Output impedance for EF86 pentode with split plate load resistors?

[jimboyogi]

Apologies if this has been covered properly elsewhere, but my searches haven't found the answers yet.

I'm trialling a design that uses an EF86 as the first gain stage, with a 100K plate load resistor. Rs = 560K, Vgain ~ 130x. Sounds great.
But…
I want to take off signal to feed a reverb driver stage, and the signal voltage at the plate is WAY too much for the driver stage. To avoid extra AC loading of the EF86, I tried using a split plate load of B+ - 33K - reverb takeoff - 68K - plate & main signal takeoff. The reverb signal voltage is now appropriate. But to help select coupling caps, and just for general understanding, i want to find the output impedance from the plate load resistor junction.

My understanding of the output impedance from the EF86 under standard conditions/component values, is that the internal plate impedance is very high, and can be treated as infinite for basic calculations. So the output impedance at the plate is ~ equal to the plate load resistor, in this case 100K ohms.

But using the split load, and taking the signal from the junction of the 33K (to B+) and 68K (to plate). If the 68K resistor is in series with the internal plate impedance, then this is also very high/treated as infinite, so the output impedance from the junction is ~ 33K?

Thanks for any help!

[pdf64]

Also consider the loading that the volume control (or whatever) puts on the main plate output. That will be in parallel with the total plate load and the tube's plate resistance.
But assuming the volume control is 1M or 500k, it's insignificant compared to the 33k, so if you go with that it will be plenty close enough.

[jimboyogi]

Thanks pdf64! The ground reference resistor for the reverb driver stage is 1M, so ~ 33K the output impedance is.

[Dave H]

The ground reference resistor for the reverb driver stage is 1M, so ~ 33K the output impedance is.

If you want to select the value of the coupling cap use 1M (or 1M + 33k) not 33k.