So I Blackfaced the Twin ,and I have a few issues.

[gtrplayr1976]

Your old voltages for the PI matched up ok with what I see for a 100W MV twin. The new voltages for the PI are low like JPB said.
If the voltage at node C of the supply (feeding PI plate resistors) is correct you should have more voltage at the PI cathodes.
Seeing as they were correct before, and the only change is the re-wiring, that must be the cause of the change.

I'll check that again this evening. So if the voltage is correct at node C it has to be the resistors ?

[Zouto]

Did you check the 470 Ohms PI cathode resistor? I saw some sort of blackfaced "broken" Twins with 470K or 4K7 Ohms etc. mistakenly used. Otherwise, checking all the ground connections thoroughly is always a good idea.

[supawah]

Hi folks,

I decided to blackface this twin. I removed the MV wiring ,and changed MOST of the amp to AB763 specs except the reverb ,and vibrato caps ,and resistors (cathode bypass ?)wired to the pedal jacks. I removed everything else that wasn't on the AB763 ,and added/changed values as indicated. I also changed the filter cap feeding the choke to a 40uf. When I first took it off standby there was a bit of a pop. That is no longer there ,but there is a sizzle for a second of two. All controls at zero it sounds like the amp is on. I can hear sound through the speaker. The thing that really bothers me is that when I was checking voltages on the preamp plates pin one was silent ,and voltage seemed normal. Pin six had really loud hum ,but voltage seemed normal. I got the same thing on V1,V2,and V4 pin six very loud hum. Also noticed when I increased the intensity pot there is static like a leaking cap. Other than that it sounds pretty good. What did I miss ? Here are the preamp voltages I took. Thanks for any input !

1 2 3 6 7 8
V1: 272 0 2 286 0 2
V2: 270 0 2 281 0 2
V3: 456 0 8 456 0 8
V4: 289 0 2 294 0 2
V5: 461 -52 0 411 -52 0
V6: 312 43 74 305 46 74

as I remember the master volume ground ties in above the 100 ohm resister in the PI circuit? might check there, you definitely have a problem in between ground and 100 ohm}22K 1watt}33k/470/33k. I'd guess. the buss wire that ties the end of that 100 ohm to the brass shield ground breaks a lot at the edge of the brass, or where the brass meets the chassis there are two to chassis ground, these crack also. might help.

[gtrplayr1976]

Did you check the 470 Ohms PI cathode resistor? I saw some sort of blackfaced "broken" Twins with 470K or 4K7 Ohms etc. mistakenly used. Otherwise, checking all the ground connections thoroughly is always a good idea.

I changed that last night with a new one even though it was a 470.

[gtrplayr1976]

I replaced the 470 Ohm resistor with a new one as well as replaced the 22k resistor to 33k. I got the cathode voltage back up 121. I know that's higher than the 100 spec. I may have to change another resistor. I still have this very loud humming even when I get near any of the XY,Z jumpers. The layout shows where the hum is located. V6 pin 2 had very loud hum ,and bias went wacko when I put the test lead on it. The others marked with H ,or LH also had hum . When I got near any of those with a test lead you can hear the hum swell.

1 2 3 6 7 8
V1 : 264 0 2 268H 0 2
V2: 260 0 2 273H 0 2
V3: 441 0H 7 441 0H 7
V4: 270 0H 0 282 0LH 0
V5: 436 -52 0 282 -53 0
V6: 272 72 121 272 78 121

Node B: 443
Node C: 432
Node D: 399
XY,and Z 0 volts ,and hum

[Jazz P Bass]

I, for one, cannot believe that you are not working from a schematic!

[gtrplayr1976]

I, for one, cannot believe that you are not working from a schematic!

I try to work from both Jazz.

[Jazz P Bass]

Sorry, it was not clear, as all you post is the layout.

[gtrplayr1976]

This is the schematic I'm using.

[Jazz P Bass]

Thankyou.

[gtrplayr1976]

Thankyou.

Well this is the one I'm using to convert to BF. The 100 watt with MV schematic ,and layout never fully matched what I have. Some things were either missing ,or added ,or just different values on some components. I still use them ,but I've marked them up to try to show what I had in this chassis.