Mod Bass Channel on A Fender Bassman AB165

[cluster]

Hello All,

Got me a drip edge Silverface Baseman AB165 at a great price. Went ahead and modified the output to AA864 spec. Put 0.047uF on the output instead of the 0.1uF. Made it a fix (adjustable) bias. 47k output plate resistor as per AA864. 820ohm and 100ohm on the negative feedback (AA864). 500pF coupling cap just before the PI. As for the Bass Channel I just basically modified the tone stack to .022uF/.022uF/56k/330pF and left everything else as is.

I was curious as to what others had done with this channel to make it more useful. I see an unused triode sitting empty that has me tempted to add a gain stage to the first channel. Has anyone used the extra gain stage here? The deep switch is waiting to be modified as is the ground switch ing the back. Also, has anyone switched out the 250k bass pot for a 1meg pot in channel 1?? Attached is a gut shot of what I have so far. Thanks

[tboy]

I guess the amp isn't squealing or you would have mentioned it, but it is on the verge of squealing if the speaker jacks are still wired as shown in the photo. You need to swap the connections of the green and black leads that go from the output transformer to the speaker jacks, so that the green wire goes to the tip lug of the closed-circuit jack and the black one to the sleeve lug of the open-circuit jack, as shown in the AA864 layout diagram. This is because the feedback connection has been moved to the opposite side of the phase inverter, where the feedback will actually be positive unless the output leads are also reversed to maintain it negative..

[cluster]

I guess the amp isn't squealing or you would have mentioned it, but it is on the verge of squealing if the speaker jacks are still wired as shown in the photo. You need to swap the connections of the green and black leads that go from the output transformer to the speaker jacks, so that the green wire goes to the tip lug of the closed-circuit jack and the black one to the sleeve lug of the open-circuit jack, as shown in the AA864 layout diagram. This is because the feedback connection has been moved to the opposite side of the phase inverter, where the feedback will actually be positive unless the output leads are also reversed to maintain it negative..

Thought I did that. Maybe it's not clear but I do remember doing that a while back. thanks

[g1]

Your photo shows black to tip and fb wire is connected to that.
So it has not been changed at the output jacks. Perhaps you flipped the phase elsewhere?
Simple to check, with test tone applied, disconnect yellow fb wire, volume should increase a bit.
If volume decreases, you need to correct the phasing.