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John
Broskie's Guide to Tube Circuit Analysis & Design

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21 May 2005

More auto bias
circuits

Broskie auto-bias circuit

One reader wanted to see how the Broskie
auto-bias circuit could be used with power output tubes; above is a more
fleshed-out version of the Broskie auto-bias circuit that would work well
with a set of power tubes like the 6L6, EL34, KT88 or 2A3—or even a single
output tube. One reader asked if the clipping or Broskie auto-bias
circuits were suitable for single-ended amplifiers. The answer is yes, as
SE amplifiers often (almost always is closer to the mark) exhibit a
rectification effect, unless a great deal of feedback is applied. The
graph below compares the generic DC servo auto-bias circuit against the
Broskie auto-bias circuit. The tube is a single 6DJ8 and it falls out of
class-A operation after 1V of input signal, so we should only look at the
first few input volts of the graph to compare the two circuits use in a
single-ended amplifier.

Below is a closeup of the first two volts of input
signal.

As the graph clearly shows, both circuits work exactly
the same up to about 0.6 volts of input signal; thereafter, the Broskie
circuit performs much better with the remaining class-A range and beyond.

Blumlein auto bias

Alan Dower
Blumlein was a genius; period. Yes, I know that today everyone is a
genius ...well, at least one out of three is a genius (unless, of course,
you have three children, which would raise the ratio to three out of
three), that the cover of the latest Esquire magazine proudly
proclaims that this is their yearly genius issue, that extraordinary
intellectual and creative power is readily available from Manpower or—via
the Web—cheaply available in a score of underdeveloped countries, that
enumerable self-help books and audio tapes are ready to release the latent
genius in you. But what a truly humbling experience it is to read of
Blumlein's life and contributions to electronics: first of all, he
invented stereo, the ultra-linear circuit, the moving-coil microphone...
over one hundred patents in audio, TV, and radar. Tragically, like Mozart,
he died when he was still in his thirties. Had he lived another forty
years, we would all be better off, as I am sure he would have helped
prevent the audio wrong turn known as quadraphonics from having inveigled
so many. And, no doubt, he would have made many more contributions to
amplifier design, both tube and solid-state based.

Shown below, Blumlein came up with an auto-bias
matching circuit that does not aim to set a specific idle
current, but works to keep both output tubes conducting the same idle
current; a current balancer, in other words. He realized that a mismatch
in idle current would steal performance from a push-pull amplifier, as it
would lead to partial magnetizing of the output transformer’s core, which
reduces its inductance, worsening its low-frequency reproduction, and
which limits its maximum output wattage, as the transformer prematurely
saturates in one direction.

His circuit, know as the “garter circuit,” cross feeds
a sampling of each output tube’s current conduction via a tap in the
cathode resistance to the other tube’s grid. As one tube conducts too
much, it raises the bias voltage on its fellow tube’s grid. Conversely, if
a tube conducts too little, it lowers the bias voltage on its fellow
tube’s grid, causing the tube to reduce its conduction.

Blumlein "garter" circuit

How well does it work? Better than you might expect, but with a few
caveats. A shootout between the Blumlein garter circuit and two
alternatives is needed. I used SPICE and I created a greatly mismatched
set of output tubes by using two tubes in parallel, effectively creating a
new tube with the same mu, but twice the gm and half the rp. (No bypass
capacitors were added, as no AC signal was used in the simulation.)

Ideally, when both output tubes are perfectly matched, the idle current
should equal 9.86mA in the Blumlein circuit.  As shown in figure (A)
below, one tube conducts 10.2mA, while the other conducts 11.07mA, a
mismatch of 0.87mA, or 8.5%; not too bad, considering the grotesque
mismatch in tubes. Using separate cathode resistors (B) yields poorer
results: a mismatch of 2.12mA, or 21.5%. And using a single common cathode
resistor (C) produces the worst results: a mismatch of 7.43mA, or
100%.

So, given these results, which topology do you think has found the
widest acceptance? The single common cathode resistor is the winner; it is
immensely popular, much more so than the dual cathode resistors, and
Blumlein's circuit is all but forgotten. Why use only one shared cathode
resistor? Because it cheaper to implement, with only one bypass capacitor
needed, and because far too many believe that using a single common
cathode resistor will transform any amplifier into a class-A amplifier,
regardless of the resulting idle current. Unbelievable, but sadly true.
Just ask a musician and see what he tells you. Of course in a world
wherein a circlotron amplifier is seen as being a dual, single-ended,
class-A-by-necessity design by many audiophiles…but I won't go on.

Unfortunately, Blumlein’s circuit displaces B+ voltage that could be
used to create more power output. Additionally, it requires the bypass
capacitors to be rated twice as high in voltage than otherwise would be
used.

A modern approach to bias balance

Blumlein didn’t have access to transistors and OpAmps; we do. (I
know that some are already getting nervous. But why not take advantage of
a technology that’s cheap, effective, doesn’t wear out, and, most
importantly, isn’t in the audio signal path? Anxiety or arrogance? Fashion
or foolishness?) A single OpAmp can be used to ensure idle current balance
between two output tubes; and nine OpAmps can be used with ten output
tubes, as each OpAmp finds the same reference voltage: the cathode voltage
of the lead tube, whose idle current is set with a potentiometer.

Because the time constant is the same for both the voltage reference’s
RC circuit (1M and 0.1µF) and the OpAmp’s RC feedback circuit (1M and
0.1µF), this circuit handles class-AB and class-B output stages quite
well.

Note that in the schematic that OpAmp connects directly to the leftmost
triode’s grid resistor. This arrangement only works when efficient output
tubes are used, which need a grid voltage that goes
down to –15 volts but no farther. Two workarounds are possible: use
an asymmetric power supply for the OpAmp or add a PNP transistor to the
OpAmp’s output, as in the Broskie auto-bias circuit shown at the top of
this page. Usually, an OpAmp is attached to a bipolar power supply of +/-
15 to 20 volts. However, we can use a skewed power supply, with +5 and –25
volts to –35 volts, as the OpAmp will only see the same voltage
differential between its power supply pins. (Actually, we might even get
away with forgoing the positive power supply rail altogether, as several
OpAmps can accept positive input signals that extend beyond their positive
power supply voltage, for example the BiMOS CA3140:

Absolute Maximum
Ratings
DC Supply Voltage (Between V+ and V-
Terminals) . . . . . . . . . . . 36V
Differential Mode
Input Voltage . . . . . . . . . . . . . . . . . . . . . . . . 8V

DC Input Voltage . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . .(V+ +8V) To (V- -0.5V)
Input Terminal
Current . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1mA
Output Short Circuit Duration. (Note 2) . . . . . . .
. . . . . . . . . . . . . .Indefinite

Alternatively, a current mirror can be used in place of
the OpAmp. In the schematic below we see the leftmost output tube
following the rightmost idle current.

The rightmost transistor is wired as a diode (and could
be replaced with a diode, such as the 1N4001), so the effective cathode
resistor used by the rightmost output tube is a 293-ohm resistor with a
solid-state diode in series with it. The leftmost transistor works to
establish the same emitter voltage that the rightmost transistor does,
which ensures that the same idle current is experienced by both output
tubes.

Or, we can replace the OpAmp with a simple transistor-based
differential amplifier, with a long tail that extends up to the output
stage’s B+ voltage. (The 360k resistor is as good as a constant-current
source in this circuit because of its large value.) Diode, D1, is a
high-voltage rectifier that exhibits a forward voltage drop of 7 volts
(internally, seven diodes are wired in series). This diode is used here as
a low-current voltage reference, which is important because we do not want
the differential amplifier to draw more than 1mA total. (Zener diodes
require far too much current conduction to break at such a low voltage.)
This diode's forward voltage drop sets the range of possible negative
voltage adjustment, as the diff-amplifier’s output cannot swing below
ground in this circuit. (If a negative power supply is available, then
much smaller cathode resistors can be used and the diff-amplifier’s out
could swing down to the negative power supply voltage.)

A more-fleshed out version is shown below. The zeners are used to
limit the rectification effect of the output tubes undergo in heavy use.
The added capacitors filter away the AC signal that might be present on
the cathodes. All in all, not a bad circuit—particularly, when no negative
power supply is available.

Of course, a differential amplifier can be made from a single
transistor (or MOSFET or JFET). Below we see one transistor doing all the
work of keeping the output tubes in idle current balance.

Since this diff-amplifier draws its power primarily through the
rightmost output tube, the leftmost tube must see an equal load current,
otherwise our balancing circuit will in itself unbalance the output stage;
thus, the connection of the 30k resistor and high-voltage diode to the
left output tube’s cathode and ground. (The 1.4k resistor should be
bypassed with a capacitor, as should be diode, D1, as these added
capacitors will slow the turn on for the output tubes and shunt away extra
AC signals.

Next time
Are we done?
There are many more circuits and many more side topics, but next time we
will move on to some interesting circlotron topologies that reader,
Johannes, sent in.

Speacial thanks
Speacial
thanks must be given to Robert McIntyre who sent me a lot of good
information on the Blumlein circuit. (You might remember that Mr. McIntyre
wrote those excellent articles on tube circuit fundamentals for Audio
Amateur back in its golden days.)

//JRB

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