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I think my tube amp might be dying

Discussion in 'Amps and Cabs [BG]' started by jman890202, Jul 19, 2007.


  1. jman890202

    jman890202

    Jun 9, 2006
    I have an old Traynor bassmaster mark II, and today when I was playing the amp started distoring a lot and the sound kept cutting out. I tried a new cable and it did the same thing.
    I know the tubes in the amp are pretty old. Should I just take it in to be retubed or what? Does this sound like an old tube problem?

    Thanks,
    Jman
     
  2. Dave W

    Dave W

    Mar 1, 2007
    White Plains
    It sounds like it could be tube related at the very least. I'd bring it in to a tech to have them take a look.
     
  3. anderbass

    anderbass

    Dec 20, 2005
    Phoenix. Az.
    +1

    And I'd let him tell you, what it needs instead of requesting new tubes.
    If you give us Your location, someone might suggest a good amp-tech in your area.
     
  4. Surly

    Surly

    Feb 2, 2007
    South Florida
    These guys are right. It may not necesarily be the tubes themselves. Could be power related; i.e. power supply.
     
  5. ihixulu

    ihixulu Supporting Member

    Mar 31, 2000
    South Shore MA
    When you say "old" how old is it? Maybe it's time for the caps to be replaced?
     
  6. jman890202

    jman890202

    Jun 9, 2006
    The amp is somewhere around 35 years old, and I think it might still have the original tubes. I'm in St. Louis and I know a good place to take the amp to.
     
  7. Bryan316

    Bryan316 Banned

    Dec 20, 2006
    Detroit
    The original power tubes????? Holy crap.... that alone is your answer!
     
  8. It COULD be, but 35 years is not above average for the service lives of old US made tubes. You're getting to be "seamonkeyesque" in calling everything tube failure every time someone here talks about amp trouble. Cap age is far more likely the culprit.
     
  9. BassJunkie730

    BassJunkie730

    Feb 3, 2005
    Brooklyn
    Caps Caps Caps Caps.

    get them replaced - and enjoy tubey goodness for years to come.

    People forget that tubes won't cut out like that unless the tube is actually broken (glass cracked- componet compromised, etc) and you can SEE when that happens

    Tubes "wear out" when you slam them for extended periods of time.

    That kind of wear is more heard than seen (although you can check the getter and the plates too)

    You'll have an output that has less HF content - and sounds dull and lifeless - kind of like strings.

    I've used the same pair of EL34EH's in a custom built 30 watt amp since 2003 and I would regularly dime the amp for hours at a time over the 5 years the head has been in service. They still sound fine to me - yes they could likely have more "life" to them - but they still work.

    Caps on the other hand WILL fail with use over time as they can't hold a charge any more. 15-20 years is a good time to get any tube amp recapped - or any amp for that matter.
     
  10. jman890202

    jman890202

    Jun 9, 2006
    What exactly are caps?
     
  11. TrooperFarva

    TrooperFarva

    Nov 25, 2004
    New City, NY
    Capacitors. When they're not used often, they can dry out, and cause the symptoms you describe. Just bring your amp to a tech and get a cap job, it doesn't cost that much, and it'll bring your amp back to life.
     
  12. anderbass

    anderbass

    Dec 20, 2005
    Phoenix. Az.
    http://en.wikipedia.org/wiki/Capacitor

    (here's the condensed version):

    Capacitor
    From Wikipedia, the free encyclopedia
    Jump to: navigation, search
    See Capacitor (component) for a discussion of specific types.

    Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. Major scale divisions are cm.A capacitor is an electrical device that can store energy in the electric field between a pair of closely spaced conductors (called 'plates'). When current is applied to the capacitor, electric charges of equal magnitude, but opposite polarity, build up on each plate.

    Capacitors are used in electrical circuits as energy-storage devices. They can also be used to differentiate between high-frequency and low-frequency signals and this makes them useful in electronic filters.

    Capacitors are occasionally referred to as condensers. This is now considered an antiquated term.

    Contents [hide]
    1 Physics
    1.1 Capacitance
    1.2 Stored energy
    1.3 Hydraulic model
    2 Electrical circuits
    2.1 DC sources
    2.2 AC sources
    2.2.1 Impedance
    2.2.2 Laplace equivalent (s-domain)
    2.3 Displacement current
    2.4 Networks
    2.4.1 Series or parallel arrangements
    2.5 Capacitor/inductor duality
    3 Capacitor types
    4 Applications
    4.1 Energy storage
    4.1.1 Power factor correction
    4.2 Filtering
    4.2.1 Signal de-coupling
    4.2.2 Noise filters, motor starters, and snubbers
    4.3 Signal processing
    4.3.1 Tuned circuits
    4.4 Other applications
    4.4.1 Sensing
    4.4.2 Pulsed power and weapons
    5 Hazards and safety
    5.1 High-voltage
    6 History
    7 See also
    8 Notes
    9 References
    10 External links



    [edit] Physics
    A capacitor consists of two conductive electrodes, or plates, separated by a dielectric.


    [edit] Capacitance

    When electric charge accumulates on the plates, an electric field is created in the region between the plates that is proportional to the amount of accumulated charge. This electric field creates a potential difference V = E·d between the plates of this simple parallel-plate capacitor.The capacitor's capacitance (C) is a measure of the amount of charge (Q) stored on each plate for a given potential difference or voltage (V) which appears between the plates:


    In SI units, a capacitor has a capacitance of one farad when one coulomb of charge is stored due to one volt applied potential difference across the plates. Since the farad is a very large unit, values of capacitors are usually expressed in microfarads (µF), nanofarads (nF), or picofarads (pF).

    The capacitance is proportional to the surface area of the conducting plate and inversely proportional to the distance between the plates. It is also proportional to the permittivity of the dielectric (that is, non-conducting) substance that separates the plates.

    The capacitance of a parallel-plate capacitor is given by:

    [1]
    where ε is the permittivity of the dielectric (see Dielectric constant), A is the area of the plates and d is the spacing between them.

    In the diagram, the rotated molecules create an opposing electric field that partially cancels the field created by the plates, a process called dielectric polarization.


    [edit] Stored energy
    As opposite charges accumulate on the plates of a capacitor due to the separation of charge, a voltage develops across the capacitor owing to the electric field of these charges. Ever-increasing work must be done against this ever-increasing electric field as more charge is separated. The energy (measured in joules, in SI) stored in a capacitor is equal to the amount of work required to establish the voltage across the capacitor, and therefore the electric field. The energy stored is given by:


    where V is the voltage across the capacitor.

    The maximum energy that can be (safely) stored in a particular capacitor is limited by the maximum electric field that the dielectric can withstand before it breaks down. Therefore, all capacitors made with the same dielectric have about the same maximum energy density (joules of energy per cubic meter).


    [edit] Hydraulic model
    Main article: Hydraulic analogy
    As electrical circuitry can be modeled by fluid flow, a capacitor can be modeled as a chamber with a flexible diaphragm separating the input from the output. As can be determined intuitively as well as mathematically, this provides the correct characteristics

    The pressure difference (voltage difference) across the unit is proportional to the integral of the flow (current)
    A steady state current cannot pass through it because the pressure will build up across the diaphragm until it equally opposes the source pressure.
    But a transient pulse or alternating current can be transmitted
    The capacitance of units connected in parallel is equivalent to the sum of their individual capacitances

    [edit] Electrical circuits

    The electrons within dielectric molecules are influenced by the electric field, causing the molecules to rotate slightly from their equilibrium positions. The air gap is shown for clarity; in a real capacitor, the dielectric is in direct contact with the plates. Capacitors also allow AC current to flow and block DC current.
    [edit] DC sources
    Electrons cannot easily pass directly across the dielectric from one plate of the capacitor to the other as the dielectric is carefully chosen so that it is a good insulator. When there is a current through a capacitor, electrons accumulate on one plate and electrons are removed from the other plate. This process is commonly called 'charging' the capacitor -- even though the capacitor is at all times electrically neutral. In fact, the current through the capacitor results in the separation of electric charge, rather than the accumulation of electric charge. This separation of charge causes an electric field to develop between the plates of the capacitor giving rise to voltage across the plates. This voltage V is directly proportional to the amount of charge separated Q. Since the current I through the capacitor is the rate at which charge Q is forced through the capacitor (dQ/dt), this can be expressed mathematically as:

    where

    I is the current flowing in the conventional direction, measured in amperes,
    dV/dt is the time derivative of voltage, measured in volts per second, and
    C is the capacitance in farads.


    For circuits with a constant (DC) voltage source and consisting of only resistors and capacitors, the voltage across the capacitor cannot exceed the voltage of the source. Thus, an equilibrium is reached where the voltage across the capacitor is constant and the current through the capacitor is zero. For this reason, it is commonly said that capacitors block DC.


    [edit] AC sources
    The current through a capacitor due to an AC source reverses direction periodically. That is, the alternating current alternately charges the plates: first in one direction and then the other. With the exception of the instant that the current changes direction, the capacitor current is non-zero at all times during a cycle. For this reason, it is commonly said that capacitors "pass" AC. However, at no time do electrons actually cross between the plates, unless the dielectric breaks down. Such a situation would involve physical damage to the capacitor and likely to the circuit involved as well.

    Since the voltage across a capacitor is proportional to the integral of the current, as shown above, with sine waves in AC or signal circuits this results in a phase difference of 90 degrees, the current leading the voltage phase angle. It can be shown that the AC voltage across the capacitor is in quadrature with the alternating current through the capacitor. That is, the voltage and current are 'out-of-phase' by a quarter cycle. The amplitude of the voltage depends on the amplitude of the current divided by the product of the frequency of the current with the capacitance, C.


    [edit] Impedance
    The ratio of the phasor voltage across a circuit element to the phasor current through that element is called the impedance Z. For a capacitor, the impedance is given by



    where

    is the capacitive reactance,
    is the angular frequency,
    f is the frequency),
    C is the capacitance in farads, and
    j is the imaginary unit.
    While this relation (between the frequency domain voltage and current associated with a capacitor) is always true, the ratio of the time domain voltage and current amplitudes is equal to XC only for sinusoidal (AC) circuits in steady state.

    See derivation Deriving capacitor impedance.

    Hence, capacitive reactance is the negative imaginary component of impedance. The negative sign indicates that the current leads the voltage by 90° for a sinusoidal signal, as opposed to the inductor, where the current lags the voltage by 90°.

    The impedance is analogous to the resistance of a resistor. The impedance of a capacitor is inversely proportional to the frequency -- that is, for very high-frequency alternating currents the reactance approaches zero -- so that a capacitor is nearly a short circuit to a very high frequency AC source. Conversely, for very low frequency alternating currents, the reactance increases without bound so that a capacitor is nearly an open circuit to a very low frequency AC source. This frequency dependent behaviour accounts for most uses of the capacitor (see "Applications", below).

    Reactance is so called because the capacitor doesn't dissipate power, but merely stores energy. In electrical circuits, as in mechanics, there are two types of load, resistive and reactive. Resistive loads (analogous to an object sliding on a rough surface) dissipate the energy delivered by the circuit, ultimately by electromagnetic emission (see Black body radiation), while reactive loads (analogous to a spring or frictionless moving object) store this energy, ultimately delivering the energy back to the circuit.

    Also significant is that the impedance is inversely proportional to the capacitance, unlike resistors and inductors for which impedances are linearly proportional to resistance and inductance respectively. This is why the series and shunt impedance formulae (given below) are the inverse of the resistive case. In series, impedances sum. In parallel, conductances sum.


    [edit] Laplace equivalent (s-domain)
    When using the Laplace transform in circuit analysis, the capacitive impedance is represented in the s domain by:



    where C is the capacitance, and s (= σ+jω) is the complex frequency.


    [edit] Displacement current
    The physicist James Clerk Maxwell invented the concept of displacement current, dD/dt, to make Ampère's law consistent with conservation of charge in cases where charge is accumulating as in a capacitor. He interpreted this as a real motion of charges, even in vacuum, where he supposed that it corresponded to motion of dipole charges in the aether. Although this interpretation has been abandoned, Maxwell's correction to Ampère's law remains valid.


    [edit] Networks

    [edit] Series or parallel arrangements
    Main article: Series and parallel circuits
    Capacitors in a parallel configuration each have the same potential difference (voltage). Their total capacitance (Ceq) is given by:



    The reason for putting capacitors in parallel is to increase the total amount of charge stored. In other words, increasing the capacitance also increases the amount of energy that can be stored. Its expression is:


    The current through capacitors in series stays the same, but the voltage across each capacitor can be different. The sum of the potential differences (voltage) is equal to the total voltage. Their total capacitance is given by:



    In parallel the effective area of the combined capacitor has increased, increasing the overall capacitance. While in series, the distance between the plates has effectively been increased, reducing the overall capacitance.

    In practice capacitors will be placed in series as a means of economically obtaining very high voltage capacitors, for example for smoothing ripples in a high voltage power supply. Three "600 volt maximum" capacitors in series, will increase their overall working voltage to 1800 volts. This is of course offset by the capacitance obtained being only one third of the value of the capacitors used. This can be countered by connecting 3 of these series set-ups in parallel, resulting in a 3x3 matrix of capacitors with the same overall capacitance as an individual capacitor but operable under three times the voltage. In this application, a large resistor would be connected across each capacitor to ensure that the total voltage is divided equally across each capacitor and also to discharge the capacitors for safety when the equipment is not in use.

    Another application is for use of polarized capacitors in alternating current circuits; the capacitors are connected in series, in reverse polarity, so that at any given time one of the capacitors is not conducting...


    [edit] Capacitor/inductor duality
    In mathematical terms, the ideal capacitor can be considered as an inverse of the ideal inductor, because the voltage-current equations of the two devices can be transformed into one another by exchanging the voltage and current terms. Just as two or more inductors can be magnetically coupled to make a transformer, two or more charged conductors can be electrostatically coupled to make a capacitor. The mutual capacitance of two conductors is defined as the current that flows in one when the voltage across the other changes by unit voltage in unit time.
     

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