12AT7 split load plate res - output impedance?

[lion]

I've been researching the topic of split load plate resistors on a valve stage. From what I found it seems applying a split load plate resistor setup change the output impedance - but I haven't been able to find the rules or how to calculate the resultant impedance.

The circuit in question is a 12AT7 in grid leak bias config (cathode to ground) and a 47k plate resistor, which according to http://www.ampbooks.com/home/amplifier-calculators/output-impedance/ has a output Z of 8k8.

I'd like to know/calculate the resultant output impedance with the plate resistor split in 15/33k. Any help advise much appreciated.

[diyDog]

With a single resistor, the output impedance is the parallel combination of the reciprocal of the tube's output conductance and its load resistor.  For a split resistor you've got to add the reciprocal of the tube's output conductance to the resistance connected to the plate, and then calculate the parallel combination of that resistance with the resistor that goes to B+.

[PRR]

> I'd like to know/calculate the resultant output impedance with

Why? Tube amp node-impedance isn't an exact science, and your design should not rely on any specific node-impedance.

First you need to know the TUBE impedance. At the *specific* operating condition you are working at.

It is interesting that Kuehnel's calculator does not ask what your supply voltage is. It isn't a big difference 200V or 400V, but if you are doing wee-Voltage work like 12V supply the plate impedance goes way up.

The calculator tells the node impedance but not the plate impedance. However you can work it out. If the 47K+12AT7 is 8.8K, what is the plate impedance?

Remember that the B+ is "ground" for signal.

Now cut-up the 47K as 15K+33K. Re-draw the circuit for the new scheme. It is a simple series-parallel circuit. Work-out the node impedance.

[lion]

Thanks for the replies

So with the 47k plate res the impedance is 47k//10.9k = 8.8k (with 250V plate voltage),
and with the split res it's 15k//33+10.9k = 11.2k

To be honest I'm a bit confused and don't fully understand the diff's between output impedance vs node impedance (?)
With a RC network after the triode - or even just a cap to ground (forming the RC with the output impedance) - what is it the cap see?