Adding negative feedback to a '57 Deluxe

[martin manning]

Thought I'd post this explanation of how a feedback loop can be added to a 5E3, including how the correct phase relationship between the OT secondary and the feedback insertion point is determined for GNFB. Several variations are possible, including switched and/or adjustable FB level.

[jbrrrrr]

Thanks for sharing this, Martin!

[psb962]

By sheer coincidence I just did this to my 5e3 clone using the Rob Robinette SPDT On/Off/On 3 way switch method. I am impressed by the change the NFB makes, its like a whole new amp, with a very different breakup/OD tone.

And of course I can switch it back and forth anytime.

[martin manning]

That mod is what prompted me to look into it. It was brought up here a couple of days ago. An interesting way to go about it might be to set it up as described, but leave the cathode bypass cap in place with a pot in series. That would allow dialing the FB level from zero (with the pot at zero) on up.

[Raoul Duke]

Thanks for looking further into this Martin.
I realize now that doing the OT polarity check on a 5E3 (or variation thereof) isn’t necessary if you’re not implementing any NFB network. However, I did the test when I built my Tweedle and I know that the brown lead attached at V4 is the one with more voltage, which I don’t think came across in how I asked the question.

Looking at your diagram, would the green text apply to my situation then? That’s what I’m thinking based on reading through it a couple of times. If the greater voltage passing lead is “in phase” and I’ve already got it at V4 already, chances are I’m good?

As always, thanks for explaining and drawing pictures. It helps us slower folks figger things out, lol.

Much appreciated!

[martin manning]

The situation is opposite to that of a LTP phase inverter since there is an inverting stage in front of the cathodyne PI. In other words, the OT's out of phase primary lead (wrt the secondary) goes to the power tube driven by the inverting side of the PI. The Fender '57 deluxe schematic shows the in phase lead connected (see phase dots on the coils), so in that case the primaries need to be reversed.

[Raoul Duke]

Gotcha, thanks again Martin!

[martin manning]

Made a couple of tweaks to the document in the first post. I'd be interesting to hear if anyone tries the variable Ck series resistor idea out.