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

[gtrplayr1976]

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

[Jazz P Bass]

Something is amiss with the PI voltages.
Pin 3 & 8 should read 100Vdc.

[gtrplayr1976]

Something is amiss with the PI voltages.
Pin 3 & 8 should read 100Vdc.

YES, that was also an issue. Forgot to mention that one. All of the resistors ,and caps were changed in that circuit to BF specs. Does that 100Vdc come from the feedback ?

[Jazz P Bass]

The 100Vdc is the cathode voltage.
Which relies on both the plate & the grid voltage.

The plate voltage is high & the cathode voltage is low.
That leaves either the grid circuit as being screwy or the plate resistor circuit is. (or both)

[gtrplayr1976]

The 100Vdc is the cathode voltage.
Which relies on both the plate & the grid voltage.

The plate voltage is high & the cathode voltage is low.
That leaves either the grid circuit as being screwy or the plate resistor circuit is. (or both)

I changed this to AB763 specs. I checked the resistors last night ,and they seem to be fine. The caps checked with tester seem fine. I've obviously missed something.

[g1]

Then it's probably a wiring error in the PI circuit. Were the voltages correct before you did the conversion?

[gtrplayr1976]

Then it's probably a wiring error in the PI circuit. Were the voltages correct before you did the conversion?

These are the voltages before I started changing things. Seems the voltage is still low compared to what it should have been for that version.

V1 pin 1 plate is low at 186 volts (before I changed the leaky cap), pin 6 is at 246.
V2 pins 1, and 6 are at 235 +/- 1 volt ,
V3 are 446 on both 1 and 6.
V4 are 245/251 ,
V5 1 is 440 ,6 is 388 ,2,and 7 are -37.
V6 1 and 6 are 328/321 , 2and 7 are 111 ,and 3 and 8 are 144.

[Alex R]

maybe check the values of the new parts around the PI? Don't mean to patronise but if I don't watch myself I'll pick up a 68k instead of a 680k most times

[gtrplayr1976]

maybe check the values of the new parts around the PI? Don't mean to patronise but if I don't watch myself I'll pick up a 68k instead of a 680k most times

No worries. I checked them all last night.

[Alex R]

next I'd go right through every change I made checking each node is wired right and the solder is good. There is an error in there somewhere and sometimes it's quicker just to check everything before trying to figure it out.

[gtrplayr1976]

next I'd go right through every change I made checking each node is wired right and the solder is good. There is an error in there somewhere and sometimes it's quicker just to check everything before trying to figure it out.

Agreed. I didn't change all that much. I removed some components also that weren't on the AB763 layout ,and of course changed the balance pot to bias like on the 763.

[Alex R]

Agreed. I didn't change all that much. I removed some components also that weren't on the AB763 layout ,and of course changed the balance pot to bias like on the 763.

I like that bias balance pot, I think it's more useful than the bias adjuster. But that's just me. Is the bias ok by the way?

[gtrplayr1976]

I like that bias balance pot, I think it's more useful than the bias adjuster. But that's just me. Is the bias ok by the way?

I like that pot also. I may try to change it back ,but not sure how it would work with current scheme ,and I also like to be able to adjust the bias. If you're speaking of the negative voltage it is at -44Vdc.

[g1]

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.

[Alex R]

I like that pot also. I may try to change it back ,but not sure how it would work with current scheme ,and I also like to be able to adjust the bias. If you're speaking of the negative voltage it is at -44Vdc.

...you can adjust the bias with the resistor to ground, generally soldered onto the pot.

You know, if the thing works and the funny noises have stopped, maybe it's fine