1987X NFB circuit help!

[Dr_Stav]

Hey there guys,

So I am building my own 1987x rockstah mod 5 50 watt plexi head.

I have the schematics of the Mod 5 for the 100 watt SLP head, and the Negative feedback resistor used is 47k, then going to the 16 Ohm tap.

My question is do I alter the NFB resistor value at all for the 50 watt version ??

If so what value resistor would you recommend for a high gain 1987x ? I want to make this amp as Raw/sizzling as I can with some good power tube grit/grind.

Plus should I keep the 16 Ohm jack for the tap or would 4 Ohm tap give me better results ?

[neikeel]

If you have to little NFB you end up with an unstable power section, particularly with high gain preamp. Mark did a lot of experimentation with NFB and generally the stock 47k on 8ohm is (IMO) best on a tweaked 50w.

[Dr_Stav]

Ok, i Dont use 8 Ohm outs ever so I won't include an 8 ohm out in my amp, just 4 and 16 Ohms. So out of the 4 ohm and 16 ohm which would you recommend and should I keep it at 47k ??

[danman]

Your amp does not need an 8 ohm output jack to hook up the negative feedback. As long as the OT you are using has the 8 ohm lead, you can simply connect it right to the turret that the purple feedback wire connects to. This will give you nfb from the 8 ohm tap no matter what cabinet you are using. You could also just connect the 8 ohm lead to the impedance selector along with the nfb wire and simply not use the 8 ohm option. Your amp still sees 8 ohm feedback even though you are not using an 8 ohm cabinet.