ampeg sb12 very high voltages

[Jazz P Bass]

If disconnecting the NFB wire lowered volume, then it was acting as positive rather than negative feedback. Disconnecting it should raise the volume rather than lowering it, that is what NFB does. So I would think you need to reverse the OT primary connections.

That!

[DRH1958]

I had a thought. What if the secondary is wired wrong(reversed)? Would the sec being reversed and the pri. being wired opposite be the same as if it were correct? Should he make sure the speaker jack is wired correctly before changing the primary? If I'm off base with this, let me know. I'm just starting to get somewhat comfortable with power amp theory.

[g1]

Here is the diagram of the OT. Black must go to the positive terminal of the speaker. It shouldn't go anywhere else The ext. spkr jack puts the external spkr in series with the internal speaker. It must be insulated from chassis which you said is the case. So with no ext. spkr connected, the internal speaker negative connects to the yellow via the switch contacts of the ext. spkr jack. Yellow goes to the switch contact, green goes to sleeve, and tip goes to negative terminal of the internal speaker (see schematic in post #49).

The reason I called out the internal speaker being wired "backwards" is so the ext. spkr when used will be in phase. So I'm assuming the schematic is exactly correct.

Would the sec being reversed and the pri. being wired opposite be the same as if it were correct? Should he make sure the speaker jack is wired correctly before changing the primary?

Yes, if primary and secondary were both reversed, it would "cancel out" and end up correct. But if it is correct, lifting NFB should cause an increase in volume, not a drop.
And yes, the speaker jack wiring should be sorted out first.

[freeformfx]

a little lazy day i've not tired reversing the plate leads from the OT yet. But you guys do think it sounds like this could be causing the strange interference with the treble control and overall excess distortion? still hung up on that tone module even though i believe I've tried bypassing it with symptoms still there.

[Justin Thomas]

a little lazy day i've not tired reversing the plate leads from the OT yet. But you guys do think it sounds like this could be causing the strange interference with the treble control and overall excess distortion? still hung up on that tone module even though i believe I've tried bypassing it with symptoms still there.

Yes. See posts 33 & 50. No guarantee that reversed plate leads will shriek loudly. Often, not always. Sometimes it's just hum, noise, low power, distortion, unpredictable controls, clicky attack... it's a cheap, quick test, nothing to lose. All learned while rebuilding a hacked Bassman 50.

Justin

[g1]

Somewhere along the line I seem to recall you saying this problem started when the OT was replaced. If that is the case, then the OT wiring seems more likely than the tone module.
Another possibility is that there is more than one problem, so they will have to be tackled one at a time.

[freeformfx]

Somewhere along the line I seem to recall you saying this problem started when the OT was replaced. If that is the case, then the OT wiring seems more likely than the tone module.
Another possibility is that there is more than one problem, so they will have to be tackled one at a time.

true, but before the OT was replaced it was unusable too obviously. I think I could get nothing but bad oscillations with the original but I recall taking measurements that proved the OT was bad.
I could take pics of how the OT is wired I guess. OKAY UPDATE:
I did try with plate leads reversed with NFB connected and disconnected(same result lowered more distorted output when disconnected) Left with same overall symptom. Amp only sounds "normal" (though still distorts way too early) with TREBLE control full on. As turned down from CW is very lowered volume then some oscilllation then distorted buzzy low volume with Treble full CCW

Are we sure it's still not in the tone module? A guy offered to sell me a replacement fairly cheap. I realize I could build it on myself fairly easy.

[DRH1958]

At this point, I would go ahead and buy the tone module. Fairly cheap sounds worth a try. Agree with g1, more than one problem. So to summarize: 1. You have high AC on all three PT secondaries. 2. Your preamp is distorting too early. 3. You have replaced the OT but never really got it sounding right after the replacement. Does that cover it? This thread is starting to get kind of long and the probability of solving the problems diminishes the longer we go. I don't see how replacing the tone module will fix the high voltage problem. If it does, then there is a problem that troubleshootong voltages in the primary should solve. And without a scope, you are limited to what you can diagnose. You really need to SEE the signal to determine exactly where it goes bad.

[Jazz P Bass]

If the amp 'only sounds good' with the Treble control dimed, then the module is certainly suspect.

Note: not to keep harping on the OT, but when you reversed the OT plate wires & the signal still dropped when you lifted the NFB, that makes no sense.

Unless, the oscillation is swamping the NFB circuit.

Just for grins & giggles and to lay the OT issue to rest, try the 'lift the NFB' when the amp is running good. (ie: Treble control dimed).
Then, hopefully, the test should prove out in a correct manner. (NFB lifted, signal climbs, when wired in the correct manner)

[freeformfx]

I disconnected the NFB with the treble maxed out and it still sort of dropped in volume, a little oscillation when digging in on the input signal too.
I measured the high AC secondaries under full load and get 387v at each which i'd say is reasonable (only 17v higher than the original schem shows)
however I notice my 12ax7 plate voltages ARE pretty high esp the PI. V1 pin 1 212v, pin 6 197v/ V2 pin 1 280v, pin 6 296. These vltgs are 30-50v higher than the schem and the large differential between pin 1 and 6 or V2 makes me wonder. It only should be 5volt difference.
trying to attach pics but having trouble with the site

[Jazz P Bass]

I disconnected the NFB with the treble maxed out and it still sort of dropped in volume, a little oscillation when digging in on the input signal too.

And then you: reversed the plate leads & all was as it should be so we can move on.

Correct?

I would hope that you have a meter hooked up to the speaker leads, set to read Volts AC, to actually measure the voltage change.

[freeformfx]

No the NFB reacts the same way no matter how the plate leads are so I didn't switch em back again
No I don't have an AC voltage test setup at the speaker output. I've never done that.
here's pics of how the OT is wired and hopefully a viewable schem for the amp. Not alot different than other ones but it shows the ext amp jack wiring.

[g1]

Can you get a few more pics with more light so we can see all the black wires?
I think with the pics, plus your description from post #47, if you get those jacks straightened out you may find a cure. Also you mentioned when you ran the ext.amp jack to another piece of gear the distortion was worse and wondered if that was a clue, I think you were on to something. The ext.amp jack is right after the tone module.
Also a pic of the wiring on the speaker itself, and the other end of the cable connected to the ext. amp jack so we can see where the shield goes.

[g1]

Going over everything again, it seems there are 2 problems.
1) The OT black is not grounded.
2) The OT primary phase is reversed.
You can ground the black by connecting the ext. amp jack's sleeve and switch lugs together.
Then, when you get the "full on" oscillation, reverse the blue and brown wires.
Once you have it properly in phase, you should find that lifting the NFB wire increases the volume a bit, connecting it back up decreases volume.

[freeformfx]

Thanks, I'll try to take some better pics but a couple things. The ext. amp wire has it's shield unconnected (heat shrinked) at the end thats connected to the control panel pcb. And the ext. amp jack has no switching tip just a lug where the black wires are connected to. And I tried connecting it to ground after reversing the plate leads, and I believe the result was lowered volume and more distortion. as opposed to huge noise/oscillation I got when grounding it previously.