Need help restoring 1966 bassman to original

[corduroyew]

I got a 1966 bassman off ebay. The seller clamed it worked just fine and that it was all original except for the addition of a 3 prong cord and the tubes were brand new. Well… This wasn’t the case.

One of my big concerns is that the bass tone knob on the guitar side doesn’t do anything and the bass tone knob on the bass side does almost nothing. I tested the pots with my multi-meter and they give me the correct readings so I was wondering if anybody know what I might need to check next.

My second concern is that I can see that some parts have been taken out. There 2 wires each of them runs from pin 3 of the 6l6GC tubes and then runs through a 220 resistor before connecting to the circuit board. Because I don’t really know how to explain it, here is a pic of the layout with the area circled where the resistors are missing.



So what I need to know is what this mod did. Is it something that I should change back? Or should I leave it alone? Will changing it back hurt anything?

My last concern is that the 3 prong plug was added but the ground switch was not disconnected. Should I disconnect the ground switch or just leave it the way it is?

[TD_Madden]

regarding the ground-switch, when rewired with a new power-cord I usually leave the switch in, but clip-out the cap.

[corduroyew]

Ok, I'll clip the cap then. Thanks for the reply.

[Sweetfinger]

Those mystery resistors are part of a negative feedback circuit that was abandoned/changed. Are you sure parts have been removed? Fender was very fond of modifying the bassman, you'll see newer revisions cobbled around earlier boards leaving unused holes or layouts that don't quite match the diagrams.

FWIW anyone using the amp for guitar would probably prefer those particular resistors to remain missing, if they were there at all.

[corduroyew]

I am sure part were removed because they just clipped them out. The tails left behind are what drew my attention to this to begin with, however, if the resistors were just for negative feedback then I think I might leave them out.

Thank you very much for the help

Now if anybody knows where I need to start looking to figure out why the bass tone control on the guitar side doesn't work... I was hoping it would be something simple like ‘put those 2 resistors back in and it’ll work fine’ but I guess I’m not that lucky.

[Bruce / Mission Amps]

Has anyone else ever noticed that when clipping those 220K resistors out and redoing the lead dress without the wires, those amps seem to hum more?

[corduroyew]

^this amp does seem to hum a lot. I just figured it had to do with the age of the amp. Do you think that putting back in those parts will get rid of some of the hum?

[capnjuan]

[QUOTE=corduroyew;24854]...this amp does seem to hum a lot. I just figured it had to do with the age of the amp...QUOTE]

I have 45 year old amps in VG electronic condition that are are dead quiet from idle to 9. You could say 'all amps hum some', you could say 'old amps hum some' but old amps that hum alot do so because they need work, not because they are old.

[corduroyew]

I just got around to putting back in those missing resistors today. All the extra hum I was getting is gone now. I also noticed that the fizz I was getting right when the amp started to break up (which sounded really bad) is now gone. Comparing the 2, I have no idea why anybody would ever want to take those resistors out. Thanks for the help you guys gave me.