Care and Maintenance of DuKane Ionovac
Tweeters
December 2003
The
DuKane Ionovac is distinctive among speakers in that it operates on a principle
very much different from the usual mechanical transducer with moving parts. In
the Ionovac there are no parts of the speaker that move. The sound is created
by the interaction of air with high temperature plasma contained inside an open
ended quartz tube.
The system is based on a radio
frequency oscillator operating at about 27 megahertz. The amplitude of this
radio frequency signal is modulated by the audio signal applied to the
oscillator. The radio frequency signal from the oscillator is greatly increased
in voltage by a coil very similar to a Tesla coil. The output of this coil is
connected to a small pointed electrode that is inserted into the quartz tube.
Energy from the high voltage radio frequency signal radiates from a sharp point
on the end of the electrode and forms a plasma inside
a small chamber in the quartz tube. As the amplitude of the audio signal
increases and decreases so does the amplitude of the RF energy. This in turn
causes the size of the plasma field to increase and decrease in direct
proportion to the audio. As it does, it compresses and rarefies the air in the
chamber causing compression waves. These waves exit the end of the tube and are
coupled to the inlet of a horn. The waves travel down the horn and exit into
the room as sound.
The
application of this simple concept is closer to a perfect speaker than just
about any other means. With no moving parts, mechanical inertia and resonances
are minimized. While it is a very good speaker it is far from perfect.
Nonlinearities in the modulation of the 27 megahertz RF and colorations added
by the transformer that couples the audio into the oscillator, called the
modulation transformer, alter the frequency response and add distortion. These
effects are relatively small when compared to the typical voice coil or
electrostatic type of speaker with the inherent mechanical drawbacks each has.
Also because of physical limitations the lower end of the frequency response is
limited to about 1.5 to 2 kilohertz. Frequency response above this extends well
into the ultrasonic range and is limited mostly by the modulation transformer.
There
are two sections to the Ionovac speaker connected together by a four conductor
cable and ground strap. They are the power supply and the oscillator/horn
assembly. The power supply contains the components that convert the 120 volt AC
power to the DC supply voltages necessary to operate the oscillator. The power
supply is a voltage doubler type with no power transformer. The main supply is
310 volts DC for the plate of the oscillator tube and 150 volts DC for the
screen grid. Two other parts complete the power supply. A small 120 volt to 6.3
volt filament transformer supplies power to operate the heater in the
oscillator tube. A modulation transformer couples the low impedance signal
coming from the audio amplifier into the screen grid of the oscillator tube.
The modulation transformer also provides ground isolation of the 120 volt AC
power line and blocks the 150 volt DC screen bias voltage from appearing at the
audio input terminals of the Ionovac. These components are all mounted on an
open face metal plate and tied together using a set of terminal strips.
The
oscillator/horn assembly contains the oscillator tube and related components,
the high voltage coil, the electrode/quartz cell and the horn, all mounted in a
shielded aluminum box.
The
biggest problem in using Ionovac speakers is that of maintenance. The
construction of the Ionovac is a great deal more complex than a magnetic or
electrostatic speaker. With consumable components involved, more can go wrong.
When a critical component fails, the speaker is useless until the part can be
replaced. The electrode is consumed during operation and the quartz tube
becomes encrusted with dirt and oxidation products of contaminates in the air
and electrode residue. A small screen inside the inlet of the horn also gets
clogged with accumulations of dust and oxidized byproducts. Eventually the tip
of the electrode gets burned away and the speaker functions poorly or ceases to
function. The vacuum tube gets weak and other components fail.
DuKane
guaranteed a minimum lifetime of 1200 hours for the cell. When it did need
replacement, DuKane sold a kit containing an electrode and a quartz tube called
a "cell replacement kit" part number 438-37, which could be bought
for a few dollars. The kit included instructions on how to remove the old cell
and install the new one. When the electrode went bad and the quartz got fouled,
installing a cell kit would restore the operation. This assembly is sometimes
called a "crystal" but that term is not used in any of the DuKane
documentation. Many years ago DuKane discontinued supplying Ionovac parts and
now they are very scarce. Fortunately very few of the components prone to
failure are absolutely irreplaceable.
Foremost in operating failures is the electrode. When
the speaker is first turned on, the oscillator tube takes some time to warm up
before it starts. When the oscillator starts, a slight click is heard and a
blue-violet glow can be seen down inside the throat of the horn. The glow
initially is unstable and sometimes takes a minute or so to fill in and settle
down as it comes up to operating temperature. As it does it may emit a buzzing
or squealing sound as the tip heats up and the unstable plasma pulsates. DuKane
called this effect “singing” and is a normal part of startup. If the plasma
does not fill in evenly or continues to make any sounds of it's
own, the electrode and quartz should be removed and examined for damage, wear
and fouling.
After it settles down, the glow should be perfectly
round and even in intensity over the entire end of the tip of the electrode
except for a small brighter spot in the exact center. An orange glow that
continues after startup indicates that the electrode should be cleaned. It is
not uncommon for a bright orange or white glow to display on startup and for
several minutes afterward. Cleaning the electrode and cell will usually
minimize this but not always. Often the abnormal coloration will burn itself
out and revert to the normal soft blue-violet glow after some operating time.
With a new electrode installed the plasma can be uneven
and produce these effects as it breaks in but should clear up within about the
first fifteen minutes of operation. If the electrode is bad it must be
replaced. New electrodes are available from Ionovac.com.
Special care should be taken when removing an electrode. Never grasp the lead wire and pull on it to retract the ceramic retaining bar. This will overstress the bar and can snap it in half. The retaining bar is made of a machinable glass bonded mica ceramic called Supramica™. I have made replacement retainers out of Supramica™, Macor™ and other machinable glass ceramics that work equally well. Machinable ceramics are relatively expensive and the fabrication of replacement retainers as well as electrodes is beyond the scope of this article. Replacement ceramic retaining bars are available from Ionovac.com.
Grasp the two ends of
the bar between the thumb and forefinger of one hand. Hook the connecting wire
with your middle finger and apply enough back tension to keep the connecting
cup seated in the retaining bar as you retract it. Holding the cell in place
with the other hand, pull straight back on the cell retaining bar far enough to
allow adequate clearance to remove the cell. When the bar is clear, remove the
cell by pulling it straight back then up over the retaining bar. Guide it back
past the tail of the horn and up. Allow
the retaining bar to return forward and park the connecting cup in the opening
where the cell goes. Remove the electrode
from the quartz for service. After cleaning replace the electrode and quartz
and place the ceramic retaining bar back in position in reverse order of
removal.
DuKane
has long since discarded their original documents on the electrode. Even though
the DuKane documents are gone, much is known about the electrode material and
dimensions. The DuKane electrode was made of a metal alloy very similar in
composition to type 416 stainless steel. They started
with a rod of material drawn down to .162 inches in diameter. This raw stock
was turned down to form the tip and shank ends and then cut to length.
Within
limits the specific material is not so much important as the dimensions and
heat conductivity. Alloys that are hard are better than softer ones. Those
having a good resistance to the effects of heat and corrosion along with a moderate heat conductivity make good choices for electrode
service. Aluminum and other soft metals are totally unsuitable for electrode
material. The inventor's original plasma speaker designs used platinum
electrodes. Platinum is a good material but totally unsuitable for mass
production because of the prohibitively high cost. Replacement electrodes have
been made from various metals and alloys. The material used determines the
operating temperature, the durability and lifetime of the electrode.
Several of the stainless steel and nickel alloys are
good candidates for electrodes. Type 304 is a commonly available stainless
alloy that is relatively easy to machine and has a moderate life comparable to
that of the factory originals. Some of the high temperature space age alloys
like Inconel™ would probably work very well also.
Tungsten
and tungsten alloys make outstanding electrode materials. Tungsten is extremely
hard and resistant to heat. It can last almost indefinitely but is very
difficult to shape due to its hardness. Another disadvantage of tungsten is its
heat conductivity. High tip temperatures are important for proper operation.
Tungsten has superior heat conductivity and because of this tungsten electrodes
can take a little more time to come up to operating temperature than nickel or
chromium alloy electrodes.
Claims
that the surface must be absolutely smooth and flawless are not accurate. I
have deliberately made electrodes with a slightly rough surface and in tests
they worked as well as highly polished samples. The proof of this is evidenced
by the fact that no matter what the finish is when first installed, the
electrodes soon become pitted and burned in service and they continue to
function satisfactorily.
Another
cause of failure is fouling of the quartz tube. When replacement parts were
available the electrode and quartz tube could be discarded and replaced with a
new set. Today since this is nearly impossible, the quartz must be reused.
Fortunately the quartz is relatively sturdy and will outlast several
electrodes. Dirt and deposits from the high plasma temperatures build up on the
inside of the plasma chamber and can obstruct the opening. As long as the
quartz tube is not broken, cracked, or has no serious pitting, it can be cleaned
and placed back in service. If it is damaged beyond use there is no alternative
at this time to hunting down an old stock replacement at great difficulty and,
no doubt, great expense. I have tried several powerful solvents and acids to
remove the crud inside the quartz tube but no chemical method has been found
that will remove the deposits effectively.
The
best method turns out to be the simplest. A gentle scraping of the inner
surface of the quartz tube will remove nearly all of the built up deposits. A
scraper having a very hard surface works best because it will not rub off onto
the rough surface of the quartz. I have used the blunt end of a number 68 solid
carbide drill bit or stainless steel wire to clean off the surface. A very
gentle scraping, one stroke at a time, in and out all the way around the inside
surface removes nearly all of the deposit. Do not gouge or attempt to use
force. This can damage the surface of the quartz. Better to leave some crud on
the quartz than to damage it. Perfect cleanliness is not necessary but remove as much of the deposit as you can. After the
scraping, a gentle swabbing out with a cotton swab soaked with a mild detergent
solution and rinse in distilled water removes the remaining loose dirt.
Following a thorough air drying, the quartz tube is ready to use. Never apply
force to push anything down through the opening of the quartz. It can be easily
cracked and broken.
Cleaning
a quartz cell should be done after about every one thousand hours of operation
or whenever a new electrode is installed. One quartz cell can last through many
electrodes if reasonable care is taken in cleaning and handling.
The
screen inside the entrance of the horn also collects dirt, which can obstruct
the opening. Remove the cell from the horn then give the horn a gentle blast of
air down the throat from a canned air duster. This will frequently remove the
accumulated dirt. If this does not remove all of it, a very gentle rubbing of
the screen with a long cotton swab will help to loosen the dirt. Be careful and
do not apply much pressure to the screen. It is very thin and can be damaged
easily by too much force.
When an
electrode becomes worn with use it can cause the plasma to become unstable or
irregular. The end of the tip becomes burned and pitted, wearing back during
normal operation. Difficult or incomplete ignition, orange coloration, buzzing,
squealing or snapping noises can result from this. New electrodes tend to do
this before they are broken in but this should clear in a short time. Used
electrodes that have not been completely consumed can be cleaned and put back
into service.
Grasping the pointed tip in a folded over piece of medium grade Scotchbrite™ and rotating it is a good way to burnish and remove deposits. If that isn’t sufficient, a gentle cleaning of the tip with 600 grit sandpaper can help to restore proper function. Place the paper on a hard flat surface. Gently rub the tip of the electrode forward and back on the paper at an angle that puts most or all of the flat side of the tip against the paper. Rotate the shank of the electrode between your thumb and forefinger as you rub it on the paper so that you get an even coverage. Just a few strokes are enough. Removal of large amounts of material or sanding out deeper pits will shorten what life is left, if any. If the end is badly pitted, swipe the tip lightly across some 240 grade sandpaper a few times to smooth it. The idea is not to remove much base material but only some of the crust that forms and smooth out the larger deformations. Do this infrequently and only when necessary.
The number of hours
between cleanings depends on the environment that the speaker is operated in.
Typically cleaning of the electrode is recommended for every two hundred hours
of operation. Sanding the tip is only necessary when the plasma continues to
show considerable persistent orange coloration after cleaning the electrode.
Replacement of the electrode should be done when about two thirds of the tip
has burned away. Electrode cleaning kits are available from Ionovac.com.
Electrical
failures in the associated electronics are common. I will describe several
types of failures that happen and what can be done to repair them.
Oscillator Tube Plate Cap
Every
single Ionovac I have seen has had the plate cap clip on the oscillator tube
badly corroded and overheated. The original construction used a plate cap clip
composed of two half sections held together by a rivet. The lead wire was put
through the hole in the rivet and soldered. After years of use the rivet
corrodes and loses electrical connection with the contact parts. The rivet
heats up and gets progressively worse. Replacement of the plate cap connector
is mandatory on all units. I have used the spiral spring clip type with good
results.
Power Supply Cable
The
connecting cable between the power supply and the oscillator/horn is insulated
with a rubber jacket. With age the rubber becomes hard and cracked. With any
movement the rubber breaks off of the wires exposing them. Replacement with a
new four conductor #18 cable will solve this problem.
The original cable is Belden type 8454 and is still available. Great care must
be taken when unsoldering the old wire and resoldering the new wire to the
feedthrough capacitors in the oscillator/horn assembly. Overheating can melt
the solder in the feedthrough causing the center lead to slip out of position.
Excessive stress on the capacitor lead can break the insulating sleeve.
Feedthrough Capacitors
Feedthrough
capacitors can get broken or arced over if the speaker is subjected to large
overloads. One speaker I repaired which was part of a DuKane column system had
the woofer blown out by seriously overdriving it. The associated Ionovac
tweeter had an arced over feedthrough capacitor in the oscillator/horn that
carried the screen bias and audio signal. It had blackened areas on it where
the arcing had burned the metallization. To prevent damage to the speaker, the
absolute maximum audio input voltage should never exceed 2.16 volts RMS
sinewave or 3 volts peak. Replacement is the only repair for an arced
feedthrough. There are two values of feedthrough capacitors, 10 picofarads for
the oscillator screen grid and 1000 picofarads for the two heater leads and the
plate supply. Replacement capacitors are still available from manufacturers on
special order.
Modulation Transformer
Modulation
transformers can also be damaged by the stress of being overdriven. They can
develop turn to turn shorts in the primary or secondary. This will cause the
speaker to work at greatly lower volume levels, add distortion or fail to
operate at all. In one speaker I worked on, the modulation transformer had
developed a primary to secondary short. This allowed a current path from the
power supply back to earth ground through the audio amplifier. The speaker
first blew it's fuse. The owner replaced the fuse with
one of a substantially higher rating. This of course prevented the fuse from
blowing again but resulted in all of the DC power supply components and
modulation transformer going up in smoke! When I first examined it, I found the
modulation transformer and all of the power supply resistors were badly
charred.
Replacement
of the modulation transformer is not too difficult. Any good quality audio
transformer of a comparable physical size and impedance ratio will work. If
only one speaker in a stereo pair needs repair it is advisable to also replace
the transformer in the other side so that any differences between the old
transformer and the new one will be matched in the other channel.
When replacing the transformer be sure to maintain the
original phase polarity. Maintaining the phase will guarantee that the speaker
will match the phase of it's mate and any other
speakers in the system. I have marked the schematic diagram with the phase of
the original modulation transformer to use as a guide for replacement.
Power Supply
The
power supply is fairly reliable. Solid state rectifiers have a very long
service life. Only an overload is likely to cause a problem. At this time all
of the Ionovacs are at least 40 years old. The electrolytic capacitors are the
most likely components to fail because of advancing age. There are two dual
section electrolytics used in the Ionovac. If they need replacement they can be
substituted with four single types having the same or similar values as the old
ones. The other components like rectifier diodes and resistors are all commonly
available parts. The original type diodes are no longer available but modern
ones are easily substituted. A set of 1N4005 silicon diodes will work well in
this application.
Filament Transformer
Filament
transformers rarely fail. If a good tube fails to light, check the tube socket
for bad connections. Measure the filament circuit with an ohmmeter and verify
that the heater RF chokes are not open. If the filament transformer has gone
bad it is easily replaced. Any transformer of a comparable physical size and
rating will work.
Oscillator Tube
A weak
tube will fail to oscillate or give only poor performance. If the cell is in
good condition and the power supply voltages are normal then a bad tube can be
suspected. Substitution with a known good tube is the best test for a bad one.
Replacing the tube easily solves tube failure. The original tube is an RCA type
6DQ6A. These are not in demand for audio work and were primarily used as a
horizontal output amplifier in black and white television sets. They can be
purchased for no more than a few dollars from a number of tube suppliers.
Oscillator Components
Overdriving
the speaker can damage components in the oscillator circuit. A failed tube can
also cause trouble. These parts are readily available and can be replaced
easily. Overheating because of the bad plate cap clip can also damage plate
resistors.
The
capacitors in the oscillator section are common types except the 12 picofarad
2500 volt silver mica. Fortunately these are very reliable and seldom fail. I
have seen one become intermittent which caused the oscillator to start only
once in awhile. Silver mica capacitors at this capacity and voltage are very
difficult to find. Since the measured
voltage across this capacitor never exceeds 850 volts I substituted one rated
at 12 picofarads at 1000 volts and had no problems.
The RF
chokes in the oscillator are very unlikely to fail. If they do they can be
replaced with modern parts that are designed for RF suppression and have about
the same inductance. The values are not critical since the chokes are used only
to suppress the RF from getting out of the oscillator cage and radiating into
the air from the interconnecting power supply cables.
The
interlock switch can become intermittent or open due to tarnish or
contamination of the contacts. The switch is a simple and inexpensive slide
type with a spring return. A rapid repeated cycling of the switch can sometimes
burnish the contacts enough to restore operation. If that doesn’t work,
cleaning of the contacts is necessary. Switch cleaning and repair is beyond the
scope of this article.
The
voltages used in this device can be dangerous and possibly lethal if not
handled correctly. Certain components like the cell and the 6DQ6A tube become
extremely hot during operation and must be allowed to cool before any work is
done. Remove all power completely from the Ionovac by unplugging it and
allowing time for the cell to cool before doing any work. The interlock switch on
the oscillator cage should never be bypassed. Dangerous voltages are inside and
present a serious shock and electrocution hazard. If you are not familiar with
electronic devices do not attempt to do the work yourself. Get help from a
qualified technician.
The
content of this article is for informational purposes only. The information
given is, to the best of the author's knowledge, accurate and correct at the
time of writing. Errors, omissions or other misleading information is purely
accidental. No guarantee is made of complete accuracy and/or freedom of error.
REPLACEMENT PARTS
Feedthrough Capacitors:
Tusonix type 357-001-X5U 100M, 10 picofarads, screen
grid
Tusonix type 357-001-X5U 102M, 1000 picofarads, heater
and plate
DuKane Ionovac Modulation
Transformer:
Turns ratio 1: 44.7
Primary impedance 8 ohms
Secondary impedance 16,000 ohms
Primary DC resistance .2 ohms
Secondary DC resistance 519 ohms
Frequency response: 30 Hz to 20,000 Hz, +1, -0.5db
Power rating: 5 watts
Suggested Substitutes:
Thordarson/Meissner 24S74, 70.7 volt line to voice coil,
using the .310 watt tap
Stancor A-8105, 70.7 volt line to voice coil, using the
.310 watt tap
The closest substitute available for the DuKane
modulation transformer is a 70.7 volt line to voice coil transformer. Use the 8
ohm speaker winding as the audio input and the .310 watt tap on the 70.7 volt
line winding as the output to the oscillator screen. Except for slight
variations in frequency response, the two listed transformers are an exact match
for the original.
Filament Transformer:
Primary 120VAC
Secondary 6.3VAC at 1.2 amperes
Magnatek/Triad type F-14X
Stancor P-6134 (center tap is unused)
Tube source:
Antique Electronic Supply
(602) 820-5411
*IMPORTANT SPECIFICATIONS:
Sound Output:
.75 volt RMS continuous sinewave input = 95db at 1.5 ft
on horn center axis
Frequency Response:
±3 db from 3500Hz to 30KHz
Operating Input Impedance:
At 200Hz = 5.1 ohms
From 1Khz to 20Khz = 8 ohms +0,
-0.8 ohms
At 30KHz = 6.30 ohms
Non-Operating Input Impedance:
At 200Hz = 8.6 ohms
At 5Khz = 182.6 ohms
At 20KHz = 43.7 ohms
Input voltage into 8 ohms:
0.75 VRMS sinewave/1.06 V Peak = recommended for maximum
undistorted output
1.50 VRMS sinewave/2.12 V Peak = 50% modulation
1.75 VRMS sinewave/2.47 V Peak = maximum input without
parasitic oscillation
2.16 VRMS sinewave/3.05 V Peak = 100% modulation,
absolute maximum input voltage
*Ratings given are for a typical speaker with a new
electrode. Your speakers may vary due to the individual characteristics of the
oscillator tube and/or cell condition.
REFERENCES:
-Popular
Electronics, May 1961, “Introducing the Ionovac” PP56, 57, 117
-U.S.
Patent #2,768,246,
-General
Instruction Manual – Model DuK-5 (14A430) Ionovac, DuKane Corporation PN
400-1141 (undated)
-Specifications
and Instructions (Tentative) Model T-3500 Ionovac, Electro-Voice PN 53518,
-“The
Ionophone Loudspeaker”,
http://home.earthlink.net/~rogerr7/ionovac.htm
(undated)
-Installation
Instructions Ionovac Cell Kit 438-37, DuKane Corporation PN 400-1184 (undated)
-Schematic
diagram, Ionovac Model 14A435A, DuKane Corporation Drawing #190-1055,