<
Back

John
Broskie's Guide to Tube Circuit Analysis & Design

Next
>

13 August 2011

RIP Allen Wright

Guido Tent of TentLabs informed me of
Allen's demise this past February. The tube world has lost an ardent
defender and supporter. I have only met Allen once, which was at the 2005 ETF in
Langenargen, Germany. I had been warned to expect a confrontation, but
Allen and I instantly got along well. I believe the first thing he said to
me was, "I have never read your Tube CAD Journal." This was fine
by me, as I had never read his books, which I was quick to point out. The
interesting thing was that Allen betrayed himself at least three times by
repeating bits of my life that I had revealed here, such as the suggestion
that the Mafia take out
spammers as a public service and the moronic audio salesman who told
me that his paper-coned speakers were "electrostatic-electrodynamic
speakers." The closest we ever got to talking audio circuits was in a
conversation wherein I lamented giving away a defective signal generator
that I once owned, as the distorted waveform was actually quite useful in
testing amplifiers. Allen knew at once where the failing component was in
the generator, which impressed me greatly. I wish that I had a chance to
have a long talk with Allen, without an audience, where we could have
talked tube circuits for a few hours; I am sure that I could have learned
a great deal.

It seems that I am always the last to hear of the death
of famous audio folk. I remember John Atwood calling me to tell me that Dr. Gizmo (Harvey
Rosenberg) had died. I could not believe it, as I had just had a long
telephone conversation with him two days earlier. I was also startled when
I heard that Charlie Kittleson, editor of Vacuum
Tube Valley magazine,
had committed suicide months ago. I had dinner with him about a year prior
and we had been e-mailing a bit afterwards. (I was pushing him in the
direction of resurrecting his magazine, albeit as an online effort.) He
seemed to me, at the time, to be more tired than depressed.

At 55 years old, I can expect a lot more bad news in
the coming years, as ours is a hobby for old men, at least here in
America.

Something New: MP3 of Blog
Post
Everyday I use a software program that converts
text into speech. Yes, I know that all computers come with such a program;
but these universally sound dreadful, with odd mechanical, robot-like
sounding voices that quickly irk and weary the listener. The program I use
makes use of new high-quality voices that sound natural. So, here is an
example. Blog210.mp3

Cathode Bias with a Constant Current
Source
A few years ago, I received many e-mails
asking for help on how tube-based, push-pull amplifiers that used a
constant-current source at the output tube's connected cathodes worked. I
believe that it was Allen Wright who caused the stir, with his Vacuum
State dpa300B power amplifiers, which the brochure describes as being:
"Not Push Pull–Not Single Ended." Okay, Allen, whatever you say ;)

Nonetheless, I am reminded of all those readers who wrote about a
decade ago to explain how the circlotron was actually two single-ended
amplifiers working in parallel. My answer was that, in that case, all
push-pull amplifiers (such as Dynaco ST-70 and the Futterman OTL
amplifiers) were also two single-ended amplifiers working in parallel. The
test is if the two tubes work in anti-current phase to each other or in
phase. If in phase, then the amplifier is a parallel single-ended (PSE)
design. If out of phase, then the amplifier is a push-pull design. It's
really that simple. Iout = Ia - Ib

Before moving on to how this auto-bias method works in a push-pull
amplifier, let's look into why this technique is never used in a
single-ended amplifier.

In the above circuit, everything seems to be kosher.
The constant-current source sets the required cathode-bias voltage and the
idle current remains steady, in spite of an inflating or deflating B+
voltage. But that is just the problem: the current remains too constant.
An output transformer relays a variation in current flow through its
primary to its secondary. No change in current, no output signal.

Any attempt to the get some output signal into the
speaker—by applying a input signal to the grid—will be countered by the
constant-current source. The grid goes up, the cathode goes up; the grid
goes down, the cathode goes down. The workaround is to place a whopping
big capacitor across the constant-current source, which will effectively
"ground" the cathode to AC signals, allowing the amplifier to amplify. In
sharp contrast, a push-pull amplifier that uses an unbypassed
constant-current source can amplify. Why? The quick answer is that the
connected cathodes do not move.

As long as the two tubes are working within the class-A
envelope, which is set by the idle current and the load impedance, the
voltage at the connection to the cathodes remains almost constant. (As no
tube is perfectly linear, there will be small voltage wiggles on the
cathodes.) Think about it this way: in a cathode-follower circuit, the
cathode tries to follow the grid; thus, if the grid moves up in voltage,
the cathode follows the upward movement. Now what would happen if two
cathode followers were placed in opposition to each other, with one trying
to pull up, while the other tried to pull down?

If the tube were the same and both saw the same input
signal (albeit inverted for one cathode follower's grid), then the result
would be a tie, with the connected cathodes remaining fixed. Well, that is
what is happening with the push-pull amplifier that uses an unbypassed
constant-current source.

Okay, so we have arrived—or have we? This is the
perfect tube output stage, right? (I was trying here to recapture some of
the breathless quality that those e-mails from days past held when asking
about this auto-bias technique for a push-pull output stage.) As long-time
readers know all too well, none of these questions admits a simple, pure,
absolute yes-or-no answer. The historical reason the
constant-current-source cathode load isn't popular is that low-voltage
constant-current sources were not available (excepting high-inductance
chokes) back in the good old days, when
tube glowed bold and transistors weren't invented; and for far too
many, if it wasn't done back then, it shouldn't be today. Additionally,
this auto bias method limits the power amplifier to class-A mode of
operation, thereby forgoing the big watts that class-AB or class-B
delivers. A manufacturer willingly walking away from potential watts is as
rare as penis reduction operations—and for the same reasons.

Class-A is expensive. It costs watts, heat, tube-life,
and sturdy parts.

When the above amplifier is asked to produce an output
voltage greater than the limit imposed by the constant-current source, the
amplifier clips, the cathodes break free of their fixed voltage and begin
to follow the most-positive grid voltage.

Well, couldn't we apply the single-ended workaround of
adding a large-value bypass capacitor across the constant-current source?

Yes, this will allow the output stage to put out brief
bursts of more power, as the capacitor will buck the cathodes' attempt to
follow the large input signal. There are few liabilities with this
workaround. First, if the bursts are not so brief and so rare events, but
instead long, sustained blasts, then the capacitor will begin to charge
up, due to the overdriven tube rectification effect, which will throw the
tubes off bias. If the capacitor charges up enough, the tube may entirely
cutoff, once the big input signal is removed, which will result in 100%
distortion, as no sound will be reproduced. Moreover, the unbypassed
constant-current source affords some nice sonic benefits, such as greatly
improved PSRR and CMRR, both of which we will lose with the addition of
the bypass capacitor.

PSRR is short for
power-supply-rejection
ratio, which is the measure of a circuit's ability to
ignore the power-supply noise at its output, which is usually measured in
dB. All power supplies contain some noise.

In the above illustration, we see the push-pull output
stage with no input signal, but with a good amount of power-supply noise.
Since the constant-current source strives to maintain a fixed current flow
throughout the set of power tubes, it cannot let the variations in B+
voltage (power-supply noise) vary the tube's current conduction, so the
cathode voltage must move 1/(mu + 1) as much as the B+ moves. The result
is noise-free output at the speaker, in spite of the plates and cathodes
bouncing about. Remember, if there is no delta, no difference in current
flow through the primary, then there cannot be any output signal at the
secondary.

CMRR is less well known. It stands for
common-mode-rejection
ratio and it is the measure of an amplifier's ability to
reject in-phase input signals relative to its amplification of uncommon
input signals, i.e. balanced input signals. Ideally, an amplifier would
entirely ignore an identical signal at its positive and negative
(non-inverting and inverting) inputs. Wait a second; why should there be
any in-phase input signals with a push-pull, balanced pair of
out-of-phase input signals? There shouldn't be...but there always are.
That's real life. Sometimes the culprit is RFI or magnetically coupled hum
or just a less-than-perfect power supply feeding the phase splitter or the
differential long-tailed amplifier that delivers its output to the output
stage. With the unbypassed constant-current source in place, the cathode
follows the common-mode signal, which prevents the tubes from seeing an
input signal; and, probably much more importantly, prevents the
common-mode signal from interacting with the desired input signal within
the output tubes.

In other words, greatly enhanced PSRR and CMRR are
wonderful attributes to have—in the real world of electronics. In a
perfect world, one free of all RFI and filled with perfect house wiring
and noiseless, absolutely fixed power supplies and flawless input signal
sources, these two highly-desired characteristics would not be necessary.
In fact, if the power supplies were perfectly clean and steady, and if the
triodes were perfectly linear, then it would make no real difference if
the constant-current source were unbypassed or bypassed or, indeed, if it
were replaced with battery or a perfect zener diode. What? Once again, as
long as the amplifier were never driven beyond its class-A envelope of
operation and as long as perfect power supplies and tube were used, the
connected cathodes would never budge a picovolt, which means that the
tubes would be entirely indifferent to what load was found between them
and ground.

But as we do live in a less-than-perfect world, we have
to prepare for the worst. We can design better power supplies and buy more
closely-matched output tubes—and we do not bypass the constant-current
source.

By the way, each output tube could see its own
connection to a negative bias voltage, so that the constant-current source
would displace less precious voltage and so each output tube could have
its idle adjusted independently from its partner. Or we could use a DC
servo loop to automatically adjust one tube's grid voltage so that the
tube matches its partner's conduction.

The most difficult correction to make, however, is
changing ourselves. Audiophiles like to crank the volume. With a huge
increase in sound pressure comes a feeling of power and an uncovering of
sonic details. Unfortunately, super loud and class-A are seldom seen
together. Unless you are running big 105dB horns, you are probably
clipping at least once during every minute of music play. It is easy to
do, as quality music presents a huge dynamic range, which would require
10kW amplifiers to reproduce faithfully with typical inefficient speakers.
But when we bought class-A glory we had to pay in potential wattage (and
in heat and shorter tube longevity). For example, the two output tubes
that could have produced 100W in a super-lean class-AB, might only produce
25W in strict class-A.

The above graph shows three idle current points and the
three resulting classes of operation. In all three examples, the peak
current draw by a single tube is 140mA. But the class-A idle current
limits the amplifier's output to one half of what the class-AB point
yields and only one forth what the class-B idle current can deliver.
Moreover, the tube will last much longer with the lower idle currents. Of
course, the lowest distortion and the flattest output impedance will be
offered by the class-A mode of operation. I believe this is a good trade;
but never forget that some tradeoff must always be made.

TCJ Workaround

I am planning on building a push-pull power amplifier and I have had
to face many design choices. Although I like the ease of the class-A,
constant-current source approach, I would like to cheat the system a tad,
by running a lower idle current and still getting an occasional burst of
wattage beyond the strict limits of class-A.

To begin with, just imagine the zener is not there,
which would leave just the constant-current source and two diodes, a
resistor, and a large-valued capacitor. Note how the capacitor is at the
same voltage as the cathodes, which makes sense as the 10k resistor and
the rightmost diode will charge up the capacitor. Once charged, the
capacitor only sees the 10k resistor connecting it to the cathodes, as the
two diodes are no longer forward biased, so they effectively fall out of
the circuit.

Now, if we never increase the volume beyond the class-A
limit, the constant-current source is only being shunted by a 10k
resistor, which can be safely ignored. On the other hand, if the crescendo
takes an output tube outside the class-A envelope, the rightmost diode
will begin conducting and effectively couple the capacitor to the
cathodes, which will greatly limit their AC movement away from the
established DC bias voltage. Of course, if the increased volume is
sustained for long enough, the capacitor will charge up to a much higher
voltage. But once the volume falls to normal, the leftmost diode will
begin conducting, allowing the capacitor to discharge through the
constant-current source.

It's not perfect, but it would allow a two-speed
operation to the amplifier. In other words, instead of having a class-A
amplifier that puts out only 25W, we could have class-A amplifier that
puts out nominally only 25W, but could be overdriven to produce clean 50W
(or more) bursts of power.

Okay, why is the zener there? The zener diode will put
the hard breaks on the cathodes climbing too high in voltage due to
rectification effects that result from overdriving the triodes. Just pick
a zener that will break at some voltage beyond what you ever expect to
find a triode biasing to at the desired idle current. In fact, we could
just use a zener and the constant-current source, but it results in sharp
transitions in the waveform—never a sonically good idea. The more complex
circuit allows a smoother transition.

I have run many SPICE simulations on the above circuit
and it performs well. I would use MUR410G rectifiers and LM317-HV
three-pin, TO-220 voltage regulators to create the constant-current
source. If an EL84 were used, then a plain LM317 could be used; if 300Bs
are used, then the TL783 is a better choice. See www.tubecad.com/february2000/page6.html
for more information.

Of course the assumption throughout has been that
carefully matched tubes would be used. Great. But what if we don't own
matched tubes or what if our once matched tubes fall out of matching over
time? The following workaround illustrates how two unmatched output tubes
could be used.

The two large capacitors effectively couple the two
cathodes in AC terms, but allow each tube to find its own DC bias voltage.
The 1M resistor establishes a polarizing voltage across each capacitor.
Why? I think electrolytics sound better when they hold a charge.

Of course, we do not have to stop at two output tubes,
as we could run eight tubes, with each tube getting its own
constant-current source and big capacitor. And, yes, the same fancy
additions of zener, resistor, big capacitor, and two diodes can be added,
but one set per tube.

Okay, one final idea (I could easily go on for twice as
long). How about a push-pull amplifier with an auto-bias constant-current
source that is set to a nominally low value, so the amplifier only puts
out 4W in class-A—until a crescendo forces the constant-current source to
climb to a higher level of current conduction; then when the music
subsides, the constant-current source falls back to previous value.

In this example, a MOSFET was used, but the three-pin
regulators could still be used, but with some slight adjustments to the
topology. The downside to this approach is that the sliding bias reacts to
changes in current demand, not anticipates the changes before they happen.

The ideal would be a computer-based sound system,
wherein the idle current through the constant-current source could be
dynamicaly adjusted by the computer. Such a setup would give us just about
everything we desire: long tube life, low heat, high power, and class-A
glory on the cheap. See blog
number 126 for more information.

Next Time

I will return to the topic of Moskido hybrid power amplifiers.

//JRB

Kit User Guide PDFs
Click
image to download

E-mail from GlassWare Customers

Hi John,

I received the Aikido PCB today - thank you for the first rate
shipping
speed.

Wanted to let you know that this is simply the best PCB I have had in
my hands, bar none. The quality is fabulous, and your documentation is
superb. I know you do this because you love audio, but I think your
price of $39 is a bit of a giveaway! I'm sure you could charge double
and still have happy customers.

Looking forward to building the Aikido, will send some comments when
I'm done!

Thank you, regards,
Gary.

And

Mr Broskie,

I bought an Aikido stereo linestage kit from you some days ago, and I
received it just this Monday. I have a few things to say about it.
Firstly, I'm extremely impressed at the quality of what I've been sent.
In fact, this is the highest quality kit I've seen anywhere, of
anything. I have no idea how you managed to fit all this stuff in under
what I paid for it. Second, your shipping was lightning-quick. Just more
satisfaction in the bag, there. I wish everyone did business like
you.

Sean H.

High-quality, double-sided, extra thick, 2-oz traces,
plated-through holes, dual sets of resistor pads and pads for two coupling
capacitors. Stereo and mono, octal and 9-pin printed circuit boards
available.

Designed by John Broskie & Made
in USA

Aikido PCBs for as little as
$24

http://glass-ware.stores.yahoo.net/

Only
$9.95
to start
designing
tube-based
crossovers
and much more...

TCJ Filter Design

The Tube CAD Journal's first companion program, TCJ
Filter Design lets you design a filter or crossover (passive, OpAmp or tube) without having to check out thick
textbooks from the library and without having to breakout the scientific
calculator. This program's goal is to provide a quick and easy display not
only of the frequency response, but also of the resistor and capacitor
values for a passive and active filters and crossovers.

TCJ Filter Design is easy to use, but not lightweight, holding
over 60 different filter topologies and up to four filter
alignments:

Bessel,

Butterworth,

Gaussian,

Linkwitz-Riley.

While the program’s main concern is active filters, solid-state
and tube, it also does passive filters. In fact, it can be used to
calculate passive crossovers for use with speakers by entering 8 ohms as
the terminating resistance. Click on the image below to see the full
screen capture.

Tube crossovers are a major part of this program; both buffered
and un-buffered tube based filters along with mono-polar and bipolar power
supply topologies are covered. Available on a CD-ROM and a downloadable
version (4 Megabytes).

Download or CD ROM
Windows
95/98/Me/NT/2000/XP

For more information...

To
purchase , please visit our Yahoo Store:

http://store.yahoo.com/glass-ware

<
Back

www.tubecad.com

Copyright © 1999-2011
GlassWare
All
Rights Reserved

Next
>