The regular Muchedumbre is an ultra-simple buffer with few parts and straightforward operation. It’s a great beginners circuit for high voltage tube applications.
If you’d like something with just a little more nerd sprinkled on top (and an extra 12AU7), try the following. This XL version uses a White Cathode Follower buffer for about half the output impedance of the vanilla version. It requires a few extra resistors and caps and the heater reference voltage is tweaked just slightly so I can sleep better.
For ideally symmetrical drive ability, the series resistor in the anode of the upper 12AU7 should be calculated as:
Rseries = (Rp + 2 * Rload) / Mu
If we break down the circuit into a cathode follower (upper triode) and a grounded cathode amplifier (lower triode), we can see that this creates a nifty push pull circuit. The cathode follower is non-inverting, so it’s pulling the output in the same direction as the input signal. The lower triode is a grounded cathode amplifier and so it inverts the input that it sees. But the input that it sees is from the anode of the upper triode, which is already inverted. You invert the inverted and you get non-inverted (same ‘direction’ as the upper triode). Tada! Push (lower) pull (upper).