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    Purpose of chokes

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    wildiowa

    Posts : 214
    Join date : 2012-03-19

    Purpose of chokes

    Post by wildiowa on Sat Jan 05, 2019 3:07 pm

    To expand the knowledge base....recently purchased a vintage guitar amp utilizing four 6L6's, split in half to deliver 40-50 watts seperately to each of two, 12 inch Jensens. On each speaker there is a choke. These are NOT field coil speakers..these parts are identified as chokes. One on each speaker. It appears the output is wired directly to the speakers, with a wire on each terminal then feeding the choke in parallel. The choke is not inserted between the output and the speakers. I have heard chokes discussed here often, but totally baffled as to their purpose, or why they are on this amp. Have a couple old field coil amps, but never seen this. What does a choke do? Thanks...
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    Peter W.

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    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: Purpose of chokes

    Post by Peter W. on Sat Jan 05, 2019 3:40 pm

    A choke "chokes" any residual DC. Transformers do not pass DC.

    wildiowa

    Posts : 214
    Join date : 2012-03-19

    Re: Purpose of chokes

    Post by wildiowa on Sat Jan 05, 2019 3:47 pm

    Ok...help me out here....what would happen if there were no chokes? Trash the speakers? Where would this DC come from? I know only enough to be dangerous. Thanks for your patience.
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    Dave_in_Va

    Posts : 254
    Join date : 2013-04-02

    Re: Purpose of chokes

    Post by Dave_in_Va on Sat Jan 05, 2019 4:25 pm

    The only guitar amp I've seen like this is the Silvertone 1483, ostensibly a bass guitar amp.
    I was told it was a device (can't remember if anyone called it a choke) to well, choke off some of the bass frequencies going into the speaker so it wouldn't just blow up.
    I'm not a tech, this is what I was told...
    Here it is.....


    wildiowa

    Posts : 214
    Join date : 2012-03-19

    Re: Purpose of chokes

    Post by wildiowa on Sat Jan 05, 2019 4:47 pm

    Exactly! I have two 1483's...killer amps, beat Marshalls in my opinion. I got a Silvertone 1474 two days ago. On one website it was theorized the choke limited speaker excursion but with my limited knowledge was not sure what elecron magic went on for that to happen. The fact they were wired in parallel had me wondering. Danelectro made rhe Silvertone stuff. Also, why not put the chokes in the chassis? Anyway...Silvertone rules!!
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    Tubes4ever

    Posts : 123
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: Purpose of chokes

    Post by Tubes4ever on Sat Jan 05, 2019 8:26 pm

    Peter W. wrote:A choke "chokes" any residual DC. Transformers do not pass DC.  

    Peter, you are correct that transformers do not pass DC. However, chokes easily pass DC because a choke only has one coil.  A choke is used to smooth out AC ripple by the use of inductive reactance.

    wildiowa

    Posts : 214
    Join date : 2012-03-19

    Re: Purpose of chokes

    Post by wildiowa on Sat Jan 05, 2019 8:43 pm

    Some research...in the case of the Silvertones, it was apparently a cheap way to eliminate some of the lows, saving relatively low watt speakers from being blown out. A high pass filter kinda thing? There was discussion on how this might change the load, could have a negative impact on power tubes etc. And others just take them out and feel they expand the range of the amp. I recall chokes in the Dynas and wondered if they play a similar role.
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    Dave_in_Va

    Posts : 254
    Join date : 2013-04-02

    Re: Purpose of chokes

    Post by Dave_in_Va on Sat Jan 05, 2019 9:13 pm

    Only thing better than a Silvertone is a Silvertone stack.
    (Totally serviced, new baffle boards, treble caps added, and NO BACKS!)

    Sorry, a little off topic, I know.....



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    j beede

    Posts : 476
    Join date : 2011-02-07
    Location : California

    Re: Purpose of chokes

    Post by j beede on Sun Jan 06, 2019 2:30 am

    Inductor basics...

    Inductive reactance or the impedance of an inductor describes the opposition to ac current flow in an inductor as a function of frequency. If a choke is added across the speaker terminals it will act as a high pass filter. At low(er) frequencies the choke will act as a resistor (R~the resistance of the wire) largely shunting the speaker. Connected this way the choke will reduce the LF content (a.k.a. boosting treble) delivered to the speaker at the rate of 6dB/octave. This is analogous to putting a bypass cap across a tweeter to reduce its exposure to HF content.

    In a power supply the choke may be used to limit transient current according to the differential equation L*di/dt. Slow changes (large dt) in current are passed, rapid changes (small dt) are attenuated.

    DarthBubba

    Posts : 88
    Join date : 2012-05-05

    Re: Purpose of chokes

    Post by DarthBubba on Tue Jan 08, 2019 2:00 am

    The purported schematic is here: http://www.prowessamplifiers.com/schematics/silvertone/Silvertone_1483.html

    Didn't think Sears made (well, sold) tube equipment, did ya Shocked

    -DB


    Last edited by DarthBubba on Tue Jan 08, 2019 2:50 pm; edited 1 time in total
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    Tubes4ever

    Posts : 123
    Join date : 2015-07-14
    Location : Star, Idaho

    Re: Purpose of chokes

    Post by Tubes4ever on Tue Jan 08, 2019 1:57 pm

    DarthBubba wrote:The purported schematic is here: http://www.prowessamplifiers.com/schematics/silvertone/Silvertone_1483.html.

    Didn't think Sears made (well, sold) tube equipment, did ya Shocked

    -DB

    Remove the period at the end of the link and it will work. Otherwise you end up at prowessamplifiers with an error. You can still then drill down manually.

    DarthBubba

    Posts : 88
    Join date : 2012-05-05

    Re: Purpose of chokes

    Post by DarthBubba on Tue Jan 08, 2019 2:51 pm

    Tubes4ever wrote:
    DarthBubba wrote:The purported schematic is here: http://www.prowessamplifiers.com/schematics/silvertone/Silvertone_1483.html

    Didn't think Sears made (well, sold) tube equipment, did ya Shocked

    -DB

    Remove the period at the end of the link and it will work.  Otherwise you end up at prowessamplifiers with an error.  You can still then drill down manually.


    Aaaarrrrggghhh!

    Okay, fixed. Embarassed

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