Presence control without PI tube

[peckjed]

Hey,

I do not have the schematic, and haven't had time to figure out how to draw it by going through the amp. My question, any idea on how to add a presence control to a SE amp without PI, or additionally, how to add a PI tube to the amplifier?

The amp in question is a Fargen Mini Plex MKII.

Thanks!

fargen1.jpeg

[thetragichero]

does it already have negative feedback coming one of the output jack taps? if so then we've got a known good place where negative feedback enters the circuit without issue and Bob's your uncle
I've been messing around with some "micro" amps using reverb transformers so something like like this nfb location would be simple


if you don't already have nfb then you'll need to do some investigating as to where to inject it back ibanez the circuit since there's a chance of oscillation if it's positive feedback. a schematic would be super helpful

[JD0x0]

'Presence' is just NFB on triode(s) with highs cut in the NFB signal so the gain stage(s) see less NFB at high frequencies than lower frequencies, and viola, a perceived treble/presence 'boost'

You can effectively do this with any triode gain stage you put NFB on. Just be aware of all the effect of the NFB being applied to the stage. For example, you generally wouldn't want to add NFB to a clipping stage as it will change the distortion and change the point that stage will start breaking up.

[peckjed]

'Presence' is just NFB on triode(s) with highs cut in the NFB signal so the gain stage(s) see less NFB at high frequencies than lower frequencies, and viola, a perceived treble/presence 'boost'

You can effectively do this with any triode gain stage you put NFB on. Just be aware of all the effect of the NFB being applied to the stage. For example, you generally wouldn't want to add NFB to a clipping stage as it will change the distortion and change the point that stage will start breaking up.

Thank you for that. Since im relatively new to going rogue from following layouts - is negative feedback something that is specific to certain transformers, or is it added to the transformer based on the circuit? Do you know of any amps that I could reference a layout (not schematic) to possibly grab and idea on how to add NFB into one of the 12ax7's. V1 is the 1st & 2nd gain stage, V2 is the cathode follower which drives the EQ stack.