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Author Topic: What is a choke? what does it do?  (Read 5028 times)

witeter

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What is a choke? what does it do?
« on: November 08, 2012, 05:10:54 PM »
 I was watching a video of the new Marshall Satriani head and noticed that inside the chasis it has a 'choke', it basically looked like a big block of something close to one of the big transformers. I have heard of other people adding 'chokes' to their amps; what is it? what does it do? and most importantly, what effect does it have on the amps sound? thanks!

BigB

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Re: What is a choke? what does it do?
« Reply #1 on: November 08, 2012, 06:26:59 PM »
I was watching a video of the new Marshall Satriani head and noticed that inside the chasis it has a 'choke', it basically looked like a big block of something close to one of the big transformers. I have heard of other people adding 'chokes' to their amps; what is it? what does it do? and most importantly, what effect does it have on the amps sound? thanks!

http://www.aikenamps.com/Chokes.html

To make a long story short: better filtering of the rectified current.
Have: Crawlers, BGF 50/52s, Mules, ABomb, RiffRaff
Had : Slowhands (n&m), Trilogy (b)

witeter

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Re: What is a choke? what does it do?
« Reply #2 on: November 08, 2012, 07:29:30 PM »
Cheers BigB, but what difference does that make to the sound of the amp?

HTH AMPS

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Re: What is a choke? what does it do?
« Reply #3 on: November 08, 2012, 07:46:35 PM »
Painting with wide brush strokes, you get a tighter bass end and slightly less fizz on the top.

witeter

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Re: What is a choke? what does it do?
« Reply #4 on: November 08, 2012, 08:23:12 PM »
Cheers man-is it a big piece of work? how much would you charge to carry it out? i dont think my amp needs it but im curious as some people who have the same model rave about it

HTH AMPS

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Re: What is a choke? what does it do?
« Reply #5 on: November 08, 2012, 11:01:43 PM »
Cheers man-is it a big piece of work? how much would you charge to carry it out? i dont think my amp needs it but im curious as some people who have the same model rave about it

Depends on the amp and the choke required, but it's not that big of a job, you'll only need the first hour of bench time to do it. 

witeter

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Re: What is a choke? what does it do?
« Reply #6 on: November 09, 2012, 08:19:55 AM »
Thanks for the info!

darkbluemurder

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Re: What is a choke? what does it do?
« Reply #7 on: November 09, 2012, 01:23:14 PM »
Painting with wide brush strokes, you get a tighter bass end and slightly less fizz on the top.

Agreed. You will also get less compression with a choke than with a resistor when the amp is run wide open. That is the exact reason why e.g. a Trainwreck has a resistor there - these single channel amps need the compression to allow you to clean up the tone with the guitar volume without losing much actual volume. In a multi-channel amp this may not be what you want.

Cheers Stephan


jpfamps

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Re: What is a choke? what does it do?
« Reply #8 on: November 09, 2012, 02:59:38 PM »
The voltage on the screen grids of the power valves used in the power stage of virtually all guitar amps is almost exclusively responsible for the acceleration of electrons from the cathode, so has a significant influence on the current flowing through a valve.

Any ripple on the screen grid will cause hum on the output, so virtually all amps employ extra filtering of the HT for the screen supply of the power valves (the exception being "ultralinear" amps).

This can be done with an RC filter (resistor then capacitor), or an LC filter (inductor aka choke and then capacitor).

The effectiveness of the filtering is determined by the resistor, capacitor and choke values used, where larger values give more filtering, ie reduce ripple.

Increasing capacitor and choke values increases cost.

Increasing resistor values is cost neutral, however resistors drop voltage in proportion to the current through them (Ohms' law), so increasing the R in an RC filter will cause a reduction in the screen voltage (especially under load), and thus a reduction in power.

When an amp is driven into clipping (does that sound familiar?), then the screen current rises dramatically, and and RC filter will drop more voltage causing the screen supply to sag giving a "compression" effect.  This may or may not be desireable depending on the users.

The situation with an LC filter is slightly more complicated. When more current is draw through the screens, the energy stored in the choke will try to maintain the screen voltage, thus preventing the screen supply sagging. However the choke can't hold the screen supply up indefinitely and under sustained overload the screen supply will start to sag. Thus an LC filter will have a slightly different dynamic than an RC filter under sustained overdrive.

As a sweeping generalization, an LC filter gives better ripple rejection and higher power but is more expensive, whereas an RC filter is cheaper and causes more compression on overdrive.

A choke is usually favoured in higher powered amps, as an RC filter wastes power (which presumably is NOT the point of having a high powered amp).

Neither is better, and great amps have been built using both RC and LC filters.


Dmoney

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Re: What is a choke? what does it do?
« Reply #9 on: November 09, 2012, 05:37:26 PM »
The VHT Deliverance has a resistor and not a choke. There is the first example of a tight sounding modern high gain head  I can think of that has no choke. The 5150 has no choke either. Both those heads can be 120watter with the full rack of output valves.

Not like either are known for being poor sounding or unpopular. Just goes to show you what is there doesn't mean the amp is good or bad sounding or feeling.

witeter

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Re: What is a choke? what does it do?
« Reply #10 on: November 11, 2012, 01:16:56 PM »
Thanks for the info guys-very interesting read JPF!

darkbluemurder

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Re: What is a choke? what does it do?
« Reply #11 on: November 12, 2012, 09:34:07 AM »
The VHT Deliverance has a resistor and not a choke. There is the first example of a tight sounding modern high gain head  I can think of that has no choke. The 5150 has no choke either. Both those heads can be 120watter with the full rack of output valves.

Different amp concept. This amp is designed to operate the power amp cleanly and get all distortion from the preamp. If the power amp is not pushed hard then the difference between a choke and a resistor is not as pronounced.

Cheers Stephan

Dmoney

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Re: What is a choke? what does it do?
« Reply #12 on: November 12, 2012, 08:26:23 PM »
Different to what?
My point was that not having a choke in a high power amp doesn't really mean that much. I'm glad we agree.

HTH AMPS

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Re: What is a choke? what does it do?
« Reply #13 on: November 12, 2012, 11:58:31 PM »
I know when I've put chokes in amps that never originally had them in the past there has been a difference in the feel/response that is noticeable, and not just at high volumes.  Thats my experience anyway.


darkbluemurder

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Re: What is a choke? what does it do?
« Reply #14 on: November 13, 2012, 04:42:37 PM »
Different to what?

To amps that rely on overdrive and compression in the power amp/power supply.

My point was that not having a choke in a high power amp doesn't really mean that much. I'm glad we agree.

Yes, we do.

Cheers Stephan