Silly Fender Schematic Question

[Pryde]

Hello all,

layout here:
http://vintagefenderamprepair.com/wp...864-layout.pdf

Can I assume the main speaker jack output on the layout is clockwise from top: SRT and the extension jack is ST clockwise?

Looks like the green OT wire should go to the tip of each jack and the sleeve and ring is just jumped on the main.
Then the black OT wire just runs to the sleeve on the extension jack correct?

Trying to remove a butchered master volume mod and get things back to normal.

Thanks.

[Randall]

I believe you've got it right. The jumpered sleeve to ring on Main is so the OT does not have an open circuit when no speaker is connected.

[Jazz P Bass]

There are not any colors on the wires but the schematic itself makes things a bit more clear.

[Pryde]

There are not any colors on the wires but the schematic itself makes things a bit more clear.

Thanks jazz. I did study the schem but noticed there is no ground on the extension speaker symbol, just shows the tip connected to the other tip on the main jack. That's where I was getting thrown as the schematic doesn't clarify where the OT black wire goes.

[Randall]

Yeah, the ground gets made through the chassis, which I never have liked, since it can be lost if the jack comes loose. No harm in running an extra ground wire between the two sleeve lugs.

[g1]

To me, the schematic implies the black wire goes to the main jack, and the Ext. jack gets its ground via chassis. Yet the layout shows the black to the ext. jack. I think black to main jack is preferable for the reason Randall mentioned, jack can come loose and main is always in use.

[Pryde]

Thanks for the great feedback. I will run the black wire to the main sleeve and run a jumper over to the extension jack sleeve.

Weirdly I found another bassman layout with the OT black wire going to the tips of each speaker jack and the green wire going to the sleeves. I guess either way is fine then?

[eschertron]

Thanks for the great feedback. I will run the black wire to the main sleeve and run a jumper over to the extension jack sleeve.

Weirdly I found another bassman layout with the OT black wire going to the tips of each speaker jack and the green wire going to the sleeves. I guess either way is fine then?

Bassman got NFB? If this other layout is using a third-party OT (like as part of a kit), then the supplier of the layout may have found that their particular OT needed the wires reversed for the NFB polarity to be correct. That's my guess. I'm assuming that otherwise it's standard for black to be the common (ground) wire color for any multi-tap OT. Correct me if I'm wrong!

[Justin Thomas]

The AB165 & later Bassmans had the OT leads switched because of changes made in the PI & NFB loops - switched where the NFB was injected back into the signal change. Swapping the speaker leads was the easiest way to fix it, apparently... It's covered in the Machine Gun Amps writeup about modding AB165s "halfway" back to AB864 specs. So, no, either way isn't fine. But if you make a mistake, it's not like it's hard to fix.

Justi

[Pryde]

OK good to know thank you. Here is what I was referring to:

AB165 with black OT wire to tip:
http://vintagefenderamprepair.com/wp...165-layout.pdf

AA864 with green OT wire to tip:
http://vintagefenderamprepair.com/wp...864-layout.pdf

This amp is a silverface but appears to have a blackface layout inside.

[g1]

You'll need to verify which way (green or black to ground) makes the feedback loop work properly (negative rather than positive).
When you disconnect the NFB resistor (820ohm) the signal should increase slightly in level. If so, the green & black are correct. If when you disconnect the NFB resistor the signal level drops, then the green and black were wrong and should be reversed.