Fretting about FETs

R

rossbalch

Active member
*Renamed the thread as it is primarily about FET designs.

Hey folks, so I've been reading Merlin Blencowe's Designing Guitar Preamp book. One thing that has interested me is the effect of essentially a poor design leading to some current leaking into the grid of the follower and therefore leading to some signal compression.

This excerpt is especially interesting

Because the ECC83 is a high ra (anode resistance) valve, it is difficult to bias properly if the grid voltage is high and the cathode load resistor is not sufficiently large—as any classic textbook will indicate. If the supply voltages in the Fender Bassman had been much lower, or if the cathode load resistor had been properly chosen, this effect might not have been realized—and the amp may not have become the icon it is today. Similarly, if a lower ra valve were plugged into this same circuit, we probably wouldn't generate any quiescent grid current, and again, we would not obtain the characteristic compression effect. (Of course, this effect can be duplicated with any valve, provided the resistor values are suitably chosen.)
Click to expand...

He then goes on to mention that bootstrapping the stage as in the following example will exaggerate this effect (show at the end of the post). My question is, would it be possible at reasonable voltages (and by this I mean up to 24v I guess) to replicate this effect with a JFET or a Depletion MOSFET like the LND150? I think that would make a pretty nice addition to a "clean" boost that still helps control transients.

I did just get some 6n16b in the mail which I would be interested to in maybe using as the basis for a preamp. Maybe adapting the Vox Cathode follower stage to bootstrapping and using the 6n16b instead for lower overall gain. But then, how much is gain a factor in the amount of compression?




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Just a hint: There is an old special tube called 12dw7, which one side 12at7 (ecc81) and the other side 12ax7 (ecc83).
Some years ago JJ released the new ecc823 and reversed version the ecc832. The latter one would be a great tube for a cathode follower circuit, which brings a ecc83 into a ecc81 (depending how the pins are wired in your amp. In most cases 6,7,8 are used for the first stage.)
 
Sounds like a good theory. IMHO definitely worthy of an LTSpice simulation or breadboard trial.

Be aware Q2 input is DC coupled from Q1 output. When Q1 is biased Q2 input bias voltage is also set.
 
I've breadboarded quite a few jfet circuits at 18v and 24v. JFet, BJT and Depletion Mosfet will all exhibit cathode follower compression/distortion, with some difference in regards to the tone/timbre. I found LND150 to sound a bit darker (or less harsh) than Jfet, with BJT being a nice middle ground. Cathode resistor value and the load that comes after will determine the degree of "compression". I did try bootstrapping at some point and didn't like the result, but I doubt I spent more than 10-15 min on it :P Mind the capacitor/resistor values, as you can end up with weird frequency boosts. As mentioned above, LTSpice is your friend!
 
Blackboarcult said:
I've breadboarded quite a few jfet circuits at 18v and 24v. JFet, BJT and Depletion Mosfet will all exhibit cathode follower compression/distortion, with some difference in regards to the tone/timbre. I found LND150 to sound a bit darker (or less harsh) than Jfet, with BJT being a nice middle ground. Cathode resistor value and the load that comes after will determine the degree of "compression". I did try bootstrapping at some point and didn't like the result, but I doubt I spent more than 10-15 min on it :P Mind the capacitor/resistor values, as you can end up with weird frequency boosts. As mentioned above, LTSpice is your friend!
Click to expand...
Have you ever done a whole preamp at high voltage, like a JCM800 or Rectifier type thing? I'm curious if there's benefits in running at higher voltage? It seems to me the initial work on these FET circuits was done at 9v just because that's the "pedal voltage", not necessarily because it was actually the best sounding or anything.
 
rossbalch said:
Have you ever done a whole preamp at high voltage, like a JCM800 or Rectifier type thing? I'm curious if there's benefits in running at higher voltage? It seems to me the initial work on these FET circuits was done at 9v just because that's the "pedal voltage", not necessarily because it was actually the best sounding or anything.
Click to expand...
Every single FET circuit I've ever played with has sounded a million times better at 18-24v.
 
Never tried higher voltages, too much faff. As drew points out, 18-24v sounds great :) And yes, 9v was initially done to keep the same "standard voltages" for 99.9% of pedals. Incidentally, J201s are one of the few jfets that sound great at 9v. 2SK208 (as seen in AMT circuits) also sound really decent at 9v, but I've tried them at 18v and they sound even better. Re-bias and overhauling the voicing components is a must, though.
 
Blackboarcult said:
Never tried higher voltages, too much faff. As drew points out, 18-24v sounds great :) And yes, 9v was initially done to keep the same "standard voltages" for 99.9% of pedals. Incidentally, J201s are one of the few jfets that sound great at 9v. 2SK208 (as seen in AMT circuits) also sound really decent at 9v, but I've tried them at 18v and they sound even better. Re-bias and overhauling the voicing components is a must, though.
Click to expand...

Do you have to measure the FETs IDSS and Vgs Off manually at the target voltage? I see the data sheets rarely specify much data after about 15v?
 
Also, I found this design interesting...

Mixing JFETs and MOSFETs. On the thread different circuits actually place the FETs and MOS in different locations in the circuit, though always a MOSFET into a JFET in a coupled "cathode" / source follower arrangement.

Why might you choose MOSFETs in this case? Is it purely texture, or are there also current / buffering considerations too do we think? There seems to be some attempt to simulate Miller capacitance here which is interesting.

Obviously this circuit has a lot more going on than the typical "conversions". But I'd love to understand more of the principles of what's going on here. For instance, the MOSFETs are Vr ground biased, but the JFETs aren't. Q3 appears to be biased in a cold clipper configuration just like V2b on a real Soldano, but I don't know how the match works out on the JFET end versus the AX7 math.

(https://www.guitarristas.info/foros/muestras-sonido-messa-soldano-krank-engl-vht/88517/pagina2)
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I tend to use jfets with tight specs (SMD J201, 2N5484, J113, 2SK208R, and the dmos LND150), and rarely bother checking their Vp or IDSS. By simulating on Ltspice, you can start with drain and source values that are in the ballpark, then check with the multimeter and do some small adjustments to get Vdrain between 1/2 and 3/4 of your voltage, depending on desired asymmetry. When dealing with cascaded gain stages, I found little difference in selecting gainier vs less gainy jfets.
I have seen that SLO emu before. For some reason I've never been very fond of N-channel mosfets for high gain, they sound noisy and rough to my ears. On the other hand, I have mixed jfets and depletion mosfets on several circuits successfully. Since the LND150 doesnt exhibit gate cutoff or blocking distortion like other fets and is not too gainy, it makes for a good first or last stage. For example, taking a Randall RG100 circuit, removing the cathode follower and replacing the clipping diodes with an LND gain stage biased more or less 1/2V has been an improvement over the original, at least to my ears.
Also, some jfets can be biased without source feedback (see some stages on the AMT Series 2), although even at 18/24v only a handful of models like J201 and 2SK208 allow it while maintaining decent gain and gate cutoff. To me, biasing it this way makes it sound closest to valves than anything else I've tried: harsh asymmetric clipping at gate, soft-then-hard clipping of positive lobe, and the negative lobe not quite reaching the rails due to the gate cutoff. Works beautifully with 2SK208 at 18v with a 10-12k drain resistor 🤘
 
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