On Wed, 22 Oct 2003 12:43:15 -0700, the highly esteemed Jim enlightened us
with these pearls of wisdom:

Hello,

Can someone explain to me how a solid-state constant current source would
be added to a Mullard-style LTP phase splitter? My amp is similar to an
Eico HF89, which is a Mullard 520 knockoff that uses a 6SN7GTB for the
phase splitter (a schematic can be seen on Ned's site at
http://www.triodeel.com/eicohf87.gif).

The amp is fixed-biased, so it already has a -50V source that I could
utilize for the CCS. If the 18K cathode resistor is replaced with a CCS, I
assume that the plate resistors would need to be balanced (currently, they
are 27K and 33K). Should the grid of the 'lower' triode be grounded
directly, instead of through a .25 cap?


You don't need the -50V supply. Just change both plate resistors to 33K
and replace the 18k resistor with the CCS. I have posted a couple suitable
CCSs on alt.binaries.schematics.electronic. I prefer the MOSFET version,
but either will work. You can use any FET or bipolar as long as it has
suitable ratings, and regardless the source/emitter resistor will probably
need to be tweaked to compensate for individual device variations, even
if you use the transistors I specified (these circuits work according to
simulations) - you may want to use an 83 ohm resistor in series with a
100 ohm potentiometer instead of the fixed resistor to allow easy
adjustment. Also, the values in the resistive divider in the mosfet
versions gate circuit (NOT including the 50 ohm stopper) can be increased
by an order of 10 (i.e. use a 12k and a 470k instead of a 1.2k and a 47k)
to reduce the wattages of the larger resistor (needs to be at least 1 watt,
but preferably 2 watt for the 47k+1.2k scheme, whereas the 470k+12K
divider would only need 1/4 watt rating). A 1uf+ film cap from the divider
center to ground is a good idea, especially if the 200V supply isn't very
well regulated. I should have included it in the schematic. Incidentally,
I prefer to use the 47k+1.2K combination since it keeps the overall
impedance lower. You don't want the voltage on the gate of that MOSFET
(or the base of the bipolar) moving around. While in theory the CMRR of
the diff-pair will prevent current variations from causing much of a
problem, the reality is that any two discrete devices (tube or SS) never
perfectly match, which hurts the CMRR of the pair.

Both sources will drift with temperature variations. The coefficent of the
bipolar CCS is positive (current decreases with increasing temp), while
the mosfet will usually be negative. In either case, the amount of
variation is relatively small, being approx. 4-6uA per degree C. Thus, the
change from room temp (25C or so) to 100C (far too hot to touch - it
shouldnt ever get that hot in real life) would be a half-milliamp or less.

Hope that helps...

--
Greg

--The software said it requires Win2000 or better, so I installed Linux.