ampeg sb12 very high voltages

[g1]

I've assumed the ext. amp jack is not insulated from chassis so the shield at that end does connect to "ground" via chassis, is this correct?
That is why I said to connect the middle lug to the sleeve lug. Now your black wire from the OT will be physically connected to chassis, therefore grounded.
Once you have done this, you should find with OT primary wired one way, you get big howl, the other way not. Without the howl is the correct way. Now with the correct wiring, disconnect the NFB wire and you should find a bit of an increase in volume. This verifies the OT wiring is all good. Re-connect the NFB wire.
Now you can carry on dealing with any other issues that are still present. The troubleshooting will no longer be complicated by incorrect OT wiring.
One other thing as far as phasing goes, the green speaker wire should go to the speakers negative terminal, black to speaker positive terminal.

[freeformfx]

No the ext amp jack isn't insulated so the shield connects at that ground lug to chassis. but as far as connecting Black secondary/Black main speaker to ground. The amp doesn't sound ok when this is done as at least one useable setting can be dialed in when it's not. confusing

[g1]

The black must be grounded. The NFB loop can not function properly without it. I realize this is not the cure for all your problems. But you can not continue with troubleshooting without it. Leaving it disconnected will just lead to false conclusions and impede progress.
It's possible this is not the correct location for it to be grounded, but it has to be grounded somewhere.

[Jazz P Bass]

"impede progress"??????

[Justin Thomas]

Oh, he meant "Con Gress."

Sorry...

Justin

[g1]

"impede progress"??????

Art thou disconcerted by my verbosity?

[Jazz P Bass]

Not at all.

78 posts & it still seems that this post is going in a circle.

[DRH1958]

Agree with Jazz. It seems time to take it to a tech. Without a scope to diagnose problems, we are indeed going in circles. It seems beyond the OP's ability to fix this. And with that photo of the jack, there seemed to be a couple of solder joints perilously close to bridging to an adjacent trace. This could also be a problem, inadvertent short.

[freeformfx]

There's no solder bridging traces , I'd just used my low quality desoldering pump before I took those pics and some little splash of solder got on the board and I hadn't c;eaned it up yet. I know what you see there and I was aware of it. I can take some more pictures If everyone can "ignore" some of my temporary lead dress. I know this has went in circles a bit but there are a few options of what could be causing the problem. ie; tone module to OT wiring.
Noone thinks the module could cause this problem with the treble pot. Audio probing all 7 pins of the module gives a pretty solid fairly undistorted signal. But probing pins 1 or 6 or V2 is completely overdriven. It's like there's an increase in gain somewhere in the preamp causing this to me.
And I unfortunately AM the only tech in my area i did all the work for all local music stores up until my brain injury 4 years ago

Everyone agrees that the black OT secondary/main black speaker lead SHOULD be grounded? It's just strange the amp only sounds decent with it ungrounded (which it appears to have been from factory) and treble pot at full. my schems shows a connection to ground on one of the pins (3), on the amp side of the plug, also but there are only two wires running to the speaker plug. I don't get that

[DRH1958]

Glad to hear the solder isn't causing shorts and you are aware of it. The statement you made about pins 1 and 6 being overdriven makes sense in regards to the NFB loop in that this where the NFB is applied to the circuit, at the phase inverter, so a positive FB would cause this overlaoding of pins 1,6. I don't think "no one thinks the treble pot is the problem" is accurate, we just think the speaker jack/NFB/Pos. FB problem needs to be fixed before you move on to other issues. If the FB is positive, then all else means nothing because the feedback affects the earlier parts of the circuit and could make it function other than it should.
If you look at your schematic, the OT secondary green is 16 ohm, yellow is 8ohm, so black is the only one left and it is grounded.

[52 Bill]

Everyone agrees that the black OT secondary/main black speaker lead SHOULD be grounded? It's just strange the amp only sounds decent with it ungrounded (which it appears to have been from factory) and treble pot at full. my schems shows a connection to ground on one of the pins (3), on the amp side of the plug, also but there are only two wires running to the speaker plug. I don't get that

I have tried to follow this thread, but I have not posted, because there are already too many voices in this choir. Stop trying to fix more than one thing at a time. The problems may be related, but one step at a time.

Do as G1 has suggested and review the output transformer wiring. If this amp was placed on my bench, the first thing that I would do would be to review the work that was already done to it. And in reviewing it, I would see that the black common wire of the OT secondary was not directly grounded. Is there another wire connected to the same lug on the jack that connects to ground?

The speaker plug wires are a hold over from the old safety interlock wiring that would basically shut down the amp if there was no speaker cabinet connected. That has been removed, so there are only two wires going to the speaker plug.

I'll leave it to G1 to help you get to the solution on this. Best of luck.

[g1]

Everyone agrees that the black OT secondary/main black speaker lead SHOULD be grounded? It's just strange the amp only sounds decent with it ungrounded (which it appears to have been from factory) and treble pot at full. my schems shows a connection to ground on one of the pins (3), on the amp side of the plug, also but there are only two wires running to the speaker plug. I don't get that

There is no reason to think the way it is now is how it left the factory. The removable speaker connector has been bypassed by someone. The ext. amp jack appears to have been replaced also. I don't think the factory would ever use a jack with the switch leaf clipped off. And I don't think they would then use that lug as a connection point. As I mentioned before, the NFB can not function without a ground reference at the OT secondary. So if they wanted the OT secondary non-grounded, they would not use NFB.
I've run out of ideas, so I will stand aside and let others comment on possible solutions.

[freeformfx]

i thought the jack situation was original because I believe before I touched it (to replace OT) the red sharpie(?) marks were still on all their solder connections.
the way the ext spk is wired if the leaf wasn't missing that would short the black speaker wire to the ext amp out signal point
Is there any other way , besides a working scope, to check that the NFB is positive instead of negative?
I've got other amps to work on so I'm about to break on this one myself.

[DRH1958]

Only other way is the output getting louder/more distorted when removing properly working NFB circuit.

[freeformfx]

Only other way is the output getting louder/more distorted when removing properly working NFB circuit.

is it possible changing the NFB resistor value could help in this situation.
I'm looking through the net to find a pic of a b15 , sb12 with output wired exactly as mine is, because I'm nearly sure I just replaced the OT with all leads going exactly where they were as it was, which looked like stock to me. As I said the red factory marks were still on those lugs' solder joints. Those were on some other areas on the crkt board too.