R.G. wrote: ↑Tue Jun 04, 2019 3:38 pm
MOSFETs can be classified in three categories by how you have to manipulate the gate-source voltage to get them to turn on (conduct current) and off. Modern MOSFETs are most commonly enhancement mode devices. Enhancement mode devices do not conduct all by themselves, and you must enhance the gate-source voltage to get them to let current through.
They also come as depletion mode devices: like vacuum tubes and JFETs, depletion mode devices conduct current if you do nothing to turn them off.
The third variety of devices are both enhancement AND depletion mode. The LND150 is one of these. An LND150 conducts some amount of current if its Vgs is zero. If you make Vgs negative, it turns progressively more off until it conducts no current. That's the depletion side. It also conducts more if you enhance the gate source voltage by making it more positive.
The LND150 is not a third type of MOSFET. It is a depletion MOSFET. It is a characteristic of depletion MOSFET's that they can operate on both sides of the 0V Gate-to-Source voltage. It is a characteristic of enhancement MOSFET's that they can only operate on one side of the 0V Gate-to-Source voltage. Which side of 0V the enhancement MOSFET operates at depends on whether it is n-channel or p-channel. What a depletion MOSFET does on each side of 0V depends on whether it is n-channel or p-channel.
It is the construction of the MOSFET that determines whether it is an enhancement or a depletion MOSFET. An enhancement MOSFET has no channel when the Gate-to-Source voltage is zero. A Gate-to-Source voltage above the threshold voltage for an n-channel (or below the threshold for p-channel) will create a channel. The magnitude of the current through the channel depends on the Gate voltage.
The depletion MOSFET has an actual channel made of either n material or p material depending on whether it is an n-channel or p-channel MOSFET. The channel can be "enhanced" by a Gate voltage on one side of 0V Gate-to-Source and "depleted" by a Gate voltage on the other side of 0V.
R.G. wrote: ↑Wed Jun 05, 2019 12:15 am
In doing your design, remember that the transconductance - yfs, or what would be mu for tubes- is very much larger for MOSFETs. This parameter is the conversion rate for changes in grid/gate voltage to plate/drain current. Transconductance is in units of Siemens - amperes per volt of control change. Power MOSFETs are commonly about, roughly 1A/V. So a 1V increase in Vgs makes a 1A increase in drain/source current - if the external power supply and loads will let it, of course.
Forward transconductance for MOSFET's is most commonly given the symbol g [sub] fs (no "sub" key on this forum?). Whereas power MOSFET's can have a large transconductance, the LND150 of interest here only has a typical transconductance of 2.0mS. 1.6mS is considered typical for a 12AX7.
So the gain of a tube is mu times Rp, the gain of a MOSFET is yfs times Rd.
The gain of a tube or MOSFET is the voltage across a component of interest divided by the voltage applied to the Grid or Gate. The transconductance will determine the voltage across the combination of internal and external resistances in the output circuit. It makes no difference if the external resistance is connected to the Plate/Drain, Cathode/Source, or both. A voltage divider is formed with the internal and external resistances and the transconductance-determined voltage is spread across this voltage divider. For a fully-bypassed triode gain stage, the only external resistance is Rp since Rk is fully bypassed. If you want to know what the voltage gain is at the Plate, you would multiply mu by the voltage divider equation rather than mu times Rp.
I've taken the ideas into consideration - and added yet another idea. I've got six preamp tube slots, and a quad of 6L6s in a fender Twin sized chassis so I figured I would do the Fender thing and do an extra channel - but this time something voiced for pedal steel (I've recently got into playing PSG)
For this extra channel, I'd like to play with using a Pentode stage like an EF86 or 6BR7. Yes, i know these tubes don't always play super happy in a combo amp. I'll do some creative shock mounting when i get around to engineering this beast.
So, extra channel is going to be a typical Pentode input stage, with an LND150 driving in source-follower configuration a typical Blackface Style tone stack, tweaked to sound good with PSG. toggle switch for Midrange control all the way up/all the way down (because i'm weird). Passively mixed with the other channel before the input to the vibrato section.
For this Emitter/Source follower setup, I'm guessing I'll need to set it up a lot like the LND150 stage driving the oscillator signal in the tremolo - does this look correct as I've drawn it in the schemo?
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I'll offer my input now, so as not to interfere with the meaningful discussion to follow.
Why not direct couple the follower in the second channel? Seems to me it'd be better to connect the gate directly to the plate of the preceding triode and get rid of drain-to-gate resistor. Maybe I'm missing something...
Those 0.047uF coupling caps in the trem, the ones from the concertina to the triodes; I'd make them at least 10x bigger. They're passing a very low frequency.
Just a thought: when it comes to the trem and preamp circuits, isolating their power supplies is probably more important than a little extra ripple filtering. I would branch the dropping resistors for B+4 and B+5 off of B+3 separately. The trem might leave a lot of LF in the preamp supply if you try to pull B+5 from B+4 as drawn. Branching them separately off the PI supply give a little extra isolation.
I has considered direct coupling the plate to Base, but was thinking that it would be prudent to trim some of the low end with a cap before moving on to the tone stack.
As for the oscillator, IIRC it is generating roughly something like an 8hz LFO tone - truth be told I aped that oscillator design from someone off of Hoffman amp forum that was trying to do something similar to I.
And as far as B+ nodes go - I apologize but that is incorrectly drawn on my schemo. They are placeholders for when I get around to squaring up all the schematic errors.
When it comes to adding SS stuff to tube stuff, and things like oscillators in general I'm still pretty green. I'm standing on other more qualified shoulders and just trying different things at this point
"It's like what Lenin said... you look for the person who will benefit, and, uh, uh..."
As far as the couplers in the phase splitter in the tremolo section, a lot of that is just pulled from the brownface vibrato schem. I don't recall offhand the LFO frequency in the original design - I may be using a lower frequency LFO than the original.
Attached is the schemo for the original circuit.
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