wrote:

Thanks Phil
That's good info. I've never compared them. But I noticed ARC used FETs
Would it be a noise issue? Or a PSRR or compliance issue? I guess I need to
build both
and actually measure and compare.

The word "" transistor "" stinks much worse amoung tubophiles than Fet,
and there are latereal and enhancement mosfets, j-fets, and so on.

For some very illogical reason ppl have said that fets of one type or another
are the
only type of SS device to use with vacuum tubes.

With all due respect to ARC who have done a stirling job in perpetuating the
myth of the fet.
ARC have dozens of various types of fets in their horridly complex preamps such
as the SP11 etc.
They are monuments to technical cleverness, but nothing sonically clever is
achieved.
The fets can fail rather sooner than the tubes, and the complexity is a
nightmare to service.


All fets are versions of 'field effect transistors' ie, a transistor that has
its
load working current changed by an applied voltage field rather like a pentode.
ie, the gate is a high impedance control point like a grid.
The mosfet is a metal oxide field effect transistor.

I suggest you spend a month studying the basics of all SS device
parameters, the behaviours, the impedances, and so on, and away from a tube
discussion group
because really not many ppl really understand exactly what happens inside any
simple transistor or 'bipolar junction transistor' as they are also called,
or the j-fet or mosfet.

Bjts require current to be supplied through their input base control
terminal to make a larger current flow in the load and through the collector to
emitter.
The base current is typically 100 times less than the collector current.
The amount of voltage change at the base to emitter junction needed to
change the base current and hence the collector current is much smaller than
that
needed in a j-fet or mosfet.

I suggest you measure a few devices set up as a one device amplifier on a
breadboard,
and see if you can draw a model of the devices you want to use.

BTW, ARC amps are no quieter than any other well designed amp you might
build at home.

I quite like fets though; I just see no point in using
so many in any preamp as ARC.
The only place I like to use a j-fet in a preamp which I would call a
"tube preamp" is at the input of a phono stage or microphone amp where a
single 2SK369, 2SK147, etc in cascode with a triode will
produce noise that is typically 20dB less than what you get with a 12AX7,
thus allowing MC cartridges to be directly connected to an amp
rather than have to be hooked up via a step up tranny.

The j-fet in such a situation can be trusted with a few millivolts
of signal without causing much distortion.

But where an SS device is used as a CCS, the current *is* constant;
no variation is measurable with say a common cathode voltage of say 5vrms.

Most folks get muddled about constant current sources in LTPs because
gee, it a bloomin transistor, and prejudice sets in about possible non
linearities.

If the CCS does have an actual finite value of 20 megohms,
then 5vrms causes a current change of only 0.00025 mA, rather a tiny amount.
And the change is substantially linear.
See http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

But I assure you that although bjts without NFB are attrociously
non linear voltage amps, they make fabulous current sources
when used with the local current FB as in the schematic above.

Probably you need to analyse exactly what dynamic current changes occur in the
LTP.
It took me awhile to understand them.
But the only the tubes and the loads have any significant change.
And the tubes can be very mismatched, and as long as the load R are the same,
there are equal outputs.
Radford used a pentode on one side of the LTP and a triode on the other.

Patrick Turner.






RonL

I'm wanting to use a FET as a current sink for a differential longtail.
This will be a balanced input front end using a 6922 with about a -60VDC
on the tail.


** So the FET connects from -60V to the two cathodes - right ?


All the FETS I have around here seem to have the reverse protection diode
built in.


** Every power MOSFET made is like that - so what ?


Can anyone recommend a good part number for this?


** Almost any MOSFET with a Vds of 100 volts.



I've been experimenting with
the MJE350 bipolar part, but I want to try the FET and see if it will be
more stable with temp and better PSRR.


** A MOSFET will be LESS stable with respect to temperature than a
bi-bolar device.

For bi-polars, the Vbe drops by 2mV per degree C rise for a given Ic -
needing one silicon diode in the bias cct to correct.

For a MOSFET , the Vgs drops by 6 mV per degree C rise for a given I ds -
needing 3 silicon diodes in the bias cct to correct.



........ Phil