5E6A Bassman Volume Pot

[Deric]

Got a 5E6A Bassman on the bench.  Amp sounds great.  Only complaint is that the Bright channel volume pot does not completely cut volume.  Measured wiper to ground at zero setting and was getting ~100 ohms.  Normal channel pot measured ~.2 ohms (normal channel does go completely silent).  Swapped bright channel pot and there was a slight improvement but still doesn't totally mute the sound zero setting (double checked that the replacement pot is good).  Swapped both 12AY7 tubes - no change.  Jumpered the wiper to ground - no change.  Jumpered the volume pot side of the 270k mixer resistor - no change.  If I ground the input side of the pot or the coupling cap feeding the pot it almost goes silent.  If I ground after the 270k mixers (grid of V2) it goes silent.  Disconnected the pot completely and ground the wire that went to the wiper - no improvement.


The first 2 tubes share the same filter cap.  Cap tests good (Blue ESR meter) but still tried paralleling a good cap with the 8uf but no change.


I'm thinking the signal is bleeding back through the power supply since the 2 tubes are not decoupled.  Or maybe conductive board....  Or...???   


http://ampwares.com/schematics/bassman_5e6a.pdf


Any suggestions or comments appreciated!

Thanks

[pdf64]

Maybe the cathode bypass cap has high esr, thereby allowing the channels to couple?

[Tony Bones]

Just last week there was this: https://el34world.com/Forum/index.php?topic=24736

[Deric]

Maybe the cathode bypass cap has high esr, thereby allowing the channels to couple?

I assume you mean the bypass cap on V1 ?  I think I checked that but will double check. 


Pretty sure I measured resistance from the filter cap grounds to the chassis and all were less than 1ohm but will double check that too.


Thanks!

[pdf64]

Yes, sorry I forgot that these guys have 2nd stage bypassed too

[Deric]

Replaced the first stage bypass cap and the preamp filter cap.  Re-did all the grounds - they measured OK but had been redone before and looked suspect.  No change.


Lifted the pot end of the coupling cap from the board.  No change unless I ground the cap - then signal almost completely disappears. If I ground the grid of the next stage it goes silent. 


Maybe a lead dress/coupling issue (the grid wire to the nest stage runs under the board very close to the bright channel plate resistor) ??

[Deric]

Disconnected the pots from the mix resistors on the board.  Soldered two mix resistors directly to the volume pots and ran a shielded wire from the resistors directly to the tube socket.  Worked perfect.


Put everything else back to normal.  Pulled the under board wire from the mix resistors to the grid and replaced with a shielded wire.


All is good.