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John Larkin John Larkin is offline
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Default Experiment (valve). - NFET.jpg

On Sun, 02 May 2010 18:39:11 -0500, flipper wrote:

On Sun, 02 May 2010 14:59:19 -0700, John Larkin
wrote:

On Sun, 02 May 2010 14:48:19 -0500, flipper wrote:

On Sun, 2 May 2010 15:20:11 +0100, "ian field"
wrote:

trim snip


The clipping characteristics would be more of a concern if it was intended
for use as a guitar amp, it is hoped that the increased gain and current
handling will mean it never goes anywhere near clipping.

I thought you were talking about a radio output stage substitute. If
so, then you're constrained by everything else already in the radio:
the OPT, B+, and so on. Neither gain nor 'more current' is going to
make much of a, if any, difference because you have an already defined
B+ into an already defined output impedance. You can't swing more
voltage across it than is there and with no more voltage you swing no
more current.

Increased idle current would likely affect the B+ filter and the
preamp stages already expect a certain output gain so I'd think the
goal would be to 'match' the designed for tube as close as practical.

BTW: why does the bipolar need to be an MJE340 ?

I said it was there simply by reflex and probably overkill.

I wasn't trying to optimize it, just demonstrate the idea.

The MJE340 is a nice 'all around' HV transistor and I do use where the
300V is needed, so that puts it in my parts bin. 300V also makes it
robust in 'low voltage' circuits under fault conditions, like say a
tube short, but, as I mentioned, that's probably moot in this circuit.

Hmm, speaking of robust, it might not be a bad idea to put a reverse
protection diode in there.

Anyway, it's just a transistor I probably don't have to 'worry much
about' so slap it in for testing, since I'll have one in the bin (as
well as the STP2NK60Z) to breadboard with, and optimize things later,
etc.

Feel free to juggle things around to suit your needs.

Which reminds me, splitting R4 into two, mentioned in my last post,
also widens the range of jfets you can use since Vgs can be catered
for by adjusting the 'gate boost' ratio. I mean, the main reason I
suggested a large Vgs device was to get some source R under it for
feedback (reducing gain & distortion) and boosting Vgate accomplishes
the same thing for a low Vgs device: you increase the source resistor
(actually the one under the MJE340 in this case) to counter the
'boost'. Should be able to tailor it to almost any desired gain by
selecting appropriate jfets and suitable 'boost' ratio/source
(emitter) resistor. Also acts as an input impedance partial boostrap.
How's that for serendipity?

The R4 split might also be the best place to put a 'bias adjust'
trimmer. Once set it shouldn't need readjusting so that doesn't need
to be user accessible.

I kinda like the 'bias boost' idea because it keeps the bias trimmer
out of high current and makes for quite a bit of flexibility.
(attachment)

I didn't look at Vgs variability in picking that adjustment range.
Again, it's 'for show'. The BF244 is lower Vgs than the previous BF246
but with 'bias boost' we end up with a 20V zener again, equivalent
source resistor feedback, and similar gain (clip point). Also note we
don't need high current jfets anymore because the NPN is doing that
work. Btw, that could be a (MOS)FET too but it's Vgs would increase
the reference voltage needed, cutting into output swing. That's why I
went bipolar.

Something I'd keep in mind when testing is potential noise from that
zener on the gate. I'm gambling it's small enough to not matter on an
output stage but if it's a problem then filter it or create a voltage
reference by other means. One might be tempted to use a simple
resistor divider but, while I haven't done an analysis, I suspect
component tolerances and B+ variation are likely too much to ensure
things work properly on the low end while staying safe for the jfet on
the high end, plus that's a hum injection point, hence the zener for a
quickie first shot.

Maybe we should patent this thing



Why not just do this? No adjustments needed.

John



I dunno. Give me some component values and we'll see.


Make the upper resistor whatever it needs to be to get +10 on the
gate. Make the source resistor whatever you like to get your preferred
idle current.


Just off hand, the 100k makes input impedance rather low and if that's
a MOSFET then gate capacitance is a problem.


Depends on what you drive it from. 100K is not going to load many
driver circuits.

If you delete the source bypass cap, Cin goes way down. At that point,
drive levels will be closer to what a toob would need, and linearity
is better.

Of course, drain impedance is high, so speaker damping is poor. Some
people like that. Local NFB is easy (just connect the upper resistor
to the drain) and helps that situation.

I was just showing a bias scheme, not a full amplifier.

John