signal, together with an error voltage.

At least one phase orientation should work properly. Actually – IT already DID work. But for some reason or another, during the last few hours – it stopped working, and I do not yet know the reason for this.

In general, transmitter tubes … well … guess WHAT !!!  They simply LOVE to transmit. Something. ANYTHING.  They will TRANSMIT, or oscillate, even if you do not want them to.

Some stray capacitance here, some stray inductance there, too long a piece of wire to top it off – and the result – will simply drive you crazy.   Fast and Dirty setups are not the best idea whilst implementing projects which are based on I-Love-to-TRANSMIT types of tubes.

Anyways, some films and some photos follow:






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