Blair wrote:I'm sorry, yes it is the grid I injected the NFB into.
I see most NFB injected between the cathode resistor and a series 100 ohm resistor on the VA stage. Should I add the 100 ohm resistor in series?
Blair
My thoughts, not knowing the source of your design elements,
Let me start with the grid injection. If you are following a proven design, I'd try to make that work right first. I can't say whether that is better or otherwise from cathode injection.
As far as the "sound" with FB goes, keep in mind (I discovered this with my new build) that the open loop FR of your amp probably drops dramaticaly from about 10K out, so it may sound "good" "OK" "normal" that way, but when you add FB the FR goes flat farther into the highs which gives the illusion of less bass, when in reality it is more realistic treble. The overall gain will be less and make the amp sound less powerfull, of course.
As far as cathode injection goes, I've seen it go in at the cathode above the resistor, I've seen it injected between two resistors like you're suggesting. But look at the ST-70 and you'll see 47R under the injection point so it's all about the precise design goals that dictate that. FR tailoring in the design and gain of the amp is what ultimately fixes those component values. I can't do the algebra using your design to give you input sensitivity/gain/FR/distortion/FB signal balance. Those are parameters for you to come up with.
One of the first things to decide after you set your power output goals is to set the input sensitivity goal (.25v, .5v., .75., 1.3v....??) and then the gain of the amp falls into the next set of decisions. In the ST-70 the open loop gain is 125 @ 1.3v input into 8R. Figure out what the output power would be under those conditions and you'll see why FB is necessary to cut the gain.
Good Luck!
Headed for Tishomingo to sing in a can...