NOS Tubes
I buy NOS tubes - I am particularly looking for
Mullard 12AT7/ECC81, Mullard 12AX7/ECC83, Telefunken(<>) 12AT7/ECC81,
Telefunken(<>) 12AX7/ECC83, Mullard 5AR4/GZ34, RCA(GT.Britain) 5AR4/GZ34,
GE(GT.Britain)5AR4/GZ34, Sylvania(GT.Britain) 5AR4/GZ34,
Mullard EL34/6CA7, RCA(GT. Britain) EL34/6CA7, GE(GT. Britain) EL34/6CA7,
Sylvania(GT. Britain) EL34/6CA7,Telefunken(<>) EL34/6CA7, General Electric 7199,
and RCA 7199. Military versions are acceptable. Send to me your your list with asking prices.
***Attention old radio and television repair shops that are about
to close forever.*** Let me know what you have in inventory. You may have the
tubes that I want to buy. Once I know what you have, I may be able to make
you an offer if you have what I want.
Tube Appraisal Service
I appraise the value of NOS tubes. I will give you a
true market value of how much each tube is worth. I have the expertise to identify
different varieties of the same tube type by the same manufacturer. Each variety
can have a different market value. Plus, I have the expertise to distinguish real
tubes from fakes and from rebranded ones. This is very important because there
are fakes and rebranded tubes out there. All the tubes I sell are appraised
and guaranteed by myself to be genuine. Inquire about the fee structure for this
very valuable tube appraisal service. Cost starts at as low as $27.50. Find out how much your tube inventory or
tube collection is really worth.
Tube Designations
Vacuum tube designations around the world have always
been a source of confusion, primarily because two different systems were
developed. Then to add insult to injury, each system had its own military
version. So we are now left with four different systems - all describing the
same tube type.
To try to clarify this bizarre nomenclature, and make it
hopefully easier to identify and collect vacuum tubes, I will attempt to put
some order into the chaos.
The two systems consist of 1)the USA one, and 2)the
British. Each has its own particular characteristics. Here is
one example : 6CA7 is the American designation, whereas EL34 is the British.
Both describe the exact same tube, so it is just a question of how you call
it. Tubes manufactured in the USA may have printed simply 6CA7 on the glass,
or they may have 6CA7/EL34 on it. Conversely, tubes made in the UK may be
printed as EL34 or EL34/6CA7. If you have managed to follow along up to this
point, congratulations. Now you are ready to get into the military
nomenclature - this is were things start to get really scary.
The USA and British military both had vacuum tubes especially
constructed for them. The tubes were used in receivers, transmitters,
telephones, etc. The military tubes were specified by the army and navy,and
built to their standards.
In order to distinguish civilian tubes from military ones,
different designations were used. In general the military version was more
heavy duty in construction. For example, the military tube may have used
thicker mica, or the inside wires were more robust.But,if you examine the
internal construction, sometimes the two versions may possibly look
identical.
The USA designation for military was JAN which stands for "Joint
Army Navy". For example, the military version of 12AX7 is 12AX7 JAN. The
British designation was more complex. Each tube was labeled with the code
CV which stands for "Civilian Valve" followed by a four digit number. For
example,12AX7 has the civilian British designation ECC83.The British
military version was called CV 4004. Is this getting complicated, or what? I
find the term "Civilian Valve" ironic because the tube was for military
purposes - not civilian ones. Could it have been a code to camouflage the
idea that it was actually for military use?
Why should you know so much about the military nomenclature?
Because the civilian form of NOS tubes is getting depleted for many types,
and all that's left is the military version.
Output Tube Bias
Bias of the output tubes in an amplifier is very
important both from an operational point of view, and from a longevity point
of view. The bias determines the class of operation of a tube amplifier. Of
course this depends on the tube type and plate voltage. By varying the bias
of a tube, you can shift the operational class(for example, from class B to
class A) of an amplifier. This affects the sound of the amp. It also affects
output tube life.
Confusion reigns for the term bias as it applies to tube amps
and solid state ones. Tube and solid state amplifiers use the term bias in
completely opposite ways. Unfortunately, this adds to the confusion in an
area which is already misunderstood.
The term bias refers to a negative voltage in tubes, whereas in
transistors it is a positive voltage. This is the source of the confusion.
Bias voltage controls current flow by applying a voltage across a grid.
Negative voltage reduces current flow. Positive voltage increases current
flow. So a "high bias" in a tube means high negative voltage applied to the
control grid which results in low current flow through the tube from the
cathode to the anode. The reverse holds true for "low bias" in tubes.
The problem is in transistor amplifiers the term bias refers to
current flow. So "high bias" in transistors means high current flow - the
exact opposite of tubes. Hence, when one uses the term bias, it is never
clear whether voltage or current is being addressed.
Since transistor amplifiers are more common, the term bias in
general refers to current. So "high bias" for tubes can also mean high
current if the transistor nomenclature is applied to tubes. Are you
following me up to this point? Good. To get high current in tubes, low
negative voltage would have to be applied to the control grid. This would
actually be called "low bias" using tube terminology.
I would suggest that to avoid confusion, the term bias should
universally apply to current as opposed to voltage, no matter what type of
amp(tube or transistor) one is referring to. This universal application
would be done with the understanding that bias voltage in tubes is the exact
opposite in transistors. For example, a tube which is biased low will have a
low cathode to plate current, but to achieve this it will be understood that
a high negative voltage will be applied to the control grid.
Vacuum Tube Rectification Versus Solid State
The power supply of an amplifier is a critical
circuit area because the supply provides both the voltage and current
demands of the amp.
Certain musical passages have great dynamic range - especially on
compact discs. These passages require a tremendous amount of current.
Although this burst of current is required for only a short time
corresponding to the brief, but intense, music sections, the power supply
must be capable of delivering this current. If it can't, then the music may
not have appropriate bass for example.
Some people have gone so far as to say that the power supply
determines the musical characteristics of the amplifier.If that is the case,
then the type of rectification can be very influential on the overall sound
of the amp. Rectifiers provide the DC voltage for the tubes and circuitry.
Up until the advent of solid state rectifiers, vacuum tube amps used tube
rectifiers.
The rectifier converts AC voltage to DC. The rectifier needs to
provide high voltage and high current. A tube amp may typically need 450-500
volts on the plate of the output tubes. At the same time, the output tubes
may conduct up to 75-100 mA of current. From this you can calculate how much
power the supply must deliver just to one output tube(500 volts X 0.10
amperes = 50 watts). So in a typical push-pull amp with four output tubes,
the total power just for the output tubes alone is 4 X 50 watts = 200 watts.
That's alot of power for just an amp!
Tube rectifiers are robust in design, and can handle high
voltages and current. Tube rectifiers which are very well designed and well
manufactured can last a long time. For example, the Mullard GZ34/5AR4 is
probably one of the best tube rectifiers ever made. It has been known to
last for many years.
The sound of an amp using a tube rectifier is characterized by a
phenomenon called sag. At high levels of music the rectifier must supply
both high voltage and high current. At high current levels, the tube drops
the B+ voltage(typically a drop of 30-50 volts). This results in a lower
voltage at the plate of the output tube. Consequently, the volume drops
proportionately. Since the high current is only required for a brief moment,
the voltage drop is only temporary. Once the current demand returns to more
moderate levels, the B+ voltage rises back up to specification, and the
volume goes back to its previous level.
The result of all this is a brief drop in volume as perceived by
the listener - this drop is called sag. It is brief, and sometimes you have
to listen very carefully for it during spirited musical passages. But it is
there, and can be demonstrated by simply hooking up a vacuum tube voltmeter
(input impedence : 10 megohms)parallel to the output terminals and watching
the voltage drop during the peak musical passages.
Vacuum tube rectifiers are slow to respond to power demands in
general. This slowness plus sag are partly responsible for the characteristic
sound of vacuum tubes.
Solid state rectifiers behave differently than their tube
counterparts. No sag is experienced with solid state. As current demand
increases, the DC voltage supplied remains constant up to the specifications
of the power supply. In addition, solid state rectifiers are quick in
response. This gives a solid state amp a faster sound compared to tube
rectification.
The million dollar question is which is better, tube rectification
or solid state? The classic tube amps had tube rectification, and people got
used to the characteristic sound of these amplifiers. Cinemas,rock concerts,
and special events all used tube rectified amps until the advent of solid
state rectification. Almost all vinyl records were mixed and cut with tube
equipment.
But before the million dollar question can be answered, one more
technical detail has to be explored. Tube rectifiers provide a slowly
increasing DC voltage to the output tube plates as the rectifier warms up.
This has an amazing beneficial effect on the output tubes - it minimizes a
phenomenon called cathode stripping. This phenomenon occurs with solid state
rectification. This is because solid state rectification provides an
instantaneous B+ voltage to the plates upon turn on. Unfortunately, this
occurs before the output tube has had a chance to warm up. Warm up takes time
as the heater increases the temperature of the tube.
The electrons do not start to get emitted from the surface of
the cathode until it is warm. In the cold state at start up the electrons are
not emitted, but the high B+ voltage is there nonetheless. This results in
stripping of the metal film on the cathode surface by the B+ voltage from
the solid state rectifier. The ultimate effect of this is that the longevity
of the output tube is reduced(by as much as 30%). This does not occur with
vacuum tube rectifiers because by the time the slowly rising B+ voltage is
present at the plate, the output tube is already itself warmed up.
This is one of the beneficial effects of the vacuum tube
rectifiers - increased lifespan of the output tubes. On the otherhand, solid
state rectifiers themselves will have a much longer lifespan than their
vacuum tube counterparts.
So returning to the million dollar question. In listening tests,
I have found that solid state rectified tube amplifiers have certain
characteristics of solid state amps - quick response time and more edge. In
contrast, tube rectified amps are slower in nature and less sharp in sound.
The music from these amps is more round and mellow. Solid state rectified
tube amps are agressive in nature whereas tube rectified amplifiers are laid
back. In general, I find solid state rectified tube amps to be excellent for
rock 'n roll. Tube rectified amps are really good for classical music. Which
type of amp you use all depends on your music taste.
My email address is optom@attglobal.net.
Last update : November 8, 2009
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