Calculating output impedance of Morgan-Jones headphone amp?

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
When I read this page:

HeadWize - Project: The Morgan Jones Mini Tube Headphone Amplifier by Chu Moy

it says that the calculated output impedance of the first amp is 6 ohms.

I'm trying to teach myself how to calculate output impedances but I can't arrive at that 6 ohm figure no matter what I try. Let me explain what I'm doing and hopefully someone can enlighten me?

Looking at that schematic, it looks like the output is seeing the Ra of one triode (I'm using Ra=2600 ohms) in parallel with (Ra of the other triode - 2600 ohms - in series with R4 which is 3300 ohms).

So I'm taking: 1/(1/2600 + 1/(2600+3300)) = 1804 ohms

Please forgive my low level of knowledge, I'm trying to learn. Thanks in advance for anyone willing to help me figure this out!
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.