EF86 troubleshooting

[Alex R]

I'd appreciate a bit of help understanding that little devil the EF86.

With a 12AX7 triode the cathode voltage tells a lot about its operation - one or two volts on there and all is probably well with it. Checking for DC on the grid, and voltage drop across the plate resistor, also helpful signs. What sort of ballpark things might we look for around an EF86 in circuit to make sure it's operating right? Sorry if it's a stoopid question...

[Steve Conner]

Exactly the same things, except you're also looking for a sensible screen voltage. But bear in mind the screen current on an EF86 is low enough that the meter might influence it.

[tubeswell]

EF86 circuit design is covered quite a bit in Chapter 3 of Merlin Blencowe's 1st book Book1.html (which is a bargain for the amount of information it covers). Mandatory reading