Choke and sound

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JackButler, Mar 5, 2008.

  1. JackButler

    JackButler Supporting Member

    Messages:
    3,656
    Joined:
    Feb 7, 2006
    Location:
    NW Ohio
    How does a choke affect the amp's sound? I never really understood them.
    The sound examples, techie stuff I am lost on!
     
  2. PRNDL

    PRNDL Member

    Messages:
    502
    Joined:
    Aug 25, 2007
    Location:
    SW Florida
    A choke in series is equivalent to a capacitor in parallel. The effect is to smooth out the AC and decrease ripple, which reduces power supply hum.

    From a guitar amp perspective, adding power supply filtering (caps and chokes) makes the power supply "stiffer" and less reactive. Some players want an amp that "breathes" with their playing.

    From a design perspective, the first preamp stages are more susceptible to power supply ripple or hum. So-called "high-gain" amps usually have multiple preamp stages (and/or a cascode), which means the power supplies need a lot of filtering.

    This leads into a discussion on power tube distortion as compared with preamp distortion. It may also be tied in with a discussion about the tone of tweed Fender amps.

    Two good examples for comparison would be the Soldano SLO 100 vs. a 6G6 Bassman.
     
  3. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Chokes and capacitors act a bit differently, but both (as noted) tend to filter/stiffen the power supply. A capacitor resists/smooths changes in voltage while a choke resists/smooths changes in current.

    Think of both as energy storage devices. Capacitance tends to hold voltage steady by discharging when voltage falls and charging when voltage rises. Inductance (e.g. from a choke) tends to hold current steady -- just replace "voltage" with "current".
     
  4. Trout

    Trout Member

    Messages:
    7,576
    Joined:
    Jun 23, 2006
    Location:
    Illinois/Far West Burbia
    One thing a lot of guys overlook is the actual location of the choke in the power supply.

    In some of the old Fender amps like the 5E7 bandmaster, the choke is before the power tube & Tansformer node, This has a much more pronounced effect than an amp that has the choke after the power tube/transformer node.

    It is more common to see the choke after the power tube & tansformer node to get additional filtering to high gain preamps as mentioned above. In this location it has very little " Stiffening" effect if any. A prime example is the Bassman 6G6.

    A 5E7 needs a choke that can handle 250-300ma Vs a Bassman 6G6's 90ma.
     
  5. Rosewood

    Rosewood Member

    Messages:
    1,875
    Joined:
    Feb 19, 2007
    Never seen a Fender amp with a choke before the output tubes, must be pretty hefty. Is that a blond or a tweed amp?
     
  6. redtoploader

    redtoploader Member

    Messages:
    643
    Joined:
    Nov 22, 2005
    Location:
    Big River
    It's tweed...the mid power tweeds like the 5e7/5e5/5f4 have the higher rated choke attached right at the center tap.
     
  7. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Actually, they're not very large, they just have low inductance (2 or 3H? I forget). A 10H choke in that application would be near the size of the PT :)
     
  8. Trout

    Trout Member

    Messages:
    7,576
    Joined:
    Jun 23, 2006
    Location:
    Illinois/Far West Burbia

    I used these as a replacement in a couple old fenders,

    Triad 200 ma 6 henries Data Sheet Nice part for the $$
     
  9. Wakarusa

    Wakarusa Member

    Messages:
    1,477
    Joined:
    May 7, 2004
    Location:
    Indianapolis, IN
    Nice and small too for the VA and inductance. The tweeds originally used Triad parts as well (tho not necessarily THIS Triad part :)). Turns out I spent some time at the Triad booth at NAMM a couple of years ago -- manned by two really surprised engineers who weren't really sure why they were there, only to find lots of people asking if they were going to reproduce their designs from the 50's. After patiently listening to them describe all of the advances in transformer/choke design, fancy new metallurgy, etc., a small group of us explained the whole vintage amp insanity to them. Still not sure if they got it :)
     
  10. Trout

    Trout Member

    Messages:
    7,576
    Joined:
    Jun 23, 2006
    Location:
    Illinois/Far West Burbia

    Very interesting, I wish someone could/would convince them to get back in the game. I have a few NOS Triad PTs here I have been saving for " just the right project"
    Hey, maybe they will start following Hammonds lead and start making a Vintage series.
     
  11. jjasilli

    jjasilli Member

    Messages:
    106
    Joined:
    May 22, 2006
    Chokes are better at ripple reduction than resistors. They are also "slow" - they work by building up a magnetic field as current flows through them. The more current, the stronger the magnetic field. Because a stronger current is forced to expend effort by building-up a stronger magnetic field as it passes through the coil of the inductor, an inductor is better than a resistor at holding its specs in the face of different levels of current flow. But it takes time for the magnetic field to build-up, and then it lasts a little longer than it "should", which makes the amp "slow down".

    With a choke, as the B+ current surges in response to strong signal demand, the coil in the choke forces the current to dissipate energy by creating a stronger magnetic field. This causes a dynamic voltage drop which contributes to compression, sustain, and saturation. This occurs over a time delay, because it takes time for the magnetic filed to build up, and then the magnetic filed is persistent and lasts for a little longer than it "should."

    Also: the resistor and the choke each function as a low pass filter. (The B+ rail is a multi-order low pass, a/k/a hi-cut filter the purpose of which is to reduce ripple toward 0 frequency = DC). But as current surges, more hi freq ripple noise gets through a resistor. But the choke is dynamic - it's resistance increases with increased current, so it better maintains its hi-cut function.

    The above is consistent with vintage tone - like BB King wringing every last drop of tone out of a note held for several bars.

    The only disadvantage I see is for shredders or heavy metal guitarists playing 16th and 32nd notes. They will not appreciate a "slow" amp.
     

Share This Page