As I try to unlock the last know issue with #Yet Another (Discrete) Clock (namely : amplification without affecting the oscillator or wasting energy), I stumble upon very nice pages about the circuits of yesteryears in the audio range. In particular I find the discussion in 8 - Cathode Follower very refreshing and useful (the kind I'm looking for in this type of WTF project with no direct usefulness). I learn about the existence of internal plate resistance (rP), the degeneration of tetrodes and pentodes when used as cathode followers...

I just have to quote a useful paragraph, about something else I didn't find elsewhere or that could discourage me :

One always has to be careful with valve circuits though, as it's easy to exceed the maximum allowable cathode to heater voltage because the heaters are nearly always ground referenced. If the voltage is exceeded the valve may be damaged, but even if it survives it may not function properly.

[...] Unlike JFETs, there is a definite limit to the upper value of the grid resistor, determined largely by the materials used and the geometry of the valve's internal structure. If the resistor value is too high, the valve will attempt to bias itself as the grid collects stray electrons. This is called 'grid leak' or 'contact' biasing, and generally uses a resistor of around 2.2MΩ to 10MΩ or thereabouts. The tiny current flow (typically less than 1µA) causes a voltage to be developed across the grid resistor (negative at the grid) which biases the valve. In general, grid leak bias is rather unpredictable and is usually a bad idea, and it should be avoided.

The page describes a simple current amplifying circuit (impendance buffer) with many details that are rarely mentioned otherwise, and precious because, since I won't build an amplifier, I won't follow well-established circuits and I'll be on my own, trying to decypher ENIAC's diagrams...

So I learn that 1M Ohms is a reasonable grid resistor. It will be fine because I'll remain in the digital domain but even there, the "contact biasing" will bite when several inputs will be connected together. Fan-in seems to be an issue since the grid resistor must be divided by the number of grids, or else "things will turn bad". But lower impedance means higher driving current, larger capacitances, more RC delays and more amplification...

As I consider a 8-bits circuit, this should not become insurmountable but will be another area to keep in mind during the design.

Then there is this link to a pretty practical page about the tubes: http://sound.whsites.net/valves/index.html

The warning about high voltages is why I chose low-voltage pentodes ;-) but this is a chilling reminder concerning the 200V needed (according to the available informations) by the 6N21B.

Thanks to Elliott Sound Products for sharing their experience :-) I'll remember this quote :

'guitar amp' and 'careful engineering' generally do not belong in the same sentence.