ampeg sb12 very high voltages

[DRH1958]

Now that I read your most recent post, I'm wondering if you have a problem in the output section because of the new OT being installed wrong and also a different problem in the preamp. These are the toughest to fix because you can fix one and not know it because you have another problem that isn't fixed. Maybe post the info that came with your replacement OT and how you hooked it up. Then we can see if there are corrections to be made.
Now that I look at the circuit again, the preamp out is after all the controls, so you probably don't have to install the 1Meg resistor to ground. I was just making sure you had the grid referenced to ground. The output is taken right off the grid of the input to your phase inverter so the circuit will be functioning as intended by the schematic without the resistor. Maybe JM Fahey was telling you to do that as a diagnostic tool. I didn't read it carefully because I didn't want to confuse the issue by posting something else. I was hoping his suggestion worked.

[freeformfx]

i tried running the ext amp out and the distortion is still there, I tried it with the tone stack bypassed then went back to the stock (CR01 module) hook up. Strangely even the distortion gets worse if the ext amp out is used. maybe a clue?
I don't know where to find the data sheet for the OT only that it's the OT 215 that Fliptops carries for sb12 replacement. The I have the green wire going to the ext speaker 8ohm jack is a switching jack which i have isolated with plastic washers from the chassis. I have the green wire going to ground lug with the negative feedback lead , yellow to the middle (switching) jack and then the green main speaker out wire is going to the tip. Now the ext speaker jack is a three lug switching style jack but with the switching tip missing. That lug is used to tie the Black OT wire and the main speaker output black wire together. They have no reference to ground. It does confuse me but i think I wired it as the factory had it wired. The other two lugs Tip and ground on this jack have the ext amp out cable signal and shield respectively.
I also notice I have the primaries reversed blue and brown to V3 and V4 reversed from my amps schematic. maybe did that while I was troubleshooting an oscillation
IF anything sounds amiss here let me know please

[freeformfx]

should I maybe try reversing the brown and blue primary leads to where they match the schematic again. surely it'd be something if that was all this trouble.

[DRH1958]

Reversing the OT primary leads will reverse phase and will change your negative FB loop into a positive FB loop and the amp will howl, squeal or something like that. You do have the correct OT for sure. Here is the schematic, do you have the jack wired like this? Is this the correct schematic?

Ampeg_SB12_1.pdf

[g1]

Agree that incorrect phasing of the OT will usually cause some strange howl or something, but sometimes it might not be so severe, or cause some other odd problem. It may be worth at least lifting the NFB wire. If the output level then drops, you know the OT phase was backwards.
And you mentioned a few times high heater voltage, just how high is it?

[freeformfx]

Agree that incorrect phasing of the OT will usually cause some strange howl or something, but sometimes it might not be so severe, or cause some other odd problem. It may be worth at least lifting the NFB wire. If the output level then drops, you know the OT phase was backwards.
And you mentioned a few times high heater voltage, just how high is it?

yeah the plate primaries are reversed as to the schem. i'll try lifting the NFB wire too then.. the heater vlgs are 5.2v and 6.7 ac iirc.

[Enzo]

Lift the NFB first. if that makes the difference, then swap the tranny leads. If the NFB doesn't change anything, then the tranny wires are not likely the issue.

[freeformfx]

Lift the NFB first. if that makes the difference, then swap the tranny leads. If the NFB doesn't change anything, then the tranny wires are not likely the issue.

didn't me testing the amp from the ext. amp output and still having the distortion or treble control weirdness prove that the problem is in the preamp though ? Or would problems at the OT wiring bleed back into the preamp stages.
I've been OVER this amp thoroughly... checked that no coupling caps are leaking , open or out of tolerance resistors. and the most I've found it a few slightly out of spec. Like a couple plate 120k,s reading 110k, a 5.6k cathode resistor reading 6.2k and another preamp cathode at 242ohm rather than 220. I wouldn't think correcting these slight deviations would make the difference I'm needing it to.

[DRH1958]

Yes you do have trouble in the preamp. I think they are trying to rule out problems with the output transformer since you replaced it. Since you have negative feedback we want to make sure you don't have the connections wrong and have positive feedback. This would be in the form of an oscillation and could cause distortion or squealing, you never know for sure. It's a 50/50 chance of getting it right. If all checks out in the power amp, then the only other thing I can think of is check for cold solder joints. Turn power on so there is some output through the speaker and use a chopstick or something like that and gently poke and prod the components and listen for static or a crackling sound. Check all components in the preamp, especially where the treble control connects to the board. BTW, I don't think 5.2v or 6.7v will do any harm to the tubes for the short term. Later on when you have tracked down the culprit in the preamp, you just might have to consider a hard look at your power tranny. Do you still have 415 AC volts coming off the main secondary? IIRC,I think it is supposed to be 375v. Refer to the schematic JM Fahey posted early like in post 6 or something like that. I know you have the larger resistor in place to bring down the B+ to the tubes but in the long run, you probably will want to bring down the 6.7v heater voltage. Maybe a small dropping resistor could fix it. 5.2 is fine to leave alone.
Actually, have you tested or replaced the preamp tubes?

[Enzo]

You need to be sure about the OT regardless. And yes, if it goes into power oscillation, that can ride the power supply back into the preamp.

Never think up reasons not to check something.

[J M Fahey]

I guess Dr Scope must give his professional opinion or we'll chase ghosts forever.

Do you have access to one?

If not, but you have a notebook, I can suggest a Windows scope program and an attenuator so you don't burn your soundcard.

Not the same as a real one, it's uncalibrated and can not show DC (Soundcards are AC coupled) nor show >20kHz signals, but we can see *shapes* at different points and we need that now.

And DC values can still be measured by a multimeter.

[freeformfx]

I've poked prodded with a stick and resoldered maybe every connection on this amp but it never hurts to keep checking. Yeah I wondered if that one heater supply is just a tad too high but it's not my main concern right now I guess. I'll recheck the high PT secondary again when I lift the negative FB in a minute
If it would come down to having to have a scope (which I've never in all my work had happen) no I don't have a working one. I've got two laying around but never had any luck with either of them working right. I have thought about using pc scopes before though.

did my explanation of how I have the OT secondary wiring sound correct to everyone? The black wire with no reference to ground confuses me

[DRH1958]

It kind of bothers me when you said the ext, jack has the switch tip missing. On the schematic, the black does go to ground. You need that ground connection for it to work properly. I wonder if you could draw up how everything is hooked up, scan it and upload it . Sometimes it's harder to explain and harder for us to picture when describing it. A sketch will be best. I must admit I can't picture exactly how it is wired from your description in that post. Sorry.

[freeformfx]

Well I can tell you that if that black wire it connected to ground the amp does nothing but a full on oscillating? racket. I was messing around and also found that connecting the lug with the green OT wire on it (ground lug which is isolated with plastic washer) connected to ground , as it would be if not isolated not only reduces the hum greatly but also makes the treble control distortion gating go away, it still doesn't work correctly but gets a pretty good tone at full CCW and at low volume setting
and the negative feedback disconnected didn't help it just made the distortion worse and lowered volume. I therefore didn't even try reversing plate leads though I'll prob still try it.
my gut tells me to look at those tone module components causing this but i may be way off.

[g1]

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.
It's difficult to understand what you are saying about the lugs and jacks the OT connects to. Can you post some pictures?
The full on oscillating when you connected the black to ground is what would be expected from an OT with reversed primary connections.
Perhaps it should be connected when the OT leads get reversed? Again, without pictures it's difficult to know how it's wired.