presence control not working

[punkykatt]

Hello all,  A local guitar player asked me to do some mods to a Weber kit Bandmaster 5E7M. (the M means a midrange pot instead of a presence pot). Drop the power with 6V6's instead of 6L6's and a NOS 5Y3 instead of the 5U4GB, also remove the midrange pot and install a presence pot, remove the normal channel and parallel V-1 and put a master volume in place of the normal volume pot. His spkr cab is 8ohm so Im using the 4 ohm spkr tap for a 8k/ 8 ohm turns ratio. The master vol scheme i used puts a 1m pot after the .02 cap that is before  the PI and the other tab to ground. there is an other .02 cap on the wiper going to a 2.2m gird resistor to node B in power rail and a 1m grid resister going to ground instead of being bootstrapped to the PI cathode. The 1.5k PI cathode resister is replaced with a jumper and just the 56k  to ground is remaining ( Paul C Mod).  Everything works fine except for the presence pot, a 5k with the NFB resistor going to one outer tab and a 0.1/250v cap from wiper to ground. I tried different value caps .01 to .2, different NFB resisters from 51k to 150k on all three spkr taps with no change. Did a listening amp  test to make sure signal is getting to the 5k pot. I put the 6l6's and 5U4GB back in and adjusted the bias still nothing.  Does anyone have any ideas what I should try next? My brain is fried at the moment. Any help is much appreciated. Thank You

[PRR]

The Presence network is a NBF scheme. If you turn-down the master volume in the middle of the power stages, NFB can't do anything. Turn that MV up FULL and report back.

Did you remove C15??

If that's not it: that's too many mods for a text description. Crayon-out an as-built schematic.

[sluckey]

Look at a real bandmaster schematic.

[punkykatt]

PRR, thanks for the reply. C15 was removed. Yes I had the master at max volume when testing. Your schematic is what I have in the amp

[pdf64]

If the feedback loop is working, then when the 56k series feedback resistor R34 is disconnected, the gain will increase. Does it?

[punkykatt]

pdf64, when I disconnect the NFB resistor the gain decreases slightly. ???

[sluckey]

That usually means the phase is wrong. Swap the OT primary plate leads to correct.

[punkykatt]

That was it!!!!! Thank you everyone for your input, my old brain has been doing a lot of farts lately. A 100k NFB resistor worked better than the 56k with the 8 ohm speaker.