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Should the OT secondary "black" or negative always be grounded to the chassis or common? I recently connected the 2 secondaries on an OT directly to the speaker terminals with no chassis/common connection. Seemed to work, but now I am second guessing that move. FYI it's a Weber 6m36 TMB kit.
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This works if the amp has no NFB from the OT secondary.
With NFB, the secondary must be grounded.
Cheers,
Albert -
Ok thanks AK.Comment
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And with an amp with NFB, the OT secondary gets grounded to where ever the NFB is applied, which is usually the phase inverter.-MikeComment
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the feedback ground would be there but the speaker ground I would put with the power tube cathodes. My thinking is high current AC return (spk.), higher current DC and AC point (cathodes), plus the respective points are sort of the same but just on the other side(s) of the output transformer. You could star these points back to the ground line individually or as in the Soldano tie them together back to the ground line (SLO uses a thick wire and I think this makes sense)--(maybe not completely so, but) I consider the two pretty much equivalent in practice. What I remember (vaguely) reading is that although in theory with no fdbk. you can float the spk. ground but in practice grounding it can help with stability.Comment
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so oscillations can be a problem when speaker isn't grounded? Having an issue w/ that, in part because of cheap Chinese preamp tubes, which I've swapped out and it helped, but still some oscillations. Probably mostly due to the gain in channel 2, but maybe grounding speaker will tame the issue.Comment
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I'm not really sure why cheap chinese tubes would make things more problematic in terms of oscillations but with oscillations I would think understanding some basic theory would help (not that I'm some sort of expert). But things that can cause or contribute such as distances, interactions, number of stages, grounding should probably be considered. As one (simple) example, I had one oscillation problem which was traced to running the input (gtr.) cord right next to the speaker cord going to my "speaker"(actually a speaker emulator with internal reactive load). I was mainly concerned with convenient physical layout, but realized that putting those paths for a few feet right next to ea. other was bad news. Because I did understand a bit, I was able to correct the situation by putting the spk. emulator load away from the amp input and towards the amp's AC input end (on the opposite side of the amp input). (Because the amp spk. output impedances are low, there isn't any problem or concern with that signal being negatively affected or overcome by the AC input, AC line in the vicinity.)Comment
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hmmm that's interesting... I can see how that may cause issues. I'm thinking the chinese tubes are exhibiting some degree of microphonics, that's all. Most likely a separate issue from the non-grounded OT secondary.Comment
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Actually, you wouldn't want the speaker "return" to share a path anything. You connect the OT "common" wire directly to the speaker jack.Originally posted by dai h. View Post
The reason you would then ground this is to provide a ground reference point, but almost no current will be going through that wire. If a NFB amp, this ground reference is important for the NFB loop to work (no reference = signal does not exist) so you ground your speaker jack at the PI ground.
That said, I've read that in a non-NFB amp, grounding the OT common can still cure some problems. Although it has no easy technical explanation, we also know that grounding is part science, part black magic. It's easy enough to do and it has no downsides (provided that everything is wired correctly).
The only non-NFB amp I've built didn't have any squealing or humming problem without the OT Secondary grounded. I still did it for the sake of it.Comment
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Any tube can be microphonic. A good vendor would usually weed these out, but you never know.Originally posted by lowell View Post
When I bought my brand new Marshall DSL50 8 years ago, it was squealing randomly for no reason. Using a spare 12AX7 that I knew was good, I swapped each preamp tube one at a time until the problem was fixed.Comment
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what I'm thinking of is the same way things are wired on the SLO100, and the OT common wire does go to the spk. jack. But would it even matter if you connected from there to a power tube cathode?Originally posted by Hardtailed View Post
I thought grounding the spk. jack back at the PI ground was incorrect. My understanding was that the feedback is grounded on the side it's returned to, so the two R fdbk.netwrk oftentimes seen (such as 27k/5k or whatever) would be grounded back towards the PI. In Marshalls (for a real life example) that is how things are wired. Also a wire is run from the spk. jacks to a power tube cathode to chassis point and not run back towards the preamp.The reason you would then ground this is to provide a ground reference point, but almost no current will be going through that wire. If a NFB amp, this ground reference is important for the NFB loop to work (no reference = signal does not exist) so you ground your speaker jack at the PI ground.Comment
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I haven't studied the SLO enough to comment.Originally posted by dai h. View Post
On a Marshall style amp, the PI is grounded via the presence knob (or a resistor parallel to it). I guess you'd ground the OT common there.
To be fair, on my JCM800 clone (which has a Plexi style presence circuit, aka scratchy pot), I simply grounded the OT common to the chassis lug where the preamp ground buss goes, although the last ground on this line is the PI so I guess it's just as good. It works, so it's fine by me.
I sorta mostly copied Ceriatone's layout.Comment
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