18 watt plexi with 6v6's has squealing problem?

[chuckb]

Hey guys, I've built a 18 watt plexi wuth 6v6's using Mark Huss' schematic. I have both channels running into one input and when I adjust the volume pass 9 on either ch I get a loud high pitch squealing. When I lower the volume below 9 it works fine. It also appears that when adjusting the presence that It has some affect on the squealing as well. I did the chopstick thing to see if there was any problems with the wiring layout and nothing seems to affect the squealing in anyway. Any suggestions on where else to go from here?

Chuck.

[Jazz P Bass]

Try reversing the output transformer primary wires (Brown & Blue).
It sounds like positive feedback is occurring.

[chuckb]

Try reversing the output transformer primary wires (Brown & Blue).
It sounds like positive feedback is occurring.

yes, i've already tried switching the ot primarys,it never helped. I turn on the amp and there is no problems with squealing without the guitar cable plugged in. when i plug in the guitar cable and adjust the volume past 9 i get the squealing.so I guess it's somewhere on the input side.I have a shielded cable on the input and it's grounded so I may need shielded cable between the 68k input resistors and pins 2&7 on V1. It seems that I may be interference from the output tranny being to close to those wires.

[Bruce / Mission Amps]

I real wild ass guess but I wonder if your pickups and or volume pot on the guitar are not grounded correctly to the guitar cord and or the amp's chassis, or or there is a coupling cap blocking DC ground for the grid of the first stage and the preamp stage is running with no grid bias... or reference to ground.. just thinking out loud....
other wise it sounds like, positive feedback, or parasitic oscillations from the driver stage, NFB stage or something like that, back to a previous stage.

[Chuck H]

With no guitar plugged in the input grid should have a near zero DCV reference. This should be the same with the guitar plugged in and the guitar volume on zero. Does the feedback happen with the guitar volume on zero?

You mention that you've tried reversing the OT leads. Did that change anything? If yes, what?