Quote from: stratitis on March 26, 2020, 07:46:15 AM
Do you know if they sound different?
I thought they are nearly the same, maybe I'm wrong.
How did you "adjust" the single stages?
Yep, the 5150 III is made by Fender, not Peavey, the schematics are still widely available via google search. It does sound a bit different, a bit more high end bite, but the underlying feel of the 5150 is pretty much the same.
I only built the high gain channel, the cleans on the original 5150 are not that impressive and the other two channels of the 5153 would require a lot more gain stages.
The adjusting was done by comparing the two circuits, LND150 and the original 12ax7 stage. The plate or drain resistor is the same, so is the grid resistor and the coupling capacitor. The diode biasing circuit is exactly the same as proposed in KMGs article, but the bias circuit (with the PNP BJT transistor) is adjustable with a trimmer. What would be the cathode resistor and bypass capacitor in the tube stage are changed slightly by comparing the KMGs Marshall preamp circuit and the original JCM800 schematic. I used what i had, so it was the next closest E24 value. Instead of a 1.8k resistor I put in a 2.2k one and so on. The bypass caps are not really that important here, as the 5150 by itself already cuts bass below 800Hz in the first stage. I think I left them at 1uF.
Also, I put in a 500k gain pot with a resistor to decrease the maximum gain. It has a lot of it.
Most of the circuit in the preamp I recorded is 5153, but I didn't have any room on the perfboard left for a source follower tone stack, so I just made it plate driven, as in the original 5150. That probably cost me some highs, as the impedance is higher, but it wasn't that bad.