Quote from: Roly on October 08, 2013, 06:02:55 PM

Quote from: Superfuzzmy old Carlsbro slave amp (wich I converted to current feedback) but for some reason I get oscillations

Now with current feedback the phase of the feedback can be a very sensitive function of the driver and cab resonances, and it is quite possible for feedback that is negative or out of phase in the mid band to rotate sufficiently at the extremes of frequency response to become in phase and cause transient instability, ringing, our outright oscillation, either low frequency putt-putt "motorboating", or high frequency shreaks or whistles.

For this reason current feedback is typically limited to around %10 of the level of the voltage feedback in the same amplifier, and attempts to apply more current feedback may result in various forms of amplifier instability.

HTH

That's a big help! Is there any formula to calculate the ratio between voltage feedback resistors and current feedback ones? Now is too late here but tomorrow I'll upload the simple schem of the mod I did to my carlsbro slave 1000, I must have messed with values too much!

Ok, thank you all for all the explainations..

Since I' spent a lot of time experimenting with stuff, I'll try to tell you something more of what I feel/need when playing since my experience as a player is a lot greater than my electronic skills (I'm only an hobbist there).
Just to find a point of what could be the direction of my research:

I've built my sound around SS amps, I play fast harsh stuff (when I say harsh I speak about playing distortions thru custom cabs with tweeters inside, it's like the end of the world) so transistors have always been perfect.
My eq. tastes are also mid-less so, tubes are useless also there..

THEN (lately) I started missing that kind of hump that comes with palm muting and stuff like that, so I first started messing with pedals (I've built a lot of them in years) and eqs but, I found that adding let's say, a peak around 100/150hz (where speakers usually have their inpedance peak) sounds close BUT then everything gets muddy and lifeless..especially when I hit my deepest distortion, it gets confused...not that kind of "breathe" that you can have with tube amps..

So, bigger experiments came:

Lately I evolved into this new setup, all my pedalboard works as a preamp, and then I go straight to power amp input of my SS amps (usually a '77 H|H ic100L). I like it a lot, very "clear" and it's also louder (to the point I can saturate my power amp input LOL)

Then while recording, I find it lacking a bit of that Hump anyway, so I first tried my old Carlsbro slave amp (wich I converted to current feedback) but for some reason I get oscillations, so I tried an Orange OR120 (power amp only also here) and, BLING. A bit softer, but so much richness, it's perfect in pair with my HH.


NOW: wich kind of power amp (SS obviously) may be the best halfway solution for my needs?

P.S.
I know I'm a lot OT, but I was considering the whole current feedback thing as a keystone for my needs before..

P.P.S.
If you're have read everything I wrote above, and you're still reading thing, it means a lot to me, thank you SO MUCH, really!  <3)

There must be a simple answer for that. Mixing current feedback on typical BJT amps has been a goal for the whole 90's, then mosfets came and no current feedback at all, I was wondering that also for class D amps..

Mosfet trannies are already sensitive to speaker dampening and stuff??

It's my main amp from years but lately it has become dull and lifeless..I also recorded some months ago thru the mixer output, and I'm pretty sure it's in the preamp.
The overall level when I play straight to the amp is much lower and lifeless, then as I put in a LOT of signal with my stompboxes the overall level is kinda right but it's very raw.
It's kinda hard to describe, it's muffled and the harmonic content is really unpleasant, plus the lows don'thave that "breathe" and fast response..

The rail voltages are fine as I get 15.7V on both

May the problem be in all the small electrolytics? I think they're still the originals, the head is from 1977..are they old enough?

Hey I was searching a thread like that from ages! I've came across that blackface with fet schematic so many times and I've never heard anyone who built it! There are some mods I'd like to do to it too! the first one would be to add a resistor on the upper fet source , they says it makes the output inpendance much lower (so you can drive a tonestack in a better way) then I'd put another resistor+bypass cap on the lower fet source (just for a sake of imitation of the real thing)..another nice thing to do would be adding a silent biasing network instead of the two big resistors ..asymmetrical biasing could be good too!
oh man I just want to be a better DIYer to make my first amp for real!

 
Hi, my name is Emanuele and I'm on the way of becoming a full time Solid State player (well I've been that since my first little amp to be honest) since a nice HH IC100L is coming here..I barely can't wait anymore of getting my hands over this baby! In the meanwhile I found this forum so I feel I must sign in being both a player and a DIYer..
Dunno but someone here should have known me as Superfuzz on Offsetguitars.org and as Emanuele on Freestompoxes.org..

Ciao!