LCR phono stages we know about

John, The input stages of the Allen Wright RTP (shown in the schemtic posted by Hiho) and of the Atma-sphere MP1 are both dual-differential cascodes , i.e., balanced topologies. Thus in both cases, a CCS is used optimally at the junction of the cathodes of the two "bottom" tubes in the cascodes and ground. (Take a look at the RTP schematic; I think AW used an LM317, which is definitely not my choice, but it's a CCS nevertheless.) Thus there is no place for more CCSs on the plate side. However, your point is relevant to a single-ended topology. But in a single-ended topology, the output Z would be the parallel sum of the impedances of the CCS and of the cascode stage below. The impedance of the CCS is ideally infinite (but is always finite in reality, albeit very high). However, would not the impedance of the cascode below be a very finite number, thus dominant in the calculation of output Z? And also subject to change as the tube(s) in the cascode age? Please correct me if I'm wrong; you probably know more about this stuff than I.

Now I re-read your post, you seem to be talking about a dual-differential cascode with CCSs between cathode and ground AND between plate and B+. AW discusses that possibility in his TPCB. Have you ever tried that?

Ddrive, That's very cool that you have the Wavac. As I recall, it's "big bucks". I forgot all about the Cole; I remember reading about its pro's and cons on Romy the Cat's blog; that was an early contender in the LCR game. I'd love to hear either. That's the only way I am going to gain a feel for these beasts.

Yes you can have a LTP differential stage with a current source tail and current source plate loads. I have one. The trick is the shunt resistor. It bleeds excess current from the plate load CCS to ground and establishes the plate voltage at the same time. This does present a problem if you are using an all vacuum tube cascode in that imbalance currents between the tubes will change as the tubes age and this will lead to differences in the two plate voltages of the pair. If you are direct connected to the second stage as in my phono stage, this is bad and will eat into the dynamic voltage "headroom" of the preamp. However if a SS/tube hybrid cascode is used, the bias is mostly determined by the SS devices used (matching is a must) which generally doesn't change over time. If the plate load current sources are mounted to the same heat sink so they track thermally the problem is solved. It's a shame that the Sonus Veritas phono pre which used this topology never got any traction in the marketplace. One TT manufacturer who will remain nameless compared in to the Ypsilon and he preferred the Sonas.

With the single ended version the cathode CCS is bypassed with a capacitor.

"With the single ended version the cathode CCS is bypassed with a capacitor." This is a sidebar to the topic, really, but are you imagining a single-ended phono input with CCSs between both the cathode and ground AND between plate and B+? Using a bypass capacitor around the lower CCS does not fit in with my understanding of the need for a bypass capacitor, the value of which needs to be inversely proportional to that of the more typical cathode resistor, in order to pass audio frequencies of interest. In this case, the CCS impedance is so high that a bypass cap would seem to be superfluous. Never seen that done.

I am guilty of going off topic in a thread that I myself started up.
I found a nice thread on Wigwam, among some English guys, one of whom built a LCR phono for another member, using inductors built by Dave Slagle. The builder explains that for an LCR, it either has to be driven by a source with an output impedance equal to its input impedance (e.g., 600R), in which case it can be terminated by an impedance that is ideally 10X higher, or the other way around. In other words, when I wrote that there needs to be 600R on either side of 600R LCR network, I was wrong. But 600R impedance is still very difficult to attain with tubes, without resorting to transformers, etc. This guy used 7K inductors made by Slagle.

You need a cathode bypass cap for the same reason you need one in an ordinary single ended triode circuit. The tube for the sake of analysis can be viewed as an AC signal generator whose output is Eg*u were Eg is the signal voltage presented to the grid times u, the gain factor of the tube. The AC signal loop with a cathode bypass cap consists of two resistors, Rp and Rl in series where Rp is the dynamic plate resistance of the tube and Rl is the load. This acts as a voltage divider on the output of our theoretical signal generator. The greater the load resistance in relation to the plate resistance, the greater the percentage of the signal is dropped on the load. With a CCS load the gain of this triode stage thus approaches u.

Now remove the bypass cap. The signal still has to return from ground to the cathode. It no longer has nice near zero impedance cap. It must pass through the cathode resistor. The AC voltage drop is -i*Rk where i is the AC signal current and Rk is the cathode resistor. The output of our tube (signal generator) which was Eg*u is now (Eg-iRk)*u. If Rk is very large this gain stage will have very little to no output.