Tyler,
no problem, take your time
.
BTW, abandoning the ultra-expensive
12AX7 (one always ends up with strong desire for a gold-pin diamond-bottom Telefunken
) and using the comparatively dirt-cheap 5687 always is a good idea, particularly if the high gain, and the sweet coloration are not asked for and the high input capacitance and plate resistance hurt.
Yes, there is also a diamond-bottom $$$ 5687, its the 6900.
Differntial cathode follower: u may have noticed i wrote "This is a diffentially working cathode follower pair." It is infact differntially working, But it is no differntial amp stage. It behaves like such with one exception: both + and - side of the "floating" input signal have to referenced somehow to ground, atleast with a high impedance, as if both sides of this input together float around DC-wise, the cathode followers will follow mercilessly.
i must admit, i haven't tried this out, i also admit, i do not dare to in a DC-coupled amp.
Okok, with output coupling cap, this does not hurt that much AC-wise, provided the stage is not yet into saturation.
But a truly differntial amp stage (or stage input like an input transformer) will not respond at its output to common mode input.
Plate resistor of T1:
differential pairs AKA long-tailed pairs have been often been described/derived as a cathode follower driving a common grid stage. Absolutely true and wonderfully suited to demonstrate a lot of math concerning CMRR and symmetry, it is not illustrating at all.
Imagine both tubes as equivalent and equal. Then use a 1st class CCS, not a resistor, between virtual ground and ground or B- . This CCS forces the current thru it to stay constant. Kirchhoffs law #1 applies: sum of currents going into a node is exactly equal to sum of currents leaving a node. Leaving current is kept constant. So if one tube increases current, the other has to decrease its current by the same amount, otherwise the CCS intervenes by changing the potential of virtual ground in a way total current remains constant. Both inputs having the same common mode signal makes the CCS rigorousy lifting or lowering virtual ground thus cancelling any current change in the plate resistors. Only a potential difference at the inputs causes an exactly inverse current change in both tubes.
Both inputs have high impedance, but not equally high. As hagtech wrote (thanx, hagtech !), one input has no Miller feedback hence lower input capacitance. Both grids used as differntial inputs, fine, and we have one input having an input impedance and capacitance differing to the other. Great
If both tubes have the same load (even if one load produces unused voltage swing, only used to heat up the universe), the both have identical input impedance.
I have not tried out much so far but this i have tried out: if you want a long-tailed pair behave symmetrical, build it as symmetrical as possible. Keep both plate loads.
The beauty of a long-tailed pair is that even if you use it single ended at input or output, it keeps its fancy properties like low THD, fantastic CMRR. You just loose 6dB of gain. As return, u have the freedom to choose inverting or non-inverting mode.
Symmetrical / SE:
going SE is not the only target one should neccesarily aim at. One way among other to get happy. REcently i had a discussion with my buddy Hartmut (AKA hifidaddy) who has tube experience but is probably as deep into SS design as Jocko or Harry and as far as sonic hedonism is concerned, probably way deeper. He compared his practical tube and SS experience concerning cascodes and long-tailed pairs, reported that he was never really happy with cascodes although he preferred tube cascodes to SS ones and that he always was happy with proplerly built long-tailed pairs.
Having the long-term listening experience with two tube preamps in mind, one of them having SE tube/SS cascodes as gain stages and the other having long-tailed pairs of identical cascode circuits, Hartmut's statement made me very alert. Because i was not happy with either amp. I did not know exactly which of them to like -- or should i say loathe -- better and found myself slightly preferring the SE unit. HIFI-wise the differential was balsting the SE one into pieces, never had such a marvelous and detailed low end before.
But way worse, i escaped listening to music with both amps, did not know whether it was the fancy complex CF output or the cascodes, so i abandoned both for my projects, no cascodes at all and no CF except where they are suited for the job and facing an output load atleast ten times their output Z and as real/resistive as possible. But long-tailed pairs i will use. Never found anything wrong with them sonically and in one case a power amp built from long-tailed pairs throughout was as magic as another SE amp, just way better low end.
