Diode Compression with JFET

[Rayman]

Ever since building the Vulcan, I've been very interested in the diode compression.  I set up a single stage JFET circuit and biased as follows:

JFET - J201
Rd - 56K
Rs - 22K
Voltage divider into gain with R1 at 4.7M and R2 at 1M.

This put Vd at about 4.6v with a 9.17v supply, and Vg at 1.6v.

Running through a Blues Jr it sounded pretty good, but then....

I put a diode (4148) between R1 and R2 (as setup in Joe Davisson's circuits).  The sound went from pretty good to really good.  I actually liked the sound better driving the Blues Jr then just the Blues Jr alone (not that the Blues Jr is the best tube amp).

Can anyone explain exactly what the diode is doing?  Is it clipping or "compressing" the ac signal?

PS - I had to re-bias Rd to 56K after adding the diode.

[petemoore]

 Is it clipping or "compressing" the ac signal?
Yupp, at the input/source.

[Rayman]

correction

I re-biased to a 47k Rd.  I also used a 22uf cap across Rs

[Joe]

It basically lowers the gain as the signal strength increases. Both BJT and JFET transistor types have an internal diode (base/emitter or gate/source). A large enough input peak puts this diode into forward-conduction, however slightly or briefly. (Keep in mind that signals in a transistor circuit are in the form of DC pulses, not AC.) This conduction causes a BJT to saturate, and a JFET to cut-off, creating bad distortion in both cases. With the input diode in place, larger signals are matched with more resistance, preventing the problem without reducing the gain much to lower-level signals.