adding an NFB into an EL84 SE amp

[ozzybui]

Hi everyone,

I'm trying to build an high gain low wattage amp (preamp like an SLO) and power amp is EL84 SE (pentode mode) but it doesn't sounds right. I think it's because of the SE power amp. I'm thinking of adding an NFB but don't know how. I looked at the Fender Champ-Amp 5E, it has an NFB goes to the cathode of the last gain stage but. I was thinking of using the same method on my but my amp has tone stack and CF so I'm wondering if there is another way to add an NFB. Should I feed the signal from the transformer secondary to the grid of EL84 through an 47K ?

[Dr. Tube]

Hey ozzy... i have an excillent schematic where you can put NFB switch or pot... I studied this thing... heres the schematic...

[eschertron]

Hey ozzy... i have an excillent schematic where you can put NFB switch or pot... I studied this thing... heres the schematic... [ATTACH=CONFIG]43335[/ATTACH]

So ozzy hasn't posted here in almost seven years, but I'm interested. Where would you put the NFB pot on the schematic you posted?

[Roberto]

Did you take care of the swing that you at the output of the CF, and the needed swing for the EL84's SE?
How do you coupled the two circuits? Do you have a schematic?

[Gingertube]

Alternate feedback scheme:
Break the 0V connection at the bottom of the cathode bias resistor and bypass cap and connect it instead to the transformer secondary speaker connection such that the tube current flows through the output tranny speaker secondary winding. That applies cathode feedback to the output tube.
If you have multiple speaker impedance taps you can get slightly different feedback levels by switching that line to the 16, 8 or 4 Ohm tap. Make sure you get the phase correct.
Just an idea - FWIW.
Cheers,
Ian

[Roberto]

I tried Ian suggestion time ago with an Hammond, it improved alot the performance and low end of the amp.

[Gingertube]

Cathode Feedback lowers the output impedance of the amp by lowering the internal impedance (rp) of the output tube.
This does 3 things:
1) the lower output impedance better drives the lowish primary inductance of the output transformer which improves bass by lowering the low end roll off AND MAYBE you can use a cheaper, lighter output tranny.
2) the lower output impedance better drives the leakage inductance and inter winding capacitance of the output transformer and improves top end, to a lesser extent than the effect on bass, because you are working against 2 high frequency roll offs.
3) the lower output impedance better electrically damps the speaker making it less "skittish", sound is "tighter" or more under control.

A caveat on this 3) point above: . REAL Speaker damping effect is calculated from one those Root Mean Squared (RMS) thingees of the Mechanical Damping (closed Box/Open Box/ported?/etc.) and the Electric damping from the amplifier. Scientifically we use Thiele / Small parameters of Qms = inverse of mechanical damping and Qes (inverse of electrical damping) to come up with a RMS value of those two which is Qts, the inverse of the Total (Electrical and Mechanical) damping.

Guitar Amps Speaker Boxes are generally unlike HiFi Speaker Boxes in that they have high Qms ratings (crappy mechanical damping).
What that means is that the reduction of amplifier output impedance effect on speaker damping is likely to be less than you expect because while you have increased electrical damping you have done "bugger all" (as we say in OZ) about mechanical damping.
As a flip side of this: sometimes the solution to your loose amp problem maybe (for example) to nail another board across the back of your speaker cabinet to convert it from 1/2 open back to 1/4 open back, thus increasing mechanical damping.

Cheers,
Ian