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What's a "Choke" on an amp?

Discussion in 'Amps and Cabs' started by tele_jas, Feb 15, 2009.

  1. tele_jas

    tele_jas Member

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    Just curious...... I was speaking to an amp builder and I mentioned I LOVED my Stangray's feel, but was looking at something a little different but with the same feel. I mentioned the 65amps London, and he said that even though they are both based off the AC30/15 & ef86 front end - that it would "feel" different because the Stangray has a choke and the London doesn't have a choke.

    What does this choke do and what amps have them? Does my Genz Benz Black Pearl have a choke, cuz I think it feels similar to my Ray.... What about an AC30 (hand wired or TBX) or a Hayseed 30 or even the Gabriel Voxer..... any of those have a choke?

    Just curious.

    Thanks, Jason
     
  2. sharpshooter

    sharpshooter Member

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    Some guys over in the tech forum would know, but for simplicity, a choke is an inductor, a coil, with a core, DC currant goes right thru easily, while any AC component, like a signal, is held back, or "choked".
     
  3. sharpshooter

    sharpshooter Member

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    They are /were most commonly used in tube rectification curcuits. Tubes have a feature, "electrical inertia", so to speak, this can allow a bit of AC ripple into the DC power. Chokes can be constructed to be tuned to different freqs, in this case, 60 hertz, so used, to serve as a last ditch effort in the power supply to block any left over AC from entering.
     
  4. SatelliteAmps

    SatelliteAmps Member

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    Not all amps need chokes. Nothing to do with cheapness. More to do with effectiveness. Some amps need them. Some amps don't.
     
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  5. Steve Dallas

    Steve Dallas Supporting Member

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    I'm gonna have to disagree with the cheapness part of this.
    Choke = $20+
    Resistor = $1

    Multiply the difference across 100 amps and the cost of a choke really starts to add up.

    It is true, however, that not all amp designs need a choke. The Marshall 18W is a very good example of this.
     
  6. Rick51

    Rick51 Member

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    There is a part of your amplifier called the power supply which converts the 60 hz. 115 volt AC wall power to various DC voltages needed by the amp. The large capacitors that you see act as filters to smooth out the power delivery while storing energy that is available to handle transient signal peaks. A choke is an inductor that performs a similar function with a key difference. Filter caps block low freq's and are more transparent to highs. Inductors block highs and pass the lows. The actual freq's involved are tuned by selecting component values.

    So what's the use? Adding a choke to your power supply will increase the amp's dynamic range and reduce sag. There's other differences, but adding a choke to a fender tweed circuit will move that amp a big step toward the brown tolex performance.
     
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  7. epluribus

    epluribus Member

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    Fosse-natin', nice synopsis. Always wondered why people say that chokes darken an amp. BTW, on blocking frequencies...I'm confused by this concept. In a DC power supply circuit, why would frequency even matter, and how does the AC signal interact with the DC power supply such that frequency becomes an issue? Just a thin spot in my theoretical understanding of the interaction between the power supply and the signal.

    --Ray
    weberbster


     
    Last edited: Dec 28, 2011
  8. sharpshooter

    sharpshooter Member

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    Let's say that the amp wants to have a 400V B+ supply, to the tubes.
    If the power supply puts out 400V, and then puts out 405V, then back to 400V, then 395V, etc,etc,. What we see here, is a 10V swing, if this occurs 100 times a second, then the tubes see this as a 10V, 100 hertz AC component, that is superimposed on the DC power. The filter choke acts to block this. SS rectification does'nt have to deal with this (generally speaking), the quality of the power supply, and its filtering make a big difference in the need for/ use of, chokes.
     
  9. Rick51

    Rick51 Member

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    Thanks, Ray.

    This is the DC power supply we are talking about - how that DC is created from the AC supply.

    Keeping it very basic... Visualize your 60 cycle AC supply as a wave that ascends to a positive peak voltage, then descends through zero to an equal and opposite negative peak voltage. The rectifier portion of your power supply either cuts off and discards all the negative parts (half-wave rectifier) or flips the negative into a positive mirror image (full-wave rectifier). Your input power is now a 60 hz. or 120 hz. wave varying from zero volts to the positive peak, 350 volts more or less depending on the (tube) amp. This is now filtered by filter caps with the object of turning that 0 - 350 - 0 wave into a nice flat continuous 350v. The filter caps are networked with resistors to tune their resonant frequency away from that 60 hz / 120 hz. supply frequency. The choke lets you tune the filter network while using less resistance and storing more energy at the same time.

    The energy storage part is where we get effects like "sag" and "bloom". Use a 5E3 as a common example. Turn the amp up, play a big fat chord, and let it ring. You'll hear the output drop (sag) then recover somewhat (bloom) as the input chord dies out on its own. The initial sounding of the chord depletes the energy stored by the power supply, then recovers as the signal decays. With the choke in a properly designed power supply, you get more power delivered to the tubes when they demand it.
     
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  10. SatelliteAmps

    SatelliteAmps Member

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    If you are buying 100 chokes, the price is a lot less than $20. Resistors are less as well. Even so, there aren't any good manufacturers that would balk at an extra $20 worth of parts if it was a necessity.

    My point was that some amps just don't need chokes, without even considering cost. Chokes will alter how an amp sounds, and some amps are made to sound the way they do without chokes. Tweed Champs & Princetons are great examples. They used chokes for a very small spot in their history and then removed them. Why? Because they didn't sound as good. Had nothing to do with cheapness.
     
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  11. epluribus

    epluribus Member

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    'Preciate the answers guys.
     
  12. Terryrocks

    Terryrocks Member

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    If they(fender) were concerned with how good the amps sound, please explain fender's silverface amps. Why did the company follow up the tone monsters (tweed/brown/black) with those icepick silverface amps?
     
  13. Terryrocks

    Terryrocks Member

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    Just realized I'm replying to a thread from 7 years ago.
     
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  14. drbob1

    drbob1 Silver Supporting Member

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    Terryrocks, you're confusing Fender (Leo) with Fender (CBS). Leo was constantly asking for feedback from his customers (country and surf players mostly) and tuning the amps to work better for them. When CBS bought the company he left and the changes that were made were not subjected to the same customer testing. It's not that some of the short lived 67-68 changes wouldn't have been made (Leo likely had already planned those changes in 65) but that they would have been dialed back to a better result quickly, rather than some of them compounding onward to the UL stuff of the late 70s.
     
  15. hogy

    hogy Member

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    Gross oversimplification and wrong. It's a design decision like any other one. Trainwreck Express and Liverpool models don't have chokes, neither do some Dumbles (Robben Ford's amp is one of them).
     
  16. Sirloin

    Sirloin Supporting Member

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    You beat me to it. I was just thinking the Trainwrecks don't have chokes.
     
  17. keithb7

    keithb7 Supporting Member

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    BF amps followed by embarassing SS near bankrupting amps. Oops we need to reintroduce BF amps but we'll make them all new SF graphics. Our smarter eningeers will make them better and cheaper.


    Thats what happened to SF amps at CBS era Fender.
     
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  18. swiveltung

    swiveltung Member

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    It may have something to do with cost, but it is incidental. Chokes are avail for less than $10 even today. Likely < $1.00 back in the day. Not all amps need them for sure. A Princeton Reverb sounds pretty good without one!
     
  19. doghouseman

    doghouseman Member

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    What about a vox ac15? I picked up one where the guy I bought it from added a choke. I couldnt really tell the difference.
     

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