In article , "Ian
Iveson" wrote:
"John Byrns" wrote
As near as I can see the Quad II uses an ordinary paraphase phase
inverter, not a "floating paraphase" phase inverter? I also can't
see any
large use of positive feedback in the Quad IIcircuit. I see a
very small
incidental amount of positive feedback that appears to be a side
effect of
the way the loop feedback is connected.
Hmm, maybe, just hovering perhaps. Certainly not "ordinary", because
the two valves are coupled in a host of different ways. Just been
looking it up. I was wrong about the "local". Here's the circuit in
case anyone doesn't know it:
http://www.triodeel.com/quad2.gif
I forget which end of the floating paraphase is supposed to float. I
think it's something of a misnomer anyway.
I may not have the precise definition of "floating paraphase" vs. ordinary
"paraphase" phase inverter, but I consider the ordinary "paraphase" phase
inverter to be one where the phase inverter valve runs open loop, with a
divider at its input to knock the gain down to -1, while I consider the
"floating paraphase" to be one that uses the phase inverter valve in a
op-amp type configuration with a closed loop gain of -1. The Quad II
circuit topology would fit the latter description, except that the 100 Ohm
resistor at the common point is so small that it effectively becomes an
ordinary "paraphase" phase inverter, if this resistor were 1000 times
larger, then it would be a "floating paraphase" in my book.
The circuit shows unbypassed 680 + 100 ohms common to the coupled
cathodes. Feedback from the secondary winding is applied to the
junction of those two resistors, so it appears as -ve to valve 1 and
+ve to valve 2.
It appears as -ve to the upper valve, but there is effectively no
feedback, either -ve or +ve to the lower valve because the feedback signal
is also applied to the grid virtually undiminished by passing backwards
through grid voltage divider for the "paraphase" phase inverter.
There is some positive feedback to the grid of the lower output valve, and
negative feedback to the grid of the upper output valve that I didn't
notice before, don't know how much feedback occurs around these loops from
the output transformer to the grids of the output valves, whether it is
significant, or is relatively insignificant? These feedback paths look
like a side effect of the main negative feedback scheme that couldn't be
easily eliminated like the positive feedback to the lower phase inverter
valve was.
Looking at the input of the power stage, the grids are connected via
680k to either side of a single 2k8. There is no central ground but
rather this 2k8 floats, with one end connected to valve 2 grid, and
the other to same cathode junction as the feedback. OK, so that's
not floating much, because there is a low resistance path to ground
via the feedback winding in parallel with the 100ohms, but the
floating paraphase is never really floating is it?
It is rarely completely floating, there is one rarely used circuit
configuration where it is completely floating, but in most circuits it is
floating for all practical purposes. In the Quad circuit the junction is
effectively grounded by the 100 Ohm resistor the feedback is applied
across, effectively making it an ordinary "paraphase" not a "floating
paraphase".
When the splitter is balanced, the signal either side of that 2k8
resistor must be equal and opposite. It may seem that the +ve fb to
V2 cathode is matched by the -ve fb to its grid.
But how do you explain how come 2k8 is such a small proportion of
680k? What is the gain of V2, and how does it compare to V1?
Remember that the grid signal for V2 is developed across just half
of that 2k8, since ac ground is half-way along it.
I don't see any AC ground half way along the 2.7 k resistor, all I see is
the bottom end of the 2.7 k resistor grounded by a 100 Ohm resistor. That
makes the loss for the feedback signal around the inverter valve something
like a factor of 0.00015, while the gain of the valve is 360 at the very
most, hardly a combination that would make a very good unity gain
inverter, especially when you consider that the input signal to the
inverter valve is scaled by about 0.004 at the input, about right for the
likely actual gain of the inverter valve operating as an ordinary
"paraphase" phase inverter.
Looking for some words from my paltry library, I find JL Hood
"...gain of V2...rather more than twice than that which would have
been expected". Expected, that is, by someone not seeing the +ve fb,
he says. Irritatingly, he fails to mention the link between the
feedback and that 2k8 resistor. It requires a fair amount of maths
to work out the whole explanation, but the gain of V2 is
indisputably much higher than it could be without +ve fb...that fact
doesn't take too much calculation.
I don't follow any of that reasoning, it doesn't ring true either.
Oh...and the screens are floating with no reference at all to
ground. Shouldn't I get half a point for that?
Yes, certainly, that's an interesting detail.
Regards,
John Byrns
Surf my web pages at,
http://users.rcn.com/jbyrns/