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Patrick Turner Patrick Turner is offline
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Default 6LU8 SET or SEUL Amp

On Mar 19, 12:56*am, John L Stewart wrote


Hey Patrick, I won't comment on your most recent except to day that
Flipper is correct in his post that it is important to attack distortion
in the hilevel stages first. If need be, find gain somewhere else, a low
level stage perhaps if there is a shortfall.


I was merely quoting RDH4 where they measured an amp with NFB and
**with** PFB and **without**
..
RDH4 had reason to believe the overall effect of adding PFB was a
reduction of IMD. The PFB raises open loop gain which allows more
global FB to be applied and the NFB reduced the IMD much more than any
increase of N&D because of PFB.

All PFB increases N&D locally at least, within a circuit, and I have
no argument with the text books. But that does not mean anyone can
conclude PFB will always increase overall N&D in the presence of NFB.

Electronic behaviour can rarely be described by any simple
generalisation and in 10 times out of 20 every generalisation is
subject to conditions of operation, behaviour A happens if condition
B, C, and D are present and if condition E. F are not present, etc.

It was the overall effect of combined PFB and NFB and not the local
which the real issue, at it was in your OP.

I also believe its best to reduce the OP stage N&D locally as much as
possible without causing the driver voltage to be come so high that it
begins to contribute more N&D to the total N&D outcome than the OP
stage.

A McIntosh OP stage in plain beam tet mode with no 50% CFB windings
and biased for largely class B op with low value RLa-a would have much
more N&D than with the 50% CFB. Even with the 50% CFB, the outcome
without other additional loop FB is rather poor compared to a class A
UL amp or triode amp.
To get 50W from 6L6 or 75W from 6550 means means high N&D.

But the McI driver has to give over half the Vac between anode and
cathode to each grid, so driver N&D is high. The global NFB enclosing
all stages reduces the total N&D of all stages about equally to an
acceptable figure which in McI's case is better than many other amps
of its day. The McI Rout is especially low because of the large total
amount of all loops of NFB and despite the inclusion of PFB in the
bootstrapping.

EAR509 follow a similar idea of having lots of voltage gain which
allows lots of FB, and aided by bootstrapping, carefully done to avoid
instability to keep Nyquist happy with resistance loads.

But EAR failed the Nyquist if a pure C load was used. Paravicini must
have thought nodody was ever going to use a 0.47uF speaker load. I
rebuilt a pair of EAR509 and was able to make them unconditinally
stable while retaining the existing high OLG and all NFB loops, and
retaining a 50kHz BW. Sometimes I think master designers
likeParavicini have **** for brains.


The short equations I gave predict the worst case of +ve FB since we
assume the triode gain to be simply equal to mu while bootstrapped. In
reality, while bootstrapped the triode gain is somewhere between the
grounded cathode value A1 & mu. So the +ve FB is always less than
predicted.


In the McIntosh the local NFB in the OP stage is 15-20 db, depending on
loading, Etc. So lots of room for a couple of db +ve FB.


The rp of the sample circuit given turns out to be about 10K since the
cathode current in each triode is 4.3 mA. That translates to a worst
case +ve FB of 2.74.


The equations can be further reduced as follows-


A1 = ( mu*Rl ) / (rp + Rl ) and


A2 = mu


Gain change is A2 / A1


So Gain change becomes ( mu ) / ( mu*Rl ) / (rp + Rl )


That reduces to Gain Change


A2 /A1 = (rp + Rl ) / Rl


Very easy to apply to various conditions.


I made some real measurements on working systems 8 or 10 years ago but
can't find the results. As I recall, all less than 2 db I think. When I
get some time, or if there is rain or snow I will make some
measurements. I've got a Circlotron, the 6AS7/6080 Amp & two different
twin coupled amps I built for publication back in the day.


Yesterday was 8K on the cross country skis, then 45K on the bicycle. Not
bad for an old fart.


Don't overdo it. It can easily dull the brain. I wish Flipper would go
for a long bike ride sometime but he's excused if he fears USA
motorists. Maybe along swim would suit him, he has the right name for
it.

I have to work. I'm not a retired old fart. But I can fart well when I
have to though.

I'm presently doing a second attemt to make a pair of 100W Ming Da
amps behave.

The Chinese were way out of their brainular cababilities when they
designed these POS amps. They are great smoke producers, and an
owner's nightmare with regard to biasing..

Anyway, I have retained the tube line up, 6SN7 SET input stage to make
about 5Vrms, 6SN7 LTP phase inverter to make two 30Vrms phases, then
300B balanced amp each with Ia 7mA and each making 120Vrms to power
grids of a pair of 845 with Ea = 1,200Vdc and Ia 43mAdc each.
Initially N&D was 5 times worse than how I have it now, stability was
poor, and the list of Chinese errors from their technical
incompetence would fill several screen fulls of description.

There is no need for bootstrapping because I have revised the +420V
rail and created a -190Vdc rail for the 300B balanced driver amp. This
300B driver stage is laughable use of 300B, but the Chinese think
westerner buyers like lots of spectacular tubes. In fact the 300B with
Ia only 7mA works fine. But Ia was less when they were expected to
work from only a 400V rail originally.

OK, the amps go back to the buyer who bought them second hand from
someone who just wanted to get rid of them. He's very happy for a
month before the attacks of N&D start again, and he returns the amps
to me. This time I find an anode resistor of a drive tube had gone
open and I have to suspect it has the Chinese Carbon Film Resistor
Disease, CCFRD where the resistor sure looks nice, and like something
ARC Corp might use, (except the paint job is worse), and the resistor
has gone open without any sign of heat stress.

Well, There are about 20 of these potentially horrible Chinese made
resistors in each monobloc and during the first re-engineering attempt
I assumed they'd be OK, but they are NOT OK. I have seen several
examples where Chinese 1W carbon film resistors just go open. Usually
its because there is more than 200Vdc across a carbon film type.
Something strange happens in the resistor. The MingDa resistors look
like 2.5W rated and I thought they would be fine. WRONG!, not OK. II
am replacing all 20 R throughout each monobloc amp with metal film
types with heavy watt ratings. Where they have 4 x 470uF caps in
series for +1,200Vdc rails, there is 300Vdc across 400V rated cap
bypassed with 220k and this looks very prone to failure.

I can't bootstrap the 50k anode loads of each 300B tube from the OPT
because there are no available taps on OPT which are suitable.
I could have taken anode R loads to the +1,200V rail after making yet
another series C RC filter, but the split rails of over 700Vdc is
enough to get the swing needed for drivng 845, and I avoid HV bothers.

But all hell breaks loose if this amp is driven well into clipping;
the 300B and 845 become badly affected by temperature changes in grids
and the biasing goes very wobbly and strange, and the best cure is to
switch off the amps, wait a minute for things to cool before switching
back on. All of which doesn't happen/isn't needed in normal OP tubes
like 6550.

To force the amp to behave when over driven I will try to make a
limiting circuit where a high resistance plus low resistance diivider
is cap coupled of V1 output and if V1 voltage rises above a threshold
due to clipping, two transistor, npn and pnp will be turns on and
there collectors will shunt the input signal preventing any V1 output
increase.

Such a scheme is fine up to clipping because the transistors have
extremely high collector resistance when fully turned off.
And if someone turns up the volume with a shorted speaker it prevents
the OP tubes being turned on fully into a shortcircuit.

I rarely cannot proceed from the start of a day to its finish without
addressing the incompetence of others I am forced to deal with in my
day to day work life.

But the Turner-Ming amps do sound very well indeed when they work.
There is no doubt the shuguang Chinese made 300B and type B 845 are
superb.

But I cross my fingers about the Chinese OPTs when I think about them.
I don't expect them to last a long time. The leads coming out of the
OPT box have insulation far too thin, nowhere as good as what you'd
find on any Hammond OPT and the the Chinese saw fit to use green wire
with yellow stripes for the CT lead to B+. This color is normally used
for chassis earth wiring. I hate to think of the Feng Shui karma
issues surrounding chinese OPT designs.

Patrick Turner.