Forgotten wire on Presence = Happy accident?

[guitar-rocker]

I have a recently new build that I completed, and was a bit in a rush to finalize it for a local amp show. I looked it over pretty good, fired it up, did my usual voltage checks. My PI voltages were a bit high, but I decided to try a guitar through it. Absolutely loved the tone, definitely a fine clean tone, and the overdrive roar is perfect for a classic rock tone. I played it off and on a few days, continued to really like the tone. Took it to the local show, and it was very well received.

The day after the show, I pulled the chassis for a final look see, and after rechecking voltages, I found that I had made an error and had forgotten to add the wire from the long tail to the Presence pot, so the Presence pot is not in the circuit at all, just the 100K feedback resistor going to the 4 ohm tap. I'm the first to admit that I'm not perfect, and I definitely forgot that wire when building it. I have tried removing the feedback and wire to the 4 ohm tap, and grounded the long tail. Still an acceptable tone with totally no feedback, but not what I would keep as is in my lineup. The Master Volume wired in the "mistake mode" really acts like an attenuator, and is an interesting tool as such. I'll try to attach a modified schematic for view. I've recently read through Aiken's, and Merlin's stuff until I'm cross eyed. Just wondering if the amp has potential failure wired in the mistake mode. Thanks.

[tubeswell]

A LTP is a differential amplifier, therefore injecting a signal from the OT secondary back to the other grid of the LTP means that sum of the LTP output is now a blend of the individual signals present at each of the inputs (at each LTP grid)

[EtherealWidow]

Strange that the PI even worked with it being disconnected from its traditional ground. I guess it was grounded through the transformer secondary. Not sure if that would then reduce or increase the headroom of the PI. Kind of surprised it was stable at all.

[guitar-rocker]

It vastly increased, or appears to have increased, the overall headroom. The overall amount of feedback allows the Master Volume to act more like a wattage control, to a portion of the pot sweep. For those who desire more total gain from their amp, this is giving the opposite effect with the massive amount of NFB, giving the PI a more linear response .

[pdf64]

The 100k tail resistor may act to limit the LTP plates max voltage swing; depending on the output tube type, this may restrict output power, with the LTP clipping before the power tube control grids have reached grid clamping (at Vg-k>0).
So it may be worth checking that the swing isn't unduly restricted.