ampeg sb12 very high voltages

[freeformfx]

Had to dig up the thread cause I believe I've gotten somewhere or a very good clue for someone to help me. Firstly I've tried wiring up a tonestack instead of the module to no avail. Went ahead and grounded the OT black/speaker lead, as it should be. I also replaced C2 a .022 cap the shows on the schem as .1
The oscillation was a wee bit better but still terrible turning the treble control. But you can actualy hear the treb pot working and the amp just has this gating distortion on top of the notes. SO I decided to pull out a scope (BK 2125) even though I've never thought it calibrated so I'm not very handy with one
I looked at the output and could immediately see it on the scope (the dist./oscill) as big wave blotches sitting in between about every cycle, then they'd be farther apart and fade away as the note dies. I followed it back to where the dist. stopped and the waveform was clean (just using a guitar for tone generator) and found it didn't stop til right before the signal enters the tone module. Clean wave (though out of calibration) Crkt looked good there, and that big c2 cap i'd just replaced

(and this is where one of the big guys around here's gonna figure this out I bet ya )Then I start chasing it forward again and touching my scope probe to pin 6 of V2 BLAMO, the amp sounds perfect with the probe on it. Tone controls work as they should no nasty distortion perfectly fine normal sounding amp I found that actually touching it to pin 1 v2 has mostly the same effect,not as clean as 6. But this is all that I want it to sound like
so what's the probe doing to the circuit and telling us. And If I have to emulate that with the circuit then I'd do it. I'm not against a band aid with this one
thanks

[Jazz P Bass]

To me, the probe tip is a 47pf capacitor to ground.

[g1]

Now is it just a coincidence that this is the side of V2 that gets the NFB ?
Did you verify with the black wire connected that the OT is now for sure phased correctly? It would be nice to settle that for certain.

[freeformfx]

so you're saying a 47pf cap from one of the plates to ground may give the same effect as the probe touching the plate.
I did notice that the probe at any of v1 v2 plates gives a similar clean signal just v2 6 was most effective.
And no I've not tried reversing the green? and black leads on the OT secondary as someone had recommended to me to see if that may resolve all. but I feel like i'm close so thanks guys. just a wierd problem I think

[g1]

You don't even need to change any wiring to check the OT phase. Just lift the NFB resistor, and the volume should increase a bit.
Try installing that 47pf cap and see if it cures the oscillation. If so, then lift the NFB resistor and see if volume goes up or down.
I think it's probably correct now as there is no howling or squealing, but it needs to be verified.

[freeformfx]

Something for tomorrow then try a 51pf 200 or 400v cap ( i think that's all I have) from the plate of v2 pin 6 to ground I guess. Then lift the nfb which has only resulted in lower volume dist tone before but I don't guess I had the Black OT grounded then though.
Oh Is there any point in reversing any secondaries though?
and I was wondering also about how I have all the wires from the OT bunched under the board toward the back of the amp toward the jacks crammed in between V2 V3 sockets, none twisted very well, I wondered if it could cause oscillation

[g1]

Yes, the lead dress of the wires from the OT could cause oscillation.
Reversing the OT secondary is generally used when the OT is phased wrong and you do not have multi-taps on the secondary. In this case you do have multi-taps, so if you need to switch phase you should do it on the primary side.

[freeformfx]

Yes, the lead dress of the wires from the OT could cause oscillation.
Reversing the OT secondary is generally used when the OT is phased wrong and you do not have multi-taps on the secondary. In this case you do have multi-taps, so if you need to switch phase you should do it on the primary side.

thanks this is what I thought was the case pretty much too.
OK something accomplished, although It should go under the heading "the wrong way to fix your amp" but hey... I used a 51pf resistor to emulate the scopes load and same distortion occurs, so I wanna move up as little as possible but found a 390pf cap laying there and boom it did it. Pretty clean signal. Played fretless bass through it and at about 1/4 volume nice smooth bass sound. Then it starts to break up a little. Not that awful distortion we're fighting though
I can't tell you exactly what I've done but it works for me at this point with this amp.

[Jazz P Bass]

I seem to recall Fender placing a small value capacitor across the negative feedback resistor.

Try the cap there.

[freeformfx]

I seem to recall Fender placing a small value capacitor across the negative feedback resistor.

Try the cap there.

I'd gladly try it there rather than grounded off a preamp plate (didn't fender put .022 caps off the plates of output tubes to ground in some of those troublesome SF amps or were they somewhere else? i forget
and you mean parallel with ,across the nfb r' right? should I stick with the 390pf ?
thanks, but at least i have an amp i can zip up put the cage on and have be useable at least now, that's a positive direction at least no matter what else to me. Though I'd like to fix the problem itself more correctly

[Justin Thomas]

Those were 2000pF caps off the plates... which I think is .002uF. And they were off the input grids. Bassman 100 shows one way, Bassman 50 shows another. Bassman 50 & AA371, etc. show 100pF in parallel w. -ve feedback resistor, along with a .1uF in series.

Just some particular Fenders to look at. The AB165 & later Bassmans also had the small caps in ][ with the preamp plate resistors on one channel.

One note: starting w. AB165, Fender also changed where they fed the -ve feedback in, so on those amps the OT wires to the speaker jack are reversed, compared to AA165 & previous. I guess that was easier than swapping the Plate wires...

Justin

[freeformfx]

thanks for clearing that up I knew they did something like that there to control Oscillations.
Haven't tried moving the cap across the NFB yet as I'm having some time with the amp in at least playable condition for a bit. But I'll try it, though It doesn't seem it would have the same effect as a cap to ground from a plate. It is still worth a try if it could clear it up even more maybe