On Oct 9, 2:03*pm, flipper wrote:
On Fri, 8 Oct 2010 06:34:00 -0700 (PDT), Patrick Turner
snip,
So the range of voltage movement means he'd need about 10V across the
CCS to allow the voltage to reduce down to 2V and up to 18V which is
OK. 10V x 1A = 10W and is slightly more Pd than the ECL86 pentode
section.
That *is* "as much" isn't it 
Yes, all things considered, it would be a good idea to have some
headroom.
Purists might argue it's really a hybrid amp and not 'all tube'.
Ah, but the CCS has infinite impedance and cannot affect the ac
current change anywhere except by indirect means of preventing the
core from saturating, so the amount it could be considered to be
hybrid = 1 / near infinity = approximately zero.
Actually, no, it isn't 'infinite'. It's a lot, all right, but not
infinite. However, that wasn't the point.
It's in the audio path.
We can argue it's transparent, and I might be inclined to agree, but
it's an active solid state thingamabob in the audio path so a 'purist'
might take umbrage with it.
Didn't say *I* would. Just threw it out for consideration.
I agree with you. I might say it IS HYBRID in the minds of those who
argue it to be so.
But I often use CCS anode loads for triode signal stages to avoiid the
triode having to produce power in a DC carrying resistor between anode
and a B+ rail. Such a "devious" practice makes the anode load maybe
20Ra rather than 3Ra and the TDH is reduced 10dB for free, and without
any external NFBb, although because the triode has more internal open
loop gain there is more internal truode NFB applyting itself hence the
reduction of THD with near maximal gain and near CCS loading with very
little current change.
Which is worse? - a triode sleeping with a whory bit of solid state
arse, or a simple country girl resistor?
Or do we insist the triode sleep with a big fat dame with an iron
character with plenty of turns?
A man can't win the arguments, but afaik, a CCS using a transistor or
mosfet *improves the sound*.
You'd need the CCS to track tube current, which might pose some
motorboating problems due to feedback through the transformer and the
cathode bias capacitor time constant.
Well, with the added C coupling to load any GNFB could cause LF
instability because of the added time constant and hence phase shift.
But phase step networks should cure all that. The CCS shoud have no
effect because it has constant current.
GNFB is a different matter. Tracking tube current means you have to
measure it and then that is fed to the secondary which is then coupled
back to the primary. That's potentially a feedback loop. Hopefully not
much of one since the pentode is a current source but, well, something
to keep in mind is all I meant.
OK, GNFB does generate some stability challenges.....
A simple approach might be to (multiplying) current mirror the cathode
into the secondary.
AAAhhhhhh........
Dern handy things, current mirrors Although, that large a
multiplier might pose a challenge or two. No matter what the mechanism
is it would need to be adjustable and then some process for
determining when you got it right. Maybe run a full power 20Hz pilot
tone and adjust for symmetry.
Btw, I almost suggested a choke loaded parafeed like you did in the
other post but that means expensive chokes and, like I mentioned
above, I presume the idea was to save the cost of such things.
The double voltage series push pull parafeed burns off about as much
(don't forget heater) as the secondary CCS so that would be to take
care of 'purist' objections to solid state thingamabobs.
If one does the amp with tubes in series it probably is best to use
only triodes. A suitable type which is cheap and doable by a beginner
is the 6CM5/EL36 which are dirt cheap because nobody wants 'em even
though they'll give a 2A3 a good contest.
6CM5 in triode has µ = 5 and Ra = 600 ohms. Pda for class A can be
18W.
The triode use means you don't have to muck around with screen
supplies.
And of course one can make a simple little SRPP with a pair of 6CM5 to
get about 7W and you only have to drive the bottom tubes. The load is
an OPT and cap coupled from the top tube cathode. Biasing is easy
although a heater winding for the top tube must be biased at the top
tube cathode potential.
Patrick Turner.