Amp Thought - Positive Feedback loop?

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rickenbackerkid
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Amp Thought - Positive Feedback loop?

Post by rickenbackerkid »

HI all,

As we all know certain amps have a negative feedback loop, which sends some of the output, out of phase, back into the start of the Phase Inverter. This keeps the amp a bit cleaner and more hi-fi. Most Fenders have a fair bit of negative feedback, Marshalls less, and Voxes none at all.

Having cleared that up -When I was building my hardwired Fender Pro Junior of epicness, when I first completed it, it was amazing BUT low in volume and very, very dirty sounding. In a good way, like Neil Young cranked Fender Tweed kind of thing.

It was so very different to how it had sounded that I spent a LONG time trying to fix it, then as is common for me, woke up in the middle of the night with an epiphany of what it could be. I figured maybe I had wired the OT primary backward and instead of sending Negative feedback into the PI, maybe I was sending Positive feedback, gaining up the PI and making it dirtier.

Turns out that was exactly what it was. Swapped the OT primary leads, and the thing sounded like a Pro Junior should (although a really good one, if I do say so myself)

It now has a switch for either the stock negative feedback, or none, which makes the amp a bit Tweed-y-er.

My question is would it be safe to send positive feedback into the PI? Would that be likely to damage the OT in someway? Is it possible to have a three way switch for three settings - Negative Feedback, No Feedback and Positive Feedback? It might be a cool way to get angry Tweed sounds and overdrive at a lower volume level

Thoughts?

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Re: Amp Thought - Positive Feedback loop?

Post by RectifiedAmps »

This is not a good idea IMO. Positive feedback from the OT usually causes oscillation -usually heard as a horrid squeal when you turn the amp on. In your case the oscillation could be below the range of hearing, but it’s still overloading the PI, which drags down the power supply and ‘browns’-out the tone.

There are other ways to add positive feedback in a more controlled way. This is usually done within the preamp, where a bit of output from one stage is fed back to an earlier stage. At the bottom here is a great example using a common cathode follower stage:
http://www.valvewizard.co.uk/dccf.html

However, strictly speaking, true positive feedback should boost gain, not reduce it. What you’re enjoying with your OT feedback is probably due to the reduced voltage- similar to a power scaling/VVR control.

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