Expected voltage drop with/without tube installed

[goldenGeek]

Hello
While messing around with a repair (covered in another thread) one technical question popped up, and I'm curious if there is a rule of thumb (or scientific answer) regarding this case. If we use a 12AX7 in lets say V1 in the attached schematic and you're reading 140 VDC at pin 1 on the socket with the tube installed - what would you expect to read if you pull the tube out..?

V1a-voltage-drop.jpg

[xtian]

First, there is a large voltage drop across the plate resistor, when a 12A?7 triode is conducting properly. So, you might see 140v at pin 1 when operating normally, but if you pull the tube out (or if the tube fails completely) you will see B+n at pin 1, like 300v, because there is no voltage drop across the plate resistor without any current running. This is a quick and easy way for you to know that a triode is conducting.

Then there is HT system sag--each time you add more load (like another tube), B+ sags a bit. I don't know any rule of thumb for this--dependent on the PT, filtering, other tubes installed, etc.

[sluckey]

If we use a 12AX7 in lets say V1 in the attached schematic and you're reading 140 VDC at pin 1 on the socket with the tube installed - what would you expect to read if you pull the tube out..?

I expect to measure the same voltage as on the top side of that 100K plate load resistor. No tube, no current flow, no voltage drop across that 100K.

[roberto]

I expect to read more than what you usually read on top of the 100k when the tube is in the socket, because you avoid the voltage drop due to that tube across all resistors upstream in the voltage supply.

[goldenGeek]

Thank you for your replies, its starting to make sense in my head

[Stevem]

This also means that the stronger / better Gm testing a tube happens to be, the greater that voltage drop across the plate load resistor will be.