As far as I know, trying to dynamically stabilize or clamp the
cathode
voltage of class AB amplifiers
using cathode biasing has never been *officially* invented before 2006,
and when
I tried to
describe a basic version of it to a news group ( rec.audio.tubes )
a couple of years ago in 2004, I don't think anyone
who saw my invention
could understand how it worked, even after posting the basic schematic
to the news group
alt.binaries.schematics.electronic.
So allow me to introduce a new idea for bias control in push pull amplifiers.
First, let's have a look at the basic sketch i submitted to the news group 2 years ago :-
The above schematic shows a typical ultralinear 35w+ amplifier
output
stage which has been used in countless
amplifiers since about 1955, except for one major addition which wasn't
possible in 1955.
There are two power transistors in the cathode circuits.
( Now just be calm, its ok, they are not bandits waiting to rob the amp
of
its marvellous sound quality!)
Let us examine the schematic for a minute and imagine that the
transistors and the 10 ohm R are
not in the schematic,
and all we have is standard a cathode bias circuit, and unlike my more
complex 300w design
shown further down this page, there is no cathode feedback windings or
multiple tubes to befuddle people when looking into the schematic.
Consider this output circuit working with a sine wave at 1 kHz.
Consider that under normal class A conditions that the signal
currents
at the cathode of each output tube
have a maximum undistorted +ve and -ve going value equal to
approximately
the idle Ik during each sine wave.
So if Ia = 50mA per tube, then we will have about +55mA, and -47mA
current swings.
The slight difference in Ia swings is due to the even order distortion
and mainly 2H currents that flow in each output tube
during normal class A operation, where each output tube acts as though
it was a single ended class A tube.
The RL a-a shown = 6.6k, so while in class A each tube's class A load =
3.3k, and with load current
equal to 52mA peak, maximum class A from each output tube = peak Ia
squared x RL / 2 = 4.5 watts.
So we will only get 9 watts in class A from this amp but class AB power
maximum can be calculated at 47 watts.
The voltage across the 1,000 uF cathode bypass caps and 750 ohm cathode R will rise slightly at 9 watts perhaps by around 5% since there is only slightly more current trying to charge the Ck than is drained out of it during current cycles of each sine wave.
While the amp works in class A there is no major bias stability
problem since the bias
changes only slightly
between the grids at 0V and cathode voltages at say +38V.
A slight change of +2 V will not displace the bias working point
of the tubes and won't increase thd very much at all.
But now let us consider class AB working. Above the 9 watt level
produced by the above output stage the tube that has its
anode current reduced by a -ve grid input signal reaches a current
change limit, and turns off completely, so no more
current reduction is possible, and since it then is effectively not
connected to the load and not giving and V x I change to the load its
contribution to power production ceases and its as if the tube was
removed from the chassis. But while one tube is cut off, the other is
being turned harder by a +ve grid signal and it is the only tube
connected to the load through the output transformer. The turn ratio
between tubes and output load winding then becomes 1/2 what it is with
class A operation and the load seen by the single tube = 1/4 of the
RLa-a value, or 1,650 ohms in this case.
Load line analysis will show us that when grid voltage = 0V is at its
maximum at clipping, anode voltage
has pulled down to just +80V and there is 390V across the 1.65k so the
RL peak flow = 390 / 1,650 = 236mA,
which is nearly 5 times the idle current or maximum peak class A
current swing.
In this case at clipping the there will be a much bigger positive peak
current
flowing in the tube than the the negative current swing when the tubes
are turned off.
So during each wave cycle at clipping, each cathode bypass cap will
be charged up by the difference in +/- cathode current to about +70V
just like the caps working off a tube rectifier, because more current
runs into the cap than drains out during
each high level wave cycle.
This is the result of large even order distortion currents in each
output tube in class AB PP amps.
This phenomena causes the tubes to then act as if the bias conditions
had been changed so that applied grid bias voltage
had
been increased from -30V to -70V, which is a case of over biased tubes,
and serious crossover distortion
is generated which cannot
be much reduced by NFB, because while the cathode voltage is at 70V,
both the tubes are in cut off where the signal is at the zero crossing
point and so for part of the wave cycle there
is no voltage gain , so large distortions occur.
Fixed bias is mostly used to avoid the problem of cathode voltage
upward
drift due to
the heavy
2H tube current flows in class AB.
This all sounds dreadful, but in practice when you only want 1 watt
average from any amp and the reserve of 47 watts
is only used for occasional transients that don't last longer than a
split second, the bias bias voltage hardly moves, since it takes
a long time to charge up the 1,000 uF cathode caps, and no music is a
constant amplitude single tone signal.
But where you want 6 watts average from an amp like this then the
bias voltage is constantly varying, wandering up and down
because of the tendency to move into class AB from class A.
Fixed bias avoids the varying bias problem with high output power,
but bias needs adjustment
and tubes will age, so bias currents will vary, although not because of
signals applied.
But what if there was a way to avoid the problems of tube ageing and
having to adjust the fixed bias currents
with adjust pots to set the grid bias voltage, and yet still be able to
use
cathode RC biasing?
Well there is a way to avoid both, and enjoy the natural excellent
quiescent bias stability of ordinary cathode bias
while the amp works in class A, and also control cathode voltages
lurching
around on musical signals
caused by class AB action.
So far we have examined the schematic without the R9, R10, R11, R12,
and two transistors and R13, R14.
Now let us add all those in and see what happens...
Consider R9, R10, 10 ohm resistors. They will have 0.5 Vdc across
them at idle since that is caused by
the 50mA
bias current dc flow from cathode to 0V.
But during heavy class AB working, the tube load becomes 1.65k, and
the Vswing at the anode
is say -390V peak, so peak current at the cathode = 50mA plus peak load
current of 236mA = 286mA,
and this would generate a feedback voltage at the 10 ohms = 2.86V peak.
The 1,000uF has a very low impedance at audio signal frequencies, so
the cathode is subject to such current feedback voltages except at very
low frequencies below the audio band.
The current feedback voltage will have a negligible effect on the tube
gain because 10 ohms is such a small value of cathode resistance.
Now consider the action of the added BD239B transistors. (We can use
almost any normal garden variety TO220 low voltage power
transistor; I used a pair of BD339B laying around.)
The emitter is taken to 0V, the base is fed through 1k, R11/12, from
the top
of the 10 ohms.
The 1k limits the base input current to the transistor by forming a
voltage divider with the base input resistance
which is quite non linear, ie, Rbin is high when no current flows, but
low when a lot of current flows from collector to emitter.
The 0.02 uF, C7/8, prevents the circuit from working when the
signal rises
above audio frequencies,
where we don't need or want the circuit to work.
The collector is taken via a 47 ohm R to the cathode.
No current ever flows in the 47 ohms unless the transistor is turned
on.
The transistor is fully turned on when base voltage is at about
+0.8V,
and fully turned off when base is at 0.5V.
When the +ve moving voltages in the 10 ohms rises in excess
of approximately 0.5V,
the transistor just begins to turn on, and cathode current peak surges
that
would normally charge up the 1,000 uF cap
are bypassed though the transistor and 47 ohms to 0V, so the voltage at
the
Ck remains at near the idle voltage.
The use of the 47 ohms is about right for an average amp and the value
can be anywhere
from say 10 ohms to maybe 100 ohms; it isn't very critical; there
must be a collector resistor to limit the collector
maximum current to prevent the
transistor from fusing.
47 ohms also seemed to work to stop the cathode voltage from *reducing*
below the idle level when large sine wave
signals were used because too much ac signal current shunting occurs
between idle and maximum class AB action.
Transistors are good, but we do have to molly coddle them and prevent
their all so easy failure when currents could exceed their SOA, safe
operational
area, so the 47 ohms will prevent excessive collector current.
When such precautions are taken, transistors can last almost
indefinitely,
and in this case to
act as slaves to make tubes work better, and without causing any extra
horrid distortions
during the tube operations; in fact the transistors act to reduce
distortions
that otherwise will occur.
If the cathode voltages were allowed to rise from say 30V to 70V
with
a sine wave at clipping then the thd
of an AB cathode bias amp ( with NFB ) can increase to say 5% easily
at clipping, which is bad when compared to a fixed bias amp with the
same
tubes, where the same load and similar power might produce only
0.3% thd.
Without my regulation the cathode bias amp will experience slightly
moving cathode bias voltages
well before clipping with music signals since drum beats and transients
cause momentary changes to cathode voltages, Ek, and hence the changes
cause the bias between Ek
and
Eg1 to change,
and the working point, so hence the distortion due to poor bias
conditions,
and momentary dc current imbalances in the halves of the OPT primary,
thus causing unwanted patches of intermodulation distortions.
With my regulation, the Ek tends to remain very steady and the
circuit can
be trimmed to give
only say 10% Ek rise with a sine wave ( worst signal condition ) where
otherwise a rise of +100% may occur.
During a heavy drumbeat or sudden transient, the excess cathode current
is shunted only as long as the transient occurs,
so the bias remains stable.
So my method renders the operation of a cathode bias amp to that of
a fixed bias type.
The bias stabilizer should produce a slight increase in Ek without
any fall in EK with
increasing power between idle and clipping, with any load down to 1/3
of that required for full class A at clipping.
Experimentors with the circuit should try 10ohms for the current sensor
resistances but may need to add an R from
transistor bases to increase the turn on threshold voltage at the 10
ohms at which current shunting begins.
The collector resistance values can also be trimmed for optimal
operation.
The two transistors don't need to be accurately matched.
What if a tube decides to conduct more dc than usual due to ageing or a fault?
Say the bias idle current increased from say 50mA to 65mA.
The dc V across the 10 ohms would rise from 0.5V to 0.65V, and the
transistor
would
just be thinking about turning on.
meanwhile the Ek would have risen from say +38V to +49V,
and if we had an active protection circuit to detect such a dramatic
Ek rise, it would
trip and turn off the B+, thus saving the user from a lounge room full
of smoke.
So dc faults in tubes will not interfere with active protection
measures.
And for those wondering just what the load would be for pure class A
operation of the schematic above,
it is easily worked out.
In class A for any single UL connected or pentode or bean tube,
the peak voltage load swing positively and negatively = 0.9 x Ea.
In this case load V swing = 0.9 x 470V = 423Vpk = 299Vrms.
The peak current swing is simply +/- the idle bias current = 50mA =
35mA in this case.
RL = V / I = 299 / 0.035 = 8.5k ohms.
Since there are two output tubes in class A acting effectively in
series on the load from anode to anode, the class A load
for both tubes is 2 x RL for one tube = 17k ohms in this case. But the
output voltage anode to anode will be twice what it is for one tube
because each anode's voltage is oppositely phased. Va-a = 598Vrms
Maximum power output in pure class A = Va-a squared / RL = 598 x 598 /
17,000 = 21 watts.
The input power to the 2 tubes is 2 x Ea x Ia = 2 x 462 x 0.05 =
46.2 watts.
The efficiency in % while in class A = 100 x output power / input power
= 100 x 21 / 46.2 = 45%.
Notice that for the above output stage the load that gives only
class A power requires a load of 3 times the 6.6k
arbitarily chosen for class AB. Many amps have been built to this
recipe to enable then to produce double the
maximum class A power with such mild bias conditions where Pda is only
23 watts per tube at idle.
Now let us move on to the 300W amps with a dozen 6550 output tubes.........
sheet 5
The schematic has the current sensing R = 1.67 ohms consisting of 6
x 10 ohm 5 watt resistors in parallel and
connected between
the ends of the cathode feedback winding and the commonly connected
cathodes on each side of the push pull
circuit.
The voltages in the 1.67 ohms are applied to the bases of the power
transistors Q1 and Q2, which
in this case are 10 amp capable TO3 or flat pack types on a small Al
angle heatsink under the amp chassis.
I used a pair of 2SD424.
R3, R4, C1, C2 act as base current limiters and a low pass filter. The collector current limiting R5 to R16 can bypass excess increase in signal currents without ever having the the peak bypass signal current rise to the rated collector current limit of about 8 amps.
All the IN4004 keep each cathode isolated from each other, but allow
the common bypass of ac currents
from the cathodes to the OPT cathode winding, so the cathode caps
do not charge up under low load class AB conditions.
Some tubes will experience more bypass currents than others because
EK
varies between unmatched tubes, and some begin to bypass before
others. But this is of little concern and I found that all Ek were
fairly
similar
at idle, and all
tubes conducted similarly, and under tests all 12 Ek voltages moved
only slightly up and to about the same
voltage. The excessive ac signal currents were being discharged from
each cathode through each
of the 100 ohm resistors.
So these 300W amps do not have to have 12 bias adjustments which
would
be a cause for anxiety for some if not many amplifier owners who will
never become
technically proficient
no matter how much teaching and training they undergo.
Some people just hate biasing rituals, they are only eager to listen to
music
without having
to worry if their amps are biased correctly, or if they are likely
to produce more smoke than Mozart.