ECC83 Load Line helper doc

[Gtr0]

Hi Folks.

I am making a load line sheet for myself to help further in creating circuits, calculate current draw etc etc. Everything I have learned has come from Merlin's book Designing Valve Preamps for Guitar and Bass, or from Rob Robinette's website. Both very helpful sources that I frequently utilize. The load line sheet has a column with notes and formulae to get me through as I am still in the learning process of load lines in general.

Here is a shared link to the file @ my Google Drive - ECC83 Load Lines - and I would like to know if someone could take a look and determine if a. it appears that I may have a grip on it and b. to post any error findings, my misunderstandings or improvement tips.

One thing I am unclear of is when drawing an AC load line: The impedance load that follows the coupling cap is obvious in some instances, but in other instances I am not sure where to stop the calculation. For example in a high gain circuit such as the SLO, there is a voltage divider following the coupling cap. Do I take both resistors in account to determine the load? Or sometimes there is a resistor in series with a gain pot - Do I count the value of the gain pot as well? I'm just not sure when to stop. I hope that's clear.(?)

From Designing Valve Preamps for Guitar and Bass:

Fig. 1.24: Coupling capacitor Co blocks the DC anode voltage. The DC load on the valve is simply Ra, but the total AC load is the parallel combination of Ra and Rl.
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From Rob Robinette's load line page: HERE

The AC load on our triode equals: plate load resistor + cathode resistor in parallel with the impedance of the following circuit. In the 5E3 the following circuit is the phase inverter with a 1M grid leak resistor and a 56k phase inverter tail resistor.

plate load 100k + cathode resistor 1.5k || 1M grid leak + 56k phase inverter tail resistor.

[Malcolm Irving]

As Rob Robinette says, what you need is the input impedance of the following circuit. This is not always easy to work out for more elaborate circuits.

For AC signal, you can take B+ as being connected to signal ground (since B+ is connected to ground via some big capacitance). You can also take input to a grid as infinite impedance (unless you are overdriving the grid). You can then combine the various resistors in series and/or parallel.

But even then it’s not that simple, because the gain of a tube can ‘bootstrap’ a resistor, for example the input impedance of the Long Tail Pair Phase Inverter is actually about 2x the grid leak resistor. (See page 163 of the 2nd edition of Merlin’s book.)