The AM Forum

(https://i.postimg.cc/rsKHtg5T/Large-4-125-TX.jpg)
Same schematic you have seen.  This time I question the Pi network inductance.  The "book" call for L 3 to be 1" diameter, 1 inch long #12 wire coil, L4 should be a 229-201 rotary inductor. To run this circuit at 1800 volts/ .150 amp or the max 2500 volts at .250 mA, the total inductance should be between 21.6 and 26 uHY.  The 229-201 is a 10 uHY inductor, The Jim Hawkins app tells me the 1 inch coil will be less than 1 uHY. and I just did the math using the ARRL Handbook method for figuring inductance and it also says .26 uHY.  What am I missing? Fire away... :)

I think what you missed is that circuit was originally designed to be multiband, and from playing around with a pi net calculator, the designer seemed to be shooting for a Q in the high teens-low 20s across all bands, probably to be able to use what was on hand, and maybe to help performance on the higher bands? Looks to me like keeping a Q in the 20s clear up to 10 meters would keep the values of plate and load C within the variable cap's range, where a lower Q as far up as 10 would require less C, probably lower than that cap would go. Higher Q is also a narrower bandwidth, could've been a measure towards preventing TVI. A Q around 12 would use about 22 uH on 80 meters like you're calculating, a Q of about 24 would be what the circuit as drawn would give. If you're building it to be a monobander, I'd just use whatever value of coil works with the value of capacitor you have or can find.

Take care with switching caps in parallel to the loading cap. This is absolutely NOT recommended though used quite a lot. The connection via the switch should be EXTREMELY low inductance, which is normally not possible. When you swith a 1000 pF and a 300pF loading cap together using a piece of wire, the resonance will be in the 10 meter band with only 0.12 uH That is a few inches or wire!!!. The result is that there is NO loading and NO suppression for 10 meters when transmitting at low frequencies, so lots of TVI , chance for oscillation etc. There WILL be a resonance in the higher HF frequencies when switching a fixed cap in parallel to the load cap, so you never know where the higher harmonics will be.
When transmitting at 80 meter, a 500 pF cap in parallel with the loading cap will result is a resonance and radiation of the 4th, 5th of 6th harmonics with only approx 0.3 uH wire inductance!!, a piece of wire.
Better use a bigger load cap that needs no fixed caps in parallel.

My plan is to use this only on 80 meters at 3,880 & 3,885.  This is good info, thanks Shelby.
Again, I plan to use this transmitter only on 75 meter AM freq's. 3,880 and 3,885. I have a 220 pf C2, and a 2000 pf C3, for the pi network, I just want to get the inductor straight. 26 uHY or the 10uHY the design asked for.