Trevor Wilson wrote:
"flipper" wrote in message
...
On Sat, 19 Aug 2006 11:47:20 +1000, "Trevor Wilson"
wrote:
"flipper" wrote in message
. ..
On Sat, 19 Aug 2006 08:33:33 +1000, "Trevor Wilson"
wrote:
snip
**It is still NFB and few deny this obvious fact. It is why you cannot
build
a triode amplifier without NFB. It's already in place.
A triode amplifier with no NFB.
**No such thing.
I gave you the circuit for one in my post, your snippage
notwithstanding.
If you mean a Triode amp with no GLOBAL NFB or added LOCAL NFB (cathode
resistors, et al), then that is a different thing. In which case, that
needs
to be spelled out.
There is no need for you to feign "if you mean" as I described the
circuit making it clear 'what I meant'. Operate a triode into a
current mirror and there is no 'feedback', global, local, or
otherwise.
**It's no longer a Triode amplifier, by the strict sense of the word, is it?
The active load is now part of the amplifier.
I routinely use a solid state CCS load for a single triode preamp gain tube or
the
input tube of a power amp.
And indeed the active CCS load makes the triode amp a hybrid.
Butt the CCS I may use is just a bjt used so that the dynamic collector
resistance
is increased from the data figure of typically 20k to about 20Meg, so that any
voltage change
at the collector does not produce any significant current change.
The huge effective collector resistance is shunted by the triode's comparitively
very low
Ra, so that any artifacts produced by the presence of the bjt are reduced
by about 3 orders of magnitude; ie, the bjt CCS acts as a totally complient
slave to the
current needs of the triode, and when there is minimal current change and
maximal voltage change
in the triode then the NFB applied within the triode is at a maximum, allowing
it to have a gain
approaching the µ on the data sheets.
This gives very low THD compared to typical R loads normally used by less keen
designers.
I suggest you view the data curves for the 6SN7, and draw the load line for
20Meg ohms through Ia = 4mA at Ea = 150V, and plot the THD for +/- 10Vrms.
Its so hard to do this accurately because there is so little THD that one must
set up
a 1/2 6SN7 and measure it with a very high input Z test probe to a THD meter.
CCS are routinely used as common cathode loads in tube circuits.
The ARC amps use depletion j-fets for CCS in cathode circuits
wherever they see fit instead of using a pentode.
The pentodes would not work as well as the depletion fets.
Ditto with bjts.
One may call such circuits hybrid, but at no point in the circuits mentioned
are SS devices used to produce voltage gain; all the circuit gain is produced
with triodes.
The active SS CCS loads remove the sonic degradation caused by resistors
because resistors load a triode to cause current change and the more current
change there
is the more distortion there is.
Go back to the data graph for 6SN7, and plot a load of 40k for the anode load.
The try RL = zero ohms, ie, no load, and that is where thd is the greatest.
Should you not understand load line analysis, read my website pages to find out
how.
Patrick Turner.
--
Trevor Wilson
www.rageaudio.com.au
--
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