Presonance issue

[Reeltarded]

Oh no you don't! I am the question asker around here. hah!

[Tonegeek]

How does the amp behave open loop? I can't see how decoupling the loop at low frequencies would be worse than decoupling it completely unless the feedback was supressing an oscillation.

Luckily I went ahead and ran the amp with no feedback and it worked just fine without it. I did not however scope it when I had the chance to see what freq. the AC voltage spike was. I have since rewired the amp back to the original presence circuit and am on to other pursuits. i should like to revisit this circuit again in the future. I have an 18W Marshall style amp I would love to try this thing on.

[Firestorm]

I think Martin may have been right to suggest that a sum or difference frequency may push the 60 degree shift past 90. I, too, would like to see this sorted out since the circuit is pretty elegant.

[martin manning]

I did a little playing with simulations and actually got an ultrasonic oscillation to happen on one whole-amp sim, but not on another. Given the differences betweeen the real world and a sim this may not be definitive. However, placing a resistor equal to the pot value, which is 2x the tail value, between the bottom of the pot and the feedback resistor kills it, and also leaves a series resistance in between the resonance cap and the OT when the pot is turned all the way down. It doesn't change the response of the presonance NW too much, with full-clockwise being essentially the same as the original presence, so it seems reasonable to make this the new baseline.

Colossal, is it possible for you to stick a resistor in there on your Livermule and see if the result is still good to your ears?

[Colossal]

Colossal, is it possible for you to stick a resistor in there on your Livermule and see if the result is still good to your ears?

PM sent

[martin manning]

Colossal has again used his mule amp to test the control with the added resistor and finds it to sound as good or better than the original. This may reduce or eliminate the stability issue described in this thread, so I've posted a revised schematic and description in the original Presonance Control thread. Thanks again Dave!

Original thread:
https://tubeamparchive.com/viewtopic.ph ... highlight=

[Colossal]

Colossal has again used his mule amp to test the control with the added resistor and finds it to sound as good or better than the original. This may reduce or eliminate the stability issue described in this thread, so I've posted a revised schematic and description in the original Presonance Control thread. Thanks again Dave!

Original thread:
https://tubeamparchive.com/viewtopic.ph ... highlight=

I did add the additional 10k resistor and noticed a slight increase in the low/mids when the pot is rotated in favor of resonance (a good thing). I did not notice any losses in clarity. I have had no stability issues and can dime all the controls on the amp and go full presence or full resonance without whistling/oscillating. Although this is a test amp and is far from neat, I spent a little time doing some lead housekeeping and replacing a few things here and there. Overall, the amp is incredible and has one of the best distortion tones I have heard. The tone is very musical and horn like. It is freakishly loud.

Great job on this mod Martin

[Reeltarded]

Ok so just throw resistance at it, which we already know is futile.

hah I made a shitty star trek joke.

[Reeltarded]

Yep. That worked Martin. No more kink, and I agree that it works better now!

[martin manning]

That sounds like progress! A little resistance thrown in at the right place can be a good thing. I put a new schematic (R2) up with the extra resistor shown as standard. Thanks for reporting on your results. What component values are you running in your amp now?

[Reeltarded]

100k and a pair of .047s again, for now. This thing is sweeter than ever now!

I put it back on the 4R tap for the moment, but I like the action of the 16R, just a little too smooth for what I am used to.

So, throw a 3-pos dpdt switch and a couple pairs of caps at it, add a variable control between the left lug and the NFB resistor.. that last bit won't work as I expected it to. I already tried. I like to vary my NFB. I'll just stick with the ppimv for that. lol oops

[martin manning]

Great, glad to hear it's working for you now!