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Aug 07 2013

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The Single Super Stage (2)

The Single Super Stage (2) …

Well, I did some drawing, and here it is …

Single Super Stage - Direct Coupled Differential

Single Super Stage – Direct Coupled Differential

 

So this is in general what I am thinking about.   The triode current sink V1 for the input triodes V2, V4 could be alternatively replaced with a pair of BC550C transistors in a silicon based current sink configuration. Then the -Vcc would suffice if it is about -12VDC.  The input tubes are directly coupled with the grids of the second set of tubes V5, V6.   The tubes V7 and V8 constitute Cascodes.  Their fixed grid voltage implies a fairly stable cathode voltage, hence the anodes V5 and V6 see a constant voltage on their anodes. The only thing they modulate is the current flowing through their anodes. The V9 and V10 are the active loads, configured as if the upper part of an SRPP circuit, but with the low-output impedance mu-follower style output tapping option, directly from the cathodes of V9, V10, rather than from the anodes of the cascodes V7, V8.

This whole circuit constitutes a single voltage amplification stage, with direct coupled “Williamson” style differential input stage.

If you really want to push it to the maximum, { but do not over-push it into oscillations :)   }, it would maybe even worth considering to replace the R1 and R2 with two  high voltage versions of constant current sources, lets say with the use of a DN2540 depletion mode mosfet with some nice current setting resistors in their source, with gate tapped from underneath of the resistor.

dn2540 ccs cascade

dn2540 ccs cascade

dn2540 ccs cascade

dn2540 ccs cascade

The above drawings show, just for the sake of completeness, how to build a current source on the DN2540 mosfet. The upper one uses a cascade of two DN2540, with possible parameters regarding dynamic impedance presented on the side.  Below, a very simple CCS, which fairs “worse” – but the funny thing is that it (probably) suits our purposes better, because we do not want to “over-do” it.  As for CCS for the pair of input tubes, I would get rid of those anode resistors an rather put in such a CCS instead.  This alone would boost the gain of the first pair of tubes, and allow them to work in a much more linear fashion (prior to feedback). The values of resistors need to be recalculated, because the pictures are from a different context, just as a visualization of the topology of the DN2540 CCS  options …

Now, where was I ? Ah, yes …

So what this really comes down to ….

If we supply this stack with a voltage of say 400VDC respective ground, then each level of the tubes shall receive a fair share of working voltage to operate upon.

For the sake of discussion, lets assume following voltages:

V1 anode = 0VDC (ground potential adjusted).V1 cathode = a reasonable -Vcc,  lets say -100VDC  (alternatively: -12VDC in case of a npn transistor based current sink).
V3 anode = +100VDC  (the second current source with elevated DC offset)
V2, V4 anodes = +90V  (as in a Williamson)
V5, V6 anodes = +180VDC (as in a Williamson)
V7, V8 grids = +176VDC ( thereby setting  a fixed voltage of their cathodes, as well as on the V5, V6 anodes as above).

Ok, till now we “consumed” 180VDC of the available headroom. If we have at our disposal say 400VDC, that leaves us with 220VDC to be reasonably distributed between the cascodes V7, V8 and the active loads V9, V10.  Lets assume that the rest is equally subdivided between the cascodes and the active loads, so we adjust the resistors so as to obtain a voltage of about 290VDC on the cathodes of V9, V10.  That gives us sufficient headroom for a healthy signal voltage swing in both directions.  If you need more output signal swing – do not sweat. The cheap electrolytic capacitors come even in a variation for 450VDC, so we can push the whole thing some 50V higher, if need be.   {{ By the way, a 450V supply … is it not a healthy voltage for KT88 based output stages ? what a nice coincidence !!!  :)   }}

We can push the whole some more, read a thing or two from Alan Kimmel, and replace the R3 and R4 with Chokes, as in the Allan Kimmel CAMUS stage (Choke Assisted MU-follower Stage).

The more I look at it, the more I like it.

Need to sort out the issue of “converting” that positive feedback into a negative one, though.

Best Regards,

zjj_wwa

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