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John
Broskie's Guide to Tube Circuit Analysis & Design

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04
November 2006

The Tube Scream

Oh, the irony of it all.

In the October 22nd blog, I mentioned how I fear
schematic typos. And then, in the October 31st blog, I made a big one. At
least it was a big one in my eyes, though many will never have spotted it.

Let me explain. The 6BQ5-based OTL headphone amplifier
circuit I originally drew up left out a key Aikido-like touch to enhance
the amplifier’s PSRR figure. I didn’t spot it, because I didn’t go through
the usual step-by-step explanation of how the circuit works; instead, I
only offered a link to an article that I had written that had explicated
the circuit. Oops.

And while I bemoaned the e-mail burden with which I daily struggle,
I didn't explain that I had fallen particularly far behind of late
because two weeks ago, my son’s teacher
and one of his classmates drowned. A week spent trying to explain to
a three-and-a-half-year-old how something like this could happen and what
it means, along with attending various funeral-related events, put me far
behind in work in general and answering e-mail in particular.

Nonetheless, I didn’t want to fail to post a blog entry, as I know many
tube afficionados look forward to reading them (just as I look forward to
downloading my weekly fix of John Derbyshire’s Derb
Radio podcasts every Saturday morning). I reasoned that in just a
little more time than it would have taken me to answer the original lost
e-mail, I could write a blog entry addressing the same issue.  But I
didn't have time to do it complete justice last week.  So I decided
to write a second part; and when I gave the original entry a second
reading, I spotted the schematic typo. (October
31’s blog now holds the corrected schematics.)

Okay,
now in greater detail.

6BQ5-based OTL headphone amplifier circuit
revisited
In spite of the little time spent writing the blog
entry, the 6BQ5-based headphone amplifier was actually carefully designed,
with the part values not casually chosen. The result is that the
headphone amplifier acknowledges and anticipates the power supply’s
failings, presenting the requisite amount of injected power-supply noise
into the output stage to cancel the power-supply noise from the output.
Below is the bad schematic that I had posted.

Notice the parts in red. This capacitor serves as a power supply
decoupler, filtering away much of the 510-volt power supply rail’s noise.
So what’s wrong? Isn’t this what 99% of tube gurus would do? (I was
tempted to write 99.9% of tube gurus, but upon some reflection, I realize
that percentage would imply that the world holds at least 1,000 tube
gurus, a number I doubt.) The problem is that if this decoupling
capacitor terminates into ground, the negative power supply rail noise
will be superimposed upon the top output triode’s grid signal. How’s that?
The differential amplifier’s common cathode resistor attaches to the
negative power supply rail and any current variations through this
resistor that result from the resistor seeing a varying voltage potential
will be relayed to the plate resistors, as they all fall on the same
current path.

But in the correct schematic below, we see the decoupling capacitor
terminated into the positive-power-supply rail, not ground.

Now the negative power-supply-rail noise will meet and cancel with the
positive power-supply-rail noise, as the cathode and plate resistors
values were chosen to ensure a noise null at the output. If the cathode
resistor were larger, then the output would see positive power-supply-rail
noise leaking out; if the resistor were smaller, then the output would see
negative power-supply-rail noise leaking out instead. (Ideally, we want
injected negative power supply rail noise to equal the positive
power-supply-rail noise, so that cancellation is complete.)

How does this trick work? Both the positive and negative power supply
rails deviate from being pure DC sources, carrying the rectification noise
residue. Fortunately, the noise on each rail is nicely related, equal in
magnitude, differing only in being 180 degrees out of phase to each other.
(This only holds true when both rails see the same current load, as they
do in this amplifier.)

The complete headphone amplifier is shown above. By the way, speaking
of noise reduction, I just reread something that I had written in the
second issue of the Tube CAD
Journal, which is worth repeating every seven years:

Noise, its causes and elimination, will continue to be explored in
future issues. The motive is not hard to find.

Noise
constitutes a large part of what is condemned as the "tube sound," by
those who favor solid-state electronics. Few tube supporters would
defend hiss or hum as being euphonic. Noise stands guilty as charged, as
it robs the musical experience of subtle nuances. Much of what is
magical in a great recording is the air, minute details, and weak hall
reverberations that make the recording come alive in our living
rooms.

By masking these subtleties, noise reminds us
that what we are hearing is not live, but only electronically
reproduced. Of course, live performances are often marred by coughing,
hacking, talking, creaking chairs, and glass clinking. But this noise is
random and sharp, unlike the continuous and dull drone of power supply
hum; no one in the audience imagines that he is hearing a reproduced
musical event in his living room because of this live noise. (In fact, a
few rare recordings so perfectly capture this live noise that its
reproduction actually helps to convince us that what we are hearing is
live.)

Given the choice between low distortion and
low noise, choose low noise. The difference between .1% distortion and
.01% distortion cannot be heard; a reduction of noise from -60 dB to -80
dB can. If you want to improve the sound of your tube equipment, lower
the noise. Of course, low noise and low distortion are not mutually
exclusive and it would definitely be better to have both.

Here is the irony of the situation. Most solid-state gear is very
quiet, because of the low distortion race among stereo manufacturers. In
order to please the total harmonic distortion meter, an amplifier must
be both clean and quiet, as the meter cannot distinguish between
distortion and noise. Whereas, the tube audio designer, having no faith
in total harmonic distortion measurements, holding to his belief that no
meter can beat his ears, forgoes the requirement of extremely low noise
operation.

Some tube audio designers are very
pessimistic about the noise given off by their designs. They tell us
that the vacuum tube is an inherently noisy device and that it is simply
not possible to match the solid-state gear in low noise operation. We
might believe them, if it were not for counterexample offered by some of
the best tube gear: tube microphones, preamps, and amplifiers that do
not hum along with the music or hiss like tea kettles. (I built a direct
coupled electrostatic headphone amplifier that is so quiet that a friend
used to borrow it to evaluate the noise of hand picked transistors for
an ultra-low noise preamp he was building. The headphones and amplifier
revealed more information about the amount and quality of the noise than
did looking into an oscilloscope.) There is simply no excuse for one
tenth of a volt of hum from the output of a tube power
amplifier.

We are not saying that the tube is just as quiet as
the transistor or FET, as, ultimately, it is not; only that much of what
is built with tubes could be much quieter.

Amplifying low
output MC cartridges reveals the absolute noise floor of a tube. The
quietest tube is much noisier than the quietest FET. The 2SK147 FET is
eerily quiet; after spending years looking at the 6DJ8's hash noise on
an oscilloscope, the flat thin line of noise from the 2SK147 makes one
check to see if it is actually connected to the probe. This praise for
the FET's low noise does not constitute a wholesale endorsement of the
FET's use for all audio circuits, only a recommendation for this limited
use: amplifying the micro-volts out of an MC phono cartridge
sufficiently to give the vacuum tube a chance to take over.

Making tube equipment quieter requires a solid grasp of where and how
the noise is interjected into the output signal.

6BQ5 issues
John Atwood has
set me straight about the old Svetlana SV83. They are relabeled 6P15P
pentodes, a Russian equivalent of an EL83, whereas the 6P14Ps are the
Russian equivalent to the EL84. In John’s own words:

I was looking at your latest Tube Cad
Journal posting, and I noticed that you implied that the SV83 is
equivalent to the EL84. It isn't. It is really a Russian version of the
EL83, the 6P15P, which is a video pentode, more similar to a 6CL6 or
6AG7. It may work in a 6BQ5 socket, but the bias points are somewhat
different. The pin-outs are also somewhat different, but if
one was aware of both pinouts, the sockets could be wired to be
compatible with both.

I have used both the conventional (i.e.
consumer-grade) and mil-spec versions of the Russian 6P14P (the mil-spec
version has the suffix "EB" in Cyrillic), and both are good, although I
especially like the EB version. They sound good, and last a long, long
time. I had a quad in my old Heathkit SA-2 amplifier, which I used as my
computer sound amp, which ran every day. Before these, I used US-made
6BQ5/7189s and they would last a year or two at the most before one
"went critical" and the others were loosing gm. I had the 6P14P-EB tubes
in for over 2-1/2 years before I took the amp out of service, and they
all tested perfect! These tubes are available on Ebay for about $2.50
each from Russian and Ukrainian sellers. I have gotten some from these
sources and have had essentially no problems with the
purchases.

And here is an interesting link SV83
Output tube warning! Below is the pinout for the 6P14P and 6P15P
pentodes.

Speaking of small, 9-pin power pentodes, I love the 6CW5/EL86, which is
a powerful, low-voltage pentode. (The EI version cost only $5 and I am
sure that you can still pick up NOS American 6CW5s for just a little
more.)

ET-T3021

POWER-AMPLIFIER PENTODE

DESCRIPTION AND RATING

FOR AF POWER AMPLIFIER APPLICATIONS
The
6CW5 is a power-amplifier pentode designed for use in the
audio-frequency power-output stage of television and radio receivers and
in high-fidelity amplifiers.

GENERAL

ELECTRICAL

Cathode—Coated Unipotential

Heater Voltage, AC or
DC
6 3 Volts
Heater
Current
0.76 Amperes

Direct Interelectrode Capacitances*

Grid Number 1 to Plate

0.6 pf

Grid Number 1 to All

12 pf

Plate to All

6.0 pf

MECHANICAL

Mounting
Position—Any
Envelope—T-6½,
Glass
Base—E9-1, Small
Button 9-Pin Outline Drawing—EIA 6-4

Maximum Diameter 7/8 Inches

Maximum Over-all Length 3-1/16
Inches
Maximum Seated Height
2-13/16 Inches

MAXIMUM RATINGS

DESIGN-CENTER VALUES
Plate Voltage

250 Volts
Screen
Voltage
200 Volts
Plate
Dissipation
12 Watts
Screen
Dissipation
1.75 Watts
Peak Screen
Dissipation
6.0 Watts
DC Cathode
Current
100 Milliamperes

Heater-Cathode Voltage

Heater
Positive with Respect to
Cathode
100 Volts

Heater Negative with Respect to Cathode

DC
Component
150 Volts
Total
DC and
Peak
300 Volts
Grid-Number 1 Circuit
Resistance
With
Cathode
Bias
1.0 Megohms

TERMINAL CONNECTIONS
Pin 1—Internal
Connection
Pin
2—Grid Number 1

Pin 3—Cathode and Grid Number 3 (Suppressor)

Pin 4—Heater

Pin 5—Heater

Pin 6—Internal Connection

Pin 7—Plate

Pin 8—Internal
Connection

CHARACTERISTICS AND TYPICAL OPERATION

AVERAGE CHARACTERISTICS
Plate
Voltage
170 Volts
Screen
Voltage
170 Volts
Grid-Number 1
Voltage
-12.5 Volts
Plate
Resistance,
approximate
23000 Ohms

Transconductance
10000 Micromhos
Plate
Current
70 Milliamperes

Screen
Current
5.0 Milliamperes

CLASS A1 AMPLIFIER
Plate
Voltage
170 Volts
Screen
Voltage
170 Volts
Grid-Number 1
Voltage
-12.5 Volts
Peak AF Grid-Number 1
Voltage
9 9 Volts
Zero-Signal Plate
Current
70 Milliamperes
Maximum-Signal Plate
Current
70 Milliamperes
Zero-Signal Screen
Current
5.0 Milliamperes
Maximum-Signal Screen
Current
22 Milliamperes
Load
Resistance
2400 Ohms
Total Harmonic
Distortion,approximate
10 Percent
Maximum-Signal Power
Output
5 6 Watts

6N1P input tube
One reader
asked if the 6N1P could be used instead of the 12AX7. It can, but at a
price. First of all, it will offer less gain, which means less feedback
will be available, which might be important to those seeking the lowest
output impedance and distortion. Second, the 6N1P should be run much
hotter than a 12AX7, 5mA per triode, not the 1mA that a 12AX7 can get away
with. So what’s the big deal, 10mA versus 2mA? The big deal comes in the
form of bigger decoupling capacitors. Where the 12AX7-based headphone
amplifier only needed 2µF capacitors, the 6N1P version will require five
times larger capacitors, 10µF decoupling capacitors. In addition, the two
bipolar power supplies may become unbalanced, as the negative power supply
rail will be taxed greater than the positive rail, as the current through
the differential amplifier starts at the negative power supply rail and
ends at the 510V rail. (The workaround might be to use more capacitance on
the negative power supply rail than the positive rail.)

In practice, this amplifier may work well enough that these added costs
are worth paying. Remember, the 6N1P can be much quieter than the 12AX7.

Another input tube that I would experiment with is the 12BZ7, which
also holds a mu of 100, but with half the rp of a 12AX7.

//JRB

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six positions, the attenuator is just a ladder attenuator, with no more
than two resistors in the signal path; thereafter, the attenuator uses
both a ladder and series configurations, with never more than eight
resistors in the signal path. With -2dB decrements, a maximum of -70dB of
attenuation is possible; with -1dB decrements, a maximum of -35dB of
attenuation.

The center knob controls both channels, and offers six
large decrements; the flanking knobs offer six fine decrements for each
channel, creating a volume control and balance control in one easy-to-use
stepped attenuator.

This clever attenuator uses fewer resistors (only 32)
than would be expected from a conventional 32-position stepped attenuator,
as two series attenuators would need a total of 72 resistors; and two
ladder attenuators would require 140 resistors.

In addition, the PCB holds dual sets of resistor pads,
one wide and one narrow, so that axial (composition, wire-wound, and film)
and radial (thick-film and bulk-foil) resistors can be used without extra
lead bending.

Although designed to go with the Aikido amplifier, it
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