Standalone EL34 Cathode Bias Power Amp - NFB Question

[dunner84]

Hello,

I am starting a stand alone power amp build, and I want something different from everything else in my collection. I have decided on a Cathode Biased EL34 power section based largely off the Matchless Chieftain, and the Hot Cat EL34 amps.

I have the perfect iron for the project, but I want to make this a bit more versatile for the various preamps I will be running through it, So the idea is to add familiar controls like "Presence" and "Depth", a pot to vary the feedback amount, and a switch to eliminate it from the circuit.

First, I have no experience with the "Brilliance" knob. Is this similar to presence? or is it a cut feature?

The Matchless and Hot Cat design has no NFB. I think some of the preamps I will run through this would benefit from a switchable NFB loop, and I have some questions about how to implement it.

Are the added depth and presence controls going to cause any issues with this design? With how I drew the switch, it looks like the "Presence" control is still part of the circuit when the NFB is disengaged. Should I use a switch with an extra throw to break the connection there?

The schematic I uploaded has the proposed NFB mods in red. Any thoughts?

[thetragichero]

yes 'brilliance' is a cut control
you've got your negative feedback going to your input cap. remove that connection and throw a 1M resistor before that cap
4k7 feedback resistor into a 4k7 tail resistor is an AWFUL lot of feedback. very quiet power amp. scope out some 50w marshall power amp schematics and borrow from known working circuits. even with a 100k pot that 4k7 is way too tiny
you might wish to lower the 47k tail resistor since you're adding another 4k7 for the negative feedback. 39k is the next lower standard value and is close enough for government work
that crossline master volume (which i have used in a 2x6v6 stand alone power amp) isn't the greatest. best bet is a dual gang pot to replace the 220k power amp grid resistors (include safety resistors)

[dunner84]

Good catch on the NFB input to the cap. That just happened to be how the NFB super imposed on the other circuit. I updated the attachment to correct that. As for the 1meg resistor. All of my preamps have this in the form of a pot on the output. I wasn't going to include it on the power amp, but it is probably not a bad idea to toss it on there. Who knows what else I will plug into this thing.

As for the 4.7K I took that value from a Dumble power section as one of my preamps is an ODS. It is really there to set the lowest possible NFB. If it's too low, I will up it after the fact.. The 47k tail makes sense for the case where the NFB is disengaged. Would a 50k pot make sense here so I can vary it based on the level of feedback?

Yes, I am not a fan of the master either. I intend to use the dual gang pot setup that Ceriatone uses in their versions of these amps, but left it off this version of the schematic for simplicity sake.

[thetragichero]

i always put the 1M resistor on the input, that way it plays nice with everything. might not be a bad idea to put a small grid stopper after the input coupling cap
i would just use a fixed resistor for the phase inverter tail. keep it simple and there's already a bunch of knobs so no need to add another