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John
Broskie's Guide to Tube Circuit Analysis & Design

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08 November 2014

New Products

Tr-PS-3: +/-12Vdc Power
Supply
The Tr-PS-3 is a regulated bipolar power supply
that puts out +/-12Vdc @ 750mA. It uses an LM317 for the positive output
and an LM337 for the negative output. The PCB-mount power transformer is
the same one used in the Tr-PS-1, a MagneTek
FS24-1500, whose specifications are as follows:

Description:
The FS24-1500 is a dual
primary and dual secondary, split bobbin design which operates with
either a parallel input of 115V or a series input of 230V. The output
voltage will be either 24.0V with a center-tap under a 1.5A load with
the secondaries
wired in series, or 12.0V under a 3.0A load with
the secondaries wired in parallel. The split bobbin design eliminates
the need for costly electrostatic shielding.

Electrical
Specifications (@25C)
1. Maximum Power:
36.0VA
2. Primary: Series: 230V; Parallel: 115V

3. Secondary: Series: 24.0V CT@ 1.5A; Parallel: 12.0V @
3.0A
4. Voltage Regulation: 25% TYP @ full load to no
load
5. Temperature Rise: 25C TYP
6.
Hipot tested 100% at 2500 VRMS

Construction:
Three flange bobbin
construction with primaries and secondaries wound side by
side
for low capacitive coupling.

The key feature to split-bobbin, flat-pack transformer
designs is their low capacitance between its primary and secondary, which
inhibits the transmission of high-frequency noise from the primary to the
secondary, as the core is too frequency-limited to allow the this coupling
on its own.

The Tr-PS-3 PCB is only 6" by 3.2 " and, like all
GlassWare PCBs, is extra thick (0.094 in), US-made, with heavy 2-ounce
copper traces. The rectifiers used are ultra-fast MUR410G types, each of
which are shunted by a 0.1µF ceramic capacitor to reduce RFI creation.

As you can see from the Tr-PS-3 schematic shown above,
using jumpers J1 & J3 places the two primary windings in parallel, for
use with 115Vac wall voltage; using only jumper J2, in series, for use
with 230Vac wall voltage.

The complete Tr-PS-3 kit is only $49 USD and is
available now at the GlassWare-Yahoo store. It
includes all the parts, capacitors, resistors, rectifiers, including the
user guide, the transformer, and four sets of standoffs and screws and
O-rings. It offers the option of a 1.5in or 2.5in tall heatsinks.

Stereo-Mute-Mono
Switch
Here is something that I have
wanted for myself for the longest time: namely, a rotary switch for phono
playback that presents three positions: mono, mute, and stereo.

Why would I want this switch? I own quite a few mono
LPs and they sound better in mono, as do a few stereo recordings, which
went a bit wild with stereo separation, offering dual mono, rather than
real stereo presentation. In addition, turntable rumble (and the cutting
lathe rumble) is out of phase between channels, so summing to mono also
eliminates much of the rumble. Moreover, I like parking the output on
mute, when cleaning the stylus or at turn-on and turn-off.

The circuit is quite simple:

The signal leaves the phono stage and enters the
Stereo-Mono switch's PCB. Then you have a choice between stereo, which
just passes the output signals out; or mute, which floats the phono stage
outputs and grounds the switch's outs to ground; or mono, which combines
the phono stage's output signals via two resistors. Since the rotary
switch is a shorting type (a make before break type), there are no popping
sounds between clicks.

Below, is the GlassWare Stereo-Mono switch in use in my
phono stage.

The complete GlassWare Stereo-Mono switch kit is only
$12 USD and is available now at the GlassWare-Yahoo
store. It includes the PCB, rotary switch, and several pairs resistors
to chose from, either carbon-film or metal-film types. Amazingly
inexpensive and amazingly effective.

The Real Grounded-Grid
Amplifier
If there is a "real" grounded-grid amplifier, is
there then an "unreal" grounded-grid amplifier? Yes, indeed, which is a
nuisance and headache for someone like me. How so? I often receive e-mails
that ask about the grounded-grid amplifier and I must figure out if they
really mean the circuit shown below, which uses one triode, or the cathode-coupled
amplifier that Bruce Rozenblit designed, which uses three triodes.

The real grounded-grid amplifier is a simple affair:
the grid is grounded, if not directly to ground, at least AC "grounded"
via a capacitor to ground; the cathode receives the input signal, either
directly or via a resistor or capacitor or coil; and the output is taken
at the plate, in the same as it is in the grounded-cathode amplifier,
where the plate meets the plate resistor, Ra. The result is no phase
inversion of the input signal and much higher high-frequency bandwidth, as
the cathode is shielded from the Miller-effect capacitance by the grounded
grid. Because of its better high-frequency response over the comparable
grounded-cathode amplifier, the grounded-grid amplifier has found its
widest use in radio designs. Indeed, its use in audio circuits has been
rare, being limited by it comparatively low input impedance.

How low an input impedance? Where a grounded-cathode
amplifier offers, at audio frequencies, an input impedance roughly equal
to the resistor that connects the grid to ground, the grounded-grid
amplifier's input impedance is equal to:

Zin = (Ra + rp) / (mu + 1) || Rk

where || stands for in parallel with, such
that a || B equals (a x b) / (a + b). On the other hand, if the cathode
resistor terminates into the signal source, rather than ground, then the
formula is:

Zin = (Ra + rp) / (mu + 1)
+ Rk

One interesting feature of the
grounded-grid amplifier is that it offers a slightly higher gain than the
comparable grounded-cathode amplifier, as the cathode offer a slightly
higher transconductance than the grid. Where the grid's transconductance
is given by

Gm = mu / rp,

the cathode transconductance equals

Gm = (mu + 1) / rp.

Thus, the formula for the AC voltage gain
of a grounded-grid amplifier is

Gain = (mu + 1)Ra / (Ra +
rp),

assuming no series resistance. If there is
a series resistance, Rs, then the formula becomes:

Gain = (mu + 1)Ra / [Ra +
rp + (mu + 1)Rs]

This is the atomic grounded-grid amplifier, which then
can be incorporated on more complex topologies. Since most small-signal
triodes usually come in pairs, such as the 6DJ8, 6SN7, 12AU7…, we always
face the problem of what to do with the second triode. Believe me, if the
twin-triode tube did not exist, the SRPP would not be as popular as it is.
(Having mentioned the darling topology of tube audio, I should, if only in
passing, that the grounded-grid amplifier can be SRPP-ed by adding a
second triode atop the plate resistor, which is just what Bruce Rozenblit
did in his "grounded-grid amplifier.") The first obvious addition would be
to follow the grounded-grid amplifier with a cathode follower, which would
offer a low output impedance and shield the grounded-grid amplifier from
the external load. I like this sort of arrangement, as I like the input
stage to have the single task of providing gain and nothing else, while
the output stage provides a low output impedance and big current
deliveries, but no voltage gain.

Note the addition of not only the cathode follower, but
the input coupling capacitor. Since the triode is cathode biased, the
input taken at the cathode includes a large DC offset, which can be a real
problem with volume control potentiometers, as they are loathe to see any
DC current flow and their wiper position will alter the bias voltage for
the grounded-grid amplifier triode; in addition, a DC coupled OpAmp as the
signal source will pull the cathode voltage down to 0V, if it can. All of
these problems are eliminated by the inclusion of the coupling capacitor.
Due to the grounded-grid amplifier's low input impedance, however, this
coupling capacitor must be large in value, far larger than the usual 0.1µF
that most tube-loving solder slingers expect to use. The formula is a
simple one:

Capacitor =
159155/Frequency/Zin,

where the capacitor's value is given in µF. For
example, if the input impedance is 1k and the low-frequency cutoff is
20Hz, then an 8µF coupling capacitor is needed.

On the other hand, almost all DACs present coupling
capacitors at their outputs, usually much larger than 8µF, as electrolytic
capacitors are used. (Icky, but common practice.) So, if no volume-control
potentiometer is used, relying on the DAC's own internal volume control
instead, and if the DAC holds a big-valued coupling capacitor, then the
DAC's output can be directly coupled to the grounded-grid amplifier's
cathode. Maybe, that is. The problem here is that if an electrolytic
coupling capacitor is used, it may become reversed biased, a bad thing; or
its voltage rating may be exceeded, also a bad thing. In other words, the
safe bet is to use an input coupling capacitor.

An additional worry is what happens at start up, when
both triodes are cold and not conducting current yet. What happens to the
cathode follower's grid when it is at the full B+ voltage and its cathode
is at ground potential? Nothing good. True the cathode follower's cathode
resistor will limit the current flow through the cold triode, but the
electrostatic attraction from having the grid be hundreds of volts more
positive than the cathode can cause cathode stripping, wherein bits of the
cathode surface are ripped away by the huge positive voltage less than a
millimeter away. The solution, as all long-time readers know, is the
inclusion of a safety diode.

High-Zin Grounded-Grid
Amplifier
We can make a trade that will give us both
lower distortion and lower output impedance and a relatively higher input
impedance. So what must we trade away? Voltage gain is the answer. If we
add a negative feedback loop, using the grounded-grid amplifier's grid as
the inverting input. At one extreme, the entire circuit would become a
unity-gain buffer, offering no voltage gain, but a very-low output
impedance.

Since the grounded-grid amplifier does not invert the
input signal, the output signal at its plate is in phase with the input
signal, which is then passed off to the cathode follower, which, in turn,
delivers the in-phase signal to the grounded-grid amplifier's grid. The
net result is a tiny signal insertion loss, say as little as -0.1dB; a
very-low output impedance, say below 10 ohms; and an input impedance just
a tad lower than the grounded-grid amplifier's cathode resistor value. If
this input impedance is still too high, which it could easily be, then the
following variation is useful.

This variation allows us to use a much larger cathode
resistor, which could be as high as 30k in value. By the way, the
assumption here is that the grounded-grid amplifier triode is a high-mu,
high-rp type, such as the 6SL7, 12AX7, 12AY7, 5751, and 6072, while the
cathode follower triode is a low-mu, low-rp type, such as the 6AQ8, 6DJ8,
6CG7, 6SN7, 12AU7, 12BH7, 5687, and ECC99. In the 12DW7, we get dissimilar
triodes, one 12AX7 and one 12AU7 triode, which is convenient.

The next step is a step away from 100% negative
feedback towards partial negative feedback, which will develop some
voltage gain. As a design example, let's use the 12DW7.

Note both the 3µF input coupling capacitor and the
grounded-grid amplifier's 10k cathode resistor. The voltage gain comes in
at about +19dB and the output impedance is a low 70 ohms. Not bad, from a
12AU7-based cathode follower.

We can replace the grounded-grid
amplifier's cathode resistor with a constant-current source.

The constant-current source would not, however, raise
the input impedance as much as it would in the unity-gain buffer design,
so we are still stuck with the 3µF input coupling capacitor. Indeed, I
much prefer the cathode-resistor-based version, as it scales beautifully
up and down with varying B+ voltage, which the constant-current source
would not. In other words, with the cathode resistor, the B+ voltage can
be increased to +300Vdc and the grounded-grid amplifier's plate voltage
will center at about 150Vdc; in contrast, with the constant-current
source, the plate voltage would rise to exactly 200Vdc, stealing
cathode-to-plate voltage from the cathode follower and making the heater
reference voltage dangerously high at 100Vdc.

What happens if we replace both the grounded-grid
amplifier's plate resistor and the cathode follower's cathode resistor
with active loads? Well, we get something like this:

(Let me interrupt this tutorial by gratuitously and
shamelessly stating that what I like best about this topology is that only
I would ever come up with it.)

No SRPP is contained herein, as nothing pushes or
pulls—only single-ended action as far as the scope can display. But,
John, don't you see that one triode sits atop another, which is what
occurs in an SRPP? Yes, I see it clearly, but SRPP does not just mean
that one triode sits atop another. What is important is how the circuit
works internally, which in this circuit is in an entirely non-SRPP
fashion.

In an SRPP, the load impedance is a critical aspect of
the design. Indeed, if there no external load, the SRPP becomes the SRSE,
as both triodes conduct current in phase, no longer in anti-phase, so no
push-pull operation. In the circuit shown above, the input stage is
effectively not loaded by the second stage, and the first stage's output
is taken at the bottom 12AX7's plate, not the top 12AX7's cathode, so the
680-ohm resistor is merely a cathode resistor, not a current-sense
resistor. In other words, no SRPP, as there is no push-pull operation.
Put differently, there is no SRPP, as there is no current delta between
top and bottom 12AX7 triodes. In sum, no SRPP.

This grounded-grid amplifier simply uses an active
plate load, which could be replaced by a resistor. The top 12AX7 defines
an active load resistance equal to rp + (mu + 1)680. The bottom 12AU7
defines an active load resistance equal to rp + (mu + 1)470. The negative
feedback attaches at the bottom 12AU7's cathode resistor. Since a smaller
sampling of the output signal is present at the top of the cathode
resistor, less negative feedback is applied, which results in a high
voltage gain, which could prove useful in some applications.

Aikido Grounded-Grid
Amplifiers
Long-time readers will recognize the
following topology, as I have shown it many times before. The idea is a
simple one: we use the grounded-grid amplifier's grid as an inverting
input and feed it a sampling of the power-supply noise, so we can create a
null at the grounded-grid amplifier's output.

Note how only one triode is needed, not the four that
are used in my grounded-cathode amplifier based Aikido circuit. Resistors
R1 & R2 define a two-resistor voltage divider that delivers the
correct portion of the power-supply noise to the grid. The capacitor that
attaches to the B+ connection shields the voltage divider from the high
voltage. No input coupling capacitor is shown, but one would probably be
needed in actual use. In fact, in actual use, more would be needed to make
a useful line-stage amplifier, such as cathode follower output stage.

Here we see a more fleshed out Aikido grounded-grid
amplifier. The 1N4007 safety diode is in place and the cathode follower
offers a low output impedance, while shielding the grounded-grid amplifier
from the external load impedance. The resistor values for the two-resistor
voltage divider depends on the input triode used.

Now let's get extra fancy. How about eliminating the
input coupling capacitor, is that possible? Sure it is; we just need to
use a negative power-supply rail and a DC servo to keep the grounded-grid
amplifier's input at ground potential.

Where to start? The 6.3Vdc negative power-supply rail
allows us to supply both the constant-current source and possibly the
tube's heater. If a 6DJ8 is used, and if a low B+ voltage is used, say
150Vdc, then we can probably get away with it, as the cathode follower's
cathode will not be at too high a voltage to exceed the cathode-to-heater
maximum voltage. The DC servo monitors the input DC offset and constantly
makes small adjustments to its output voltage to keep the input centered
at 0Vdc. At the same time the two-resistor voltage divider terminates into
the DC servo's output, which in audio frequency AC terms is the same as
ground. The 1N4001 diode is a safety device that is normally reversed
biased and falls out of the circuit. But at startup, when the input triode
is still cold, the diode becomes forward biased and conducts, allowing the
DC servo to pull against the constant-current source and bring the input
up to 0Vdc. As long as the input sign does not swing below about -2.7V, in
this example, the diode will not be a problem.

I know that many are thinking that this circuit would
be ideal to connect to a DAC's current-output. Well, actually, it would
work better with a voltage-out DAC, as the input impedance, while low, is
not low enough. For a current-out DAC, the Broskie
I-to-V converter is a better choice.

Since we are in a fancy mood, why not design an MC
phono pre-preamp? A moving-coil phono cartridge requires a low-impedance
shunting resistance, sometimes as little as 100 ohms. So, here is an
example of where the grounded-grid amplifier's low input impedance can
become a virtue, not a liability.

Once again, where to start? The unmarked resistor can
be whatever value is needed to work in parallel with the 390-ohm cathode
resistor for loading the cartridge. The two funny looking diodes are
Schottky diodes, which unlike the typical rectifier diode are super-fast
and offer a very low forward voltage drop, which will prove helpful in
protecting the delicate MC from the coupling capacitor discharge at turn
off. Speaking of the coupling capacitor, I would actually use a
non-polarized Panasonic capacitor, not the electrolytic capacitor shown.
The cathode follower stage may not be needed at all, as the typical
moving-magnet phono preamp resents an input impedance of 47k.

What if we want to use the grounded-grid amplifier with
a step-up transformer? First of all, the transformer must be able to
sustain a DC current flow through its secondary, as all of the triode's
current must flow through the secondary. In addition, the secondary is not
likely to present enough DCR to bias the triode correctly.

The workaround shown above will trouble many readers,
so I will down shift a gear or two. Since the secondary DC resistance is
too low to bias the triode, we must make the grid more negative than the
cathode by some other means. The battery provides the negative bias
voltage needed. Wait a minute, John, batteries wear out and are a pain
to replace and expensive. Audio playback requires linear electronics,
which is a tad ironic, as most audiophiles are nonlinear thinkers. I know
many audiophiles, who think nothing of buying a new $5,000 phono cartridge
every other year, but complain and whine about the expense and hassle of
replacing a battery every ten years. Ten years? Yes, ten years, which is
the average shelf-life of the typical high-quality, alkaline battery. In
this circuit, the battery does not deliver any DC current, acting only as
a self-powered voltage reference. In other words, the battery is not used
in any real sense, so its output voltage is roughly equal to its published
shelf-life. (The only real use occurs at turn-on and turn-off, as the
capacitor that attaches to the the two-resistor voltage divider must first
charge up and then then discharge; that's it.)

If you are wondering why the two Schottky diodes are
used, particularly as the transformer shields the MC cartridge from the
cathode voltage, the answer is that at shut-off, the transformer can
develop a voltage pulse at the primary, which the Schottky diodes will
shunt to ground.

While we are still feeling fancy, let's design a
four-triode Aikido grounded-grid amplifier.

Much like the typical Aikido line-stage amplifier, this
design uses two tubes and four triodes. A negative feedback loop attaches
after the output coupling capacitor, which allows easy adjustment of the
feedback ratio. From here, it's a small step to an Aikido push-pull
grounded-grid amplifier for driving headphones.

We have added a current-sense resistor, Rs; and we are
using a much larger valued output coupling capacitor. The intended load
impedance is 300 ohms and lower gain is established by the feedback ratio
resistors. The 5687 output tube idles at a hot 15mA and can deliver twice
that current into the high-impedance headphones. It is a push-pull,
class-A output stage after all. The unmarked resistor's value would have
to be found either through some complex math or via SPICE simulations or
actual hands-on testing in reality with the real grounded-grid
amplifier.

Okay, one last circuit: a balanced Aikido grounded-grid
amplifier:

Note how the two-resistor voltage divider is shared by
both input triodes. Also note that with certain signal source, the two
input coupling capacitors could be eliminated, as the the two cathode
inputs should be at roughly the same value.

Before ending, I must point out that all of these
Aikido grounded-grid amplifier designs assume that a low-impedance signal
source is used and that no intervening resistance is added, such as a
volume potentiometer would add.

One problem with the above circuit is that it presents
a poor CCRR, which is really what balanced circuit are all about. We must
cross-couple the input grounded-grid amplifiers to get an improved CMRR.

The above circuit offers a much improved CMRR, but a
much poorer PSRR, so we must chose which is more important. To see how the
CMRR obtains, imagine the same, in phase, signal being applied to both
inputs. All the cathodes and grid would see the same signal, which would
greatly reduce the gain realized. Another problem with this circuit is
that it not really a grounded-grid amplifier. It is, in fact, a hybrid
between the grounded-grid amplifier and the grounded-cathode amplifier.
Indeed, other than increased gain, we seem to have gotten the worst of
both worlds: low input impedance and Miller-effect capacitance. The next
circuit offers a better solution.

This balanced grounded-grid amplifier delivers both a
fine CMRR and stellar PSRR. Actually, the stellar PSRR figure is the
result of the fine CMRR, as any power-supply noise that leaks into the
output will be equal and common to both outputs, so both input triodes see
the same unwanted signal strive to eliminate it. The CMRR is interesting.
If the same, in-phase, input signal is presented to both input cathodes,
the output gain will be 1 (or unity or 0dB). Thus, the CMRR will equal
20Log(1/gain) in dBs.

All that is really missing is two 1N4007 safety diodes.
What could this circuit be used for? Most standalone DACs offer balanced
outputs along with the RCA jacks. The balanced outputs could drive this
balanced grounded-grid amplifier, which in turn would drive a power
amplifier with balanced inputs.

Next Time
Many new
developments.

Since I am still getting e-mail asking how to buy these
GlassWare software programs:

For those of you who still have old computers running
Windows XP (32-bit) or any other Windows 32-bit OS, I have setup the
download availability of my old old standards: Tube
CAD, SE
Amp CAD, and Audio
Gadgets. The downloads are at the GlassWare-Yahoo store and the price
is only $9.95 for each program.

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

So many have asked that I had to do it.

WARNING: THESE THREE PROGRAMS WILL
NOT RUN UNDER VISTA 64-Bit or WINDOWS 7 & 8 or any other 64-bit
OS.

One day, I do plan on remaking all of these programs
into 64-bit versions, but it will be a huge ordeal, as programming
requires vast chunks of noise-free time, something very rare with children
running about. Ideally, I would love to come out with versions that run on
iPads and Android-OS tablets.

//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:

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