Is my OT fried?

[Enzo]

No to #2. negative feedback is a loop within the power amp. The output to the speaker is sampled back to the phase inverter or someplace close to the inverter. What happens before that has nothing to do with the relationship of the speaker output to the phase inverter. The absolute phase of your guitar signal might be inverted, it is, but all the NFB cares about is that it is opposite phase from the PI.


Phase or polarity just means which direction the signal is changing. A signal starts at zero, and moves both positive and negative from there, exactly as your speaker cone starts at rest and moves in and out. SO if we send a positive going signal into a stage, and it comes out negative going, then we say its phase is reversed - its polarity is reversed. A gain stage can be made with a tube like a triode, or a regular transistor, or a MOSFET, or a JFET. In all those cases, the current through the device is controlled by some control input - the grid, the base, the gate, whatever the device uses.In that regard they all work the same. SO if a positive going signal at the grid/base/gate increases conduction of the part, then the voltage across the stage load will increase. So if I replace a tube stage with a MOSFET, that shouldn't affect phase relations.

[Gainzilla]

Thanks guys! This build has been such a great learning experience for me.

First off, that's great news about the two stages. I was starting to resign myself to having to cut the switch. Makes sense now, though. It matters when running two channels in that you don't want them out of phase when they hit the PI. For single channel, it doesn't matter what the phase is doing before it hits the PI.

Clarifying question about that: does that not matter because the phase inverter flips the signal to whatever orientation it lives on? Or is it because as long as the PI and OT are in sync, and the NFB connects to a negative side of the PI, that's the only thing that matters?

[Tom Phillips]

...Clarifying question about that: does that not matter because the phase inverter flips the signal to whatever orientation it lives on? Or is it because as long as the PI and OT are in sync, and the NFB connects to a negative side of the PI, that's the only thing that matters?

Well...I'd say that your first sentence is incorrect and the second sentence more on track.

[Enzo]

Let me try this:

Your kid is bouncing on a trampoline, maybe bouncing too high. SO you go over to it, and when the trampoline is going down, building energy, you pull up on the stretchy ropes so it can;t go down as far. Then as it rebounds and tries to throw the kid into the air, you push down on the ropes so it can't spring up as much. You are applying negative feedback...sorta... opposing the motion of the trampoline.

Now it doesn't matter if the kid is standing on his feet or on his head, either way YOU do the same pushing and pulling to slow him down.

In your amp, the PI doesn't do anything but split the signal into two opposing signals for the power tubes. The NFB holds it back some. NFB is just a part of the power tubes and the PI working together. It goes no further than that.

[Gainzilla]

That was a great non-technical explanation. Totally get what's going on now. I've actually been wanting to understand how that works for a while now. It helps me personally to be able to conceptualize how a thing works visually.

I tried the phase test last night, and the volume went down when I alligator clipped the presence circuit back on. I used the bias drain clip (with 50r resistor inline). I'm assuming that resistor isn't big enough to produce an audible volume drop, or bring a louder signal down that far, so I think I can safely say I've got negative feedback.

[Tom Phillips]

...I tried the phase test last night, and the volume went down when I alligator clipped the presence circuit back on. I used the bias drain clip (with 50r resistor inline)...

I'm not sure what you mean by that. It is different than the test I described.

[Gainzilla]

Sorry, I think I just made up some new words. What the heck is a bias drain clip? Lol

I just meant that I used an alligator clip to reattach the NFB line to the transformer. That way I could check a couple times back/forth without soldering. However, the only clip I had were the ones I made to discharge filter caps (not bias or whatever it was I said). They have a small resistor in line, but I didn't think it was large enough to invalidate the test (listen to how the NFB effects the signal. Louder means positive feedback, quitter means NFB).

That's correct, yes?

[Enzo]

Yes, that is fine, 50 ohms won't upset the test either way.

[Gainzilla]

Cool, I didn't think so, but then I've made incorrect assumptions before that have complicated things. I know exactly enough to be dangerous!

But I learn from each mistake.