EL84 amp hum in triode mode

[punkykatt]

Hello all, a friend of mine asked me to install a pentode/triode switch in his VHT 36 watt head.  In the triode mode  I get a low pitch hum, in pentode its fine. The switch is located on the back panel right between the four  EL84`s and all wire runs a short as possible. Attached is a schematic how I have the switch wired.  Has anyone else had this problem?  Thanks in advance for any help or comments.

[sluckey]

Swap the two wires on the center lugs of the switch.

[punkykatt]

Thank you Steve.  still have the hum in triode mode. 

[VMS]

Just thinking out loud here but maybe the noise in triode mode is because b+1 node has less filtering than B+2 node 

[shooter]

Maybe also just hardwire triode mode for testing to eliminate wire noise

[punkykatt]

VMS, I tried clipping in a 33uf/450v  over the 1st filter cap to make 66uf no change in hum.   I have put  Pentode /triode switches in other amps but they either had 6V6`s or 6L6`s with no problems.

[punkykatt]

Found the source of the hum in triode mode.  The quad of el84`s  were not matched very well   one pair was drawing almost twice the currant of the other pair.  I put in my test quad  of JJ`s , hum gone.  Im glad I built  bias checkers for el84`s.  Thanks to all for your replies.

[tubeswell]

Yep it helps to have quads that are reasonably closely matched (same with sextets  )

[sluckey]

Swap the two wires on the center lugs of the switch.

Did you leave these wires swapped? I mentioned it because your schematic shows them crossed.

[punkykatt]

Steve, I have to correct that schematic. The screens are connected to the wrong plates in that schematic.

[punkykatt]

Corrected schematic