Choke related question...

[flood]

so i went to my local transformer winder and seems she had data for chokes in her dusty old catalogue, but can only wind them to 10H. amperages between 50mA and 250mA are possible.

I'm a bit confused with regards to chokes - could i actually use a 10H choke in place of, say, a 40H JCM 800 choke? or a 30H AC30 choke? i don't fully understand how chokes work to magically improve the SNR in the power supply - this is my basic understanding. the choke along with the filter caps arranged in a CLC config would reach maximum impedance at a resonant frequency - the larger the choke, the smaller the caps need to be. i believe EZ80s and EZ81s need to see a minimum capacitance of 50uF?

so... if i use a 10H choke dimensioned adequately for the correct current and change the caps, would it work with no damage to the amplifier/choke?

thanks, as always!

[mbeldyga]

not MINIMUM but MAXIMUM capacitance of 50uF. Using caps with bigger capacitance may damage rectifier tube, also proper resistance of b+ winding is important.

Here's datasheet for ez80 and ez81
http://tubedata.milbert.com/sheets/155/e/EZ80.pdf
http://tubedata.milbert.com/sheets/010/e/EZ81.pdf

[dynaman]

JCM800s didn't have 40H chokes. More like 3-5H.

[flood]

oops, my bad on both counts... i took choke data from here: http://www.hammondmfg.com/guitarLineCHK.htm

[Cliff Schecht]

I've used anywhere from 1H to 16H chokes in different SE amps and can't say that I noticed a big difference tonally. Even at 1H you are going to have a pretty stiff B+ voltage because of the amount of energy that choke is storing after warmup.

[tubeswell]

i don't fully understand how chokes work to magically improve the SNR in the power supply -

Simply and crudely put, the choke is a wire wound around an iron core and the iron has an inductance when a current is passed through the wire. When that current is sitting on a slightly waivering AC voltage - after the first (reservoir) filter cap in a CLC filter - the winding and the iron core together build up an EMF (sought of like a surplus field of electrons in storage within the electromagnetic field) which surrounds the winding, and which in-turn steadies the otherwise waivering voltage by induction back into the winding to stabilise the flow. It does this without hardly an voltage drop because of the relatively low DC resistance of the winding in the choke. At least that's how I figure it. Anyone with a better or more accurate explanation is welcome to chirp in. 2CW

[C Moore]

i don't fully understand how chokes work to magically improve the SNR in the power supply -

Simply and crudely put, the choke is a wire wound around an iron core and the iron has an inductance when a current is passed through the wire. When that current is sitting on a slightly waivering AC voltage - after the first (reservoir) filter cap in a CLC filter - the winding and the iron core together build up an EMF (sought of like a surplus field of electrons in storage within the electromagnetic field) which surrounds the winding, and which in-turn steadies the otherwise waivering voltage by induction back into the winding to stabilise the flow. It does this without hardly an voltage drop because of the relatively low DC resistance of the winding in the choke. At least that's how I figure it. Anyone with a better or more accurate explanation is welcome to chirp in. 2CW

I appreciate this also,..... explaining chokes in the power supply. Some amps do and some don't. Why just one, and why is it always in that position ? I am happy to read a link(s) and do some searching, but I don't get much further than a GENERAL detailed explanation of what a choke/inductor is. I do not read much that is specific to guitar amplifiers.
Thank You (and sorry for high-jacking the thread)

[flood]

I appreciate this also,..... explaining chokes in the power supply. Some amps do and some don't. Why just one, and why is it always in that position ? I am happy to read a link(s) and do some searching, but I don't get much further than a GENERAL detailed explanation of what a choke/inductor is. I do not read much that is specific to guitar amplifiers.
Thank You (and sorry for high-jacking the thread)

seconded - but tubeswell's explanation was excellent too! i think i need to pull out my hughes electronics textbook and do an electromagnetism/inductor refresher.

i do understand this better though - the idea i have now is that there is a "dynamic" AC resistance of sort, so ripple is minimized and there is no significant drop in the DC component of the rectified AC. it's a lot easier to visualize it happening... pixar needs tomake a 3D animated movie on electronics!

so i can safely assume that 10H will be all right for my needs with, say, 47uF caps?

also, how does the current rating of the choke make a difference? from the little that i know, the choke's current rating is integral to it's correct function - but comparing the sizes of the two hammond chokes - 40H at 50mA for the JCM 800 and 30H at 250mA left me totally confused - i always assumed that the choke's current rating should be the maximum current expected to flow through to the plates...?

[Structo]

What I remember about inductors is they oppose a change in current.

Sort of like a resistor for alternating current, if you will.

Just another tool to smooth the DC.

[Andy Le Blanc]

The first thing you should do is figure the % ripple of the capacitor input.

then use the result to find the % ripple of the second section which includes
the inductor (choke) and next capacitor.

The real issue is whether or not you know what the load is, the assumption
that your designing the filter around.

Its a graded filter with taps along the way, you use the first node for the push-pull
which cancels the higher ripple figure any way, and the choke can be small
because it doesn't pass a lot of current.

The piece of data that's often missing is the impedance figure of the PT.
which disallows you the ability to find the peak rectifier plate current,
which is a basic consideration in choosing your rectifier.

[tubeswell]

..... explaining chokes in the power supply. Some amps do and some don't. Why just one, and why is it always in that position ?

Most production amps use a resistor in the filter because it is cheaper, and/or a resistor drops more voltage in the power rail, and in the case of some amps they want this voltage drop to set the screens up to 50-60V below the plate voltage (which you could also do with a choke and a separate screen supply resistor, but less parts is less money and resistors are cheaper than chokes).

You wanna use more chokes? Be my guest. But its kinda like a waste of money if you use more than one, because they do such a good job of filtering the power supply, that you really only need one (along with a few decoupling filter caps and dropper resistors to get the power rail voltage progressively lower and smooth going further towards the start of the signal path in the pre-amp).

As to the position in the supply rail, well the screen voltage ideally needs to be smooth to provide the constant current in the output tubes.

In a PP amp the opposite sides of the primary provides a hum-cancelling/inductor function, for the output tubes, so the choke ideally needs to go between the plate/OT supply point and the rest of the amp supply.

Whereas in a SE amp, there is no hum-cancelling property in the OT, so chokes are only useful in that case if they are filtering the whole amp (i.e. including the B+ for the plates), which means you need a bigger choke (than you otherwise would if you were only filtering the pre-amp and screens) because it is doing more inducting (bigger AC swings- more current).

In early Fender tweed amps, (Super. Pro, Bandmaster etc) there are big chokes filtering the whole amp, but his concept was later dropped by Fender as cost-cutting mentality took hold. Still there's nothing wrong with using a choke to filter the whole amp if you want to put in a big enough one to do the job.

[C Moore]

tubeswell, et al .....
Thanks for all the info. I have a much better understanding of this now. Just another piece of a very big puzzle for me.
Thank You

[FunkyE9th]

seconded - but tubeswell's explanation was excellent too! i think i need to pull out my hughes electronics textbook and do an electromagnetism/inductor refresher.

i do understand this better though - the idea i have now is that there is a "dynamic" AC resistance of sort, so ripple is minimized and there is no significant drop in the DC component of the rectified AC. it's a lot easier to visualize it happening... pixar needs tomake a 3D animated movie on electronics!

so i can safely assume that 10H will be all right for my needs with, say, 47uF caps?

also, how does the current rating of the choke make a difference? from the little that i know, the choke's current rating is integral to it's correct function - but comparing the sizes of the two hammond chokes - 40H at 50mA for the JCM 800 and 30H at 250mA left me totally confused - i always assumed that the choke's current rating should be the maximum current expected to flow through to the plates...?

For a typical guitar amp with a choke (i.e. capacitor input supply, Cap, Choke, Cap etc...) you do not need a lot of current. You are only filtering the DC supply to the screens and preamp tubes, so plate current is not included.

As for understanding how it gets rid of the ripple, you do not need to review electromagnetism. A simple LC circuit analysis will give you a good feel for how it works. You can assume the choke to be an "ideal" inductor (i.e. ignore the DC resistance) to simplify calculation.

You'll find that the choke and the cap form a low pass filter. The higher inductance the lower the cutoff freq. So if your AC is 120VAC @ 60Hz, you are dealing with a 120Hz ripple from the rectified AC. The lower the cutoff the more you are able to reject the ripple. The thing is you have to balance your cutoff, current and voltage drop requirements and still have a small enough choke to fit your chassis.