DC on the grid of 2 of the 4 el84 tubes

[Guy77]

Hi everyone. I am having an issue with 3 volts of DC voltage on the grid of 2 of the 4 el84 tubes in my liverpool amp. It happens only when I turn up my ppimv master volume almost all the way (basically the master is bypassed and coupling caps are seeing 220k to ground). This is causing the tubes to red plate. The other 2 el84 tubes are seeing 0 volts DC on there grids and working fine and the coupling cap feeding those 2 tubes also has 0 volts dc. But the other coupling cap is passing 3 volts dc when I open up (bypass) my ppimv master volume.
I thought it was a bad coupling cap so I swapped it out several times with caps that are working fine in my other amps and still I am seeing about 3 volts dc on el84 grids of the 2 tubes that feed off of that coupling cap!

I will try to remove the ppimv tonight and see how that goes.
Just wondered if anyone else ever experienced this.

Thanks

Guy.

[JazzGuitarGimp]

Try pulling both el84's on the side in question, and see if you still have DC on the grid pins. If you do not, put one tube at a time back in and re-check. This will tell you if you have a bad tube, and which one it is.

[martin manning]

I think leaky coupling cap on that side.

[Guy77]

In the end it was a bad tube. I replaced a pair of tubes ( out of the 4 tubes) and all is good now. One tube in the matched pair of JJel84 was defective and causing dc to leak. The coupling caps have 0 DC volts on them now. I am running 2 sets of matched tubes as opposed to a matched quad. Each pair has its own cathode resistor with capacitor.


Thanks JazzGuitarGimp.

Cheers
Guy

[JazzGuitarGimp]

Ah, glad you got it sorted!

[martin manning]

I guess I should have read to the end... In this situation a good way to see if the tube(s) are at fault is to swap the pairs side-to-side and see if the problem follows the tubes. If it does, then you can swap one-for-one and find the bad one.