FET input

[AcePepper]

Howdy,
Newbie here! Built lots of amps, workin' on 1st D-type. What's up with the FET input-thingy? I've seen this in several of the clones & layouts, but not in any of the schems. What's it do? How crucial is it to the D-tone? Do I really need one?
Thanx!
Ace Pepper

[jaysg]

The DC at the plate of the first triode is affected by the FET circuit...apparently. Search on discussions of putting a resistor in place of the FET circuit. I think Scott explains it in a couple of places. The value might be about 150k, though that could just be a starting point.

hth

[Luthierwnc]

The FET (Field Effect Transistor) is a input boost stage on some D-models. It is in addition to the basic input. Here is an older schematic:

https://tubeamparchive.com/files/jfet_156.pdf

Search for layouts and jack descriptions from regular contributors. I haven't ever built one but you can get some pretty good sounds without it.

Jaysg mentioned a resistor. The FET is at the end of the power supply string. Whether you use the FET or not, putting a 150k (ish) power resistor to ground after the last dropping resistor will help keep the preamp plate volts closer to the right levels. Mine average 202VDC on the clean tube and 219 on the OD. If yours come in where you want, cool. If they are too high, this is a good way to knock them down without changing values upstream.

Skip

[AcePepper]

Cool, that's what I wanted to know. It sounds like the dropping resistor is the way to go.
Thanx!