68 Bassman Just Hums after Blackface Mod

cabkab

Member
Messages
234
So I ordered all the parts to blackface my 68 Bassman and went carefully through all of the steps. After I finished the mod I biased the amp at 35mv (using a dummy speaker load) and everything looked good up to that point.

But...
When I plugged in the speaker cabinet and power the amp on I get a low hum and no signal from the guitar comes through the amp. Volume or tone knobs don't do anything to change the hum. Can you check the pics and let me know if you see an issue? I removed the hidden ground wire that used to run under phase inverter section.

The only other thing I did is I bypassed the ground switch on the AC power. Did I do that incorrectly? I ran the white to the power switch and the black to the fuse.

Here are some pics:











Any ideas?

-Ken
 

Blue Strat

Member
Messages
30,204
Yikes, you made a LOT of changes if you did the typical conversion. All it takes is ONE wrong connection.

In the future, it's best to do only a few changes at a time and test after each one. Like remove the 220K negative feedback (if this is an AB165)and test. Reconfigure bias circuit and test, etc. This way you can isolate the problem quickly.You made a common rookie mistake by doing everything all at once and being overconfident.


Nothing glaring jumps out but we can't see things like your "under the board" change, etc.

Usually, the only thing to do is restore each section to it's original state...one at a time, testing after each.

Do both channels do the same thing? Take a bias measurement on the power tubes. This might tell you if there's a problem in the bias or feedback area. Take voltage readings for all voltages marked on the schematic. Report back.

Oh yeah, did the amp work perfectly before you started?
 

cabkab

Member
Messages
234
Amp worked perfectly before. I notice there is two 220k resistors missing where those two empty eyelets are near phase inverter. They are marked X and Y in the fender layouts. I removed 2 capacitors from there (ac578) but it shows resistors there in AB165 and blackface layouts. Is that the culprit?
 

Blue Strat

Member
Messages
30,204
Which circuit is your amp? Did it have 220Ks mounted on terminal strips off the board?

If you don't have an AB165 amp DON'T LOOK AT AB165 SCHEMATICS! :)
 

cabkab

Member
Messages
234
I only removed the one hidden wire that runs to ground from where the negative feedback wire is connected in the above pics.
 

pdf64

Member
Messages
7,940
I removed 2 capacitors from there (ac578) but it shows resistors there in AB165 and blackface layouts. Is that the culprit?
There doesn't seem to be anything connected to the phase splitter input coupling cap?
Something is needed to establish the signal path there.
 

fngrstyl1

Member
Messages
21
did you check for cracks in the screen resistors?
did you do a voltage check around the power section?
 

Blue Strat

Member
Messages
30,204
Yes, this was the AC568.

Did you catch the comment above about the PI input cap not being connected to anything on the non PI end? This is crucial, though the hum makes me think you still have another issue. Correct this first though.
 

cabkab

Member
Messages
234
Did you catch the comment above about the PI input cap not being connected to anything on the non PI end? This is crucial, though the hum makes me think you still have another issue. Correct this first though.

Can you point this out? What two points? I can look and see what was there before.
 

Blue Strat

Member
Messages
30,204


Little orange disk cap, 2/3 of the way from the left side of the photo ^^^ with 2 empty eyelets above it... check your layout drawing (I can't find one, post a link if you have one). There should be components between those eyelets and the cap. Try one at a time which will connect only one channel. Once you get that one working, install the other component.
 

cabkab

Member
Messages
234
I took out that ceramic and put the .1uf above it and works like a charm. Nice and quiet too. Thanks guys :) very appreciative of the help here.
 






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