Winding specs for 3H, 5H, 7H inductor on E20 lams and bobbin

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Spencerleehorton

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Joined
May 12, 2012
Messages
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Location
Felixstowe, Suffolk, UK
Hi All,

Wanted to wind my own inductor to try out different values for a 50 watt valve amp.
I have quite a few left over lams which I think are EI- 75s E20, I have some bobbins but could make a few more if needed.
Just needed to know wire gauge and winds for getting a 3H @ 250ma at about 112ohms dcr.
If anyone could help please?
 
Most guitar amps that use a choke include it in the screen supply , 10-20 hy would often be a typical value but usually at much lower current than you specify. A choke input filter or even a CLC filter to handle the entire plate current of the amp plus the ac current would probably need a larger core . The EI-75 core would make a convienient size for a screen choke , the kind included in vintage marshall  or Ac-30 , I'd say 20 hy or so at around 70 ma might be better for the cores you have handy .

Some more details about the vox type screen choke here ,

https://groupdiy.com/index.php?topic=71082.msg907067#msg907067
 
Spencerleehorton said:
Hi All,

Wanted to wind my own inductor to try out different values for a 50 watt valve amp.
I have quite a few left over lams which I think are EI- 75s, I have some bobbins but could make a few more if needed.
Just needed to know wire gauge and winds for getting a 3H @ 250ma at about 112ohms dcr.
If anyone could help please?
A choke is submitted to significant DC current. It must be gapped. The minimum gap is butt-stacked, but it's not enough, so it is necessary to add cauls. The thickness of the cauls modify significant the Al coefficient. There are probably tables that give the Al as a function of the core size and gap thickness.
 
Well I'm building a marshall jtm 45 which I will mod to plexi 68.
I will measure the E lams just to be sure as they might be bigger than EI75.
At a glance they are about 75mm long.

The 3H chokes that I've seen for marshall plexi are 250ma at 112ohm Dcr.
Hammond do a version I think it's a 194G.
 
The vox choke I re-wound just used random wound turns , over the years the wire insulation had broken down almost completely,  to the point where I could only measure 100mH,  practically a dead short across the coil.  One of the more heavily insulated types of magnet wire is probably best for that job , the marshall style has paper interleave which helps insulation but makes winding a bit more complex a job .



 
Well from what little info I have to go on these I've found out the following

For an inch square core, using 0.35mm with approx 1800 turns and a crate paper gap in core it's meant to give 300ma, 3H and be 46ohms.

It's a starting point I suppose.
I'll experiment and see what I can get on this core with what I've got as I think I have some 0.35mm
 
Just measured my bobbin and lams and have found some 0.4mm copper enammelled wire, might be a touch big for what i need but could it work?
My lams measure 75mm long with 25mm middle of E and 38mm wide
Bobbin core is 31mm x 25mm (when i say 25mm its 1inch, so 25.4mm)
I will throw on some winds and see how many i can get on there, ive found a couple of calculators but they are only for round core, this core is 31mmx25mmx38mm so will need to find another calculator somewhere!!
 
Also going by the Hammond 194G spec it looks like it could be around the E20 size, so a fair bit smaller than ive got.
Also current wise if i use this 0.405mm i can go up to 2.2A!! so i could get away with using this 0.255mm wire i have as it is up to 860mA.
i do also have some other cores and bobbins i could use which are small and more to the size of a E20, so might give them a go.
Hammond 194G is 3H @ 250mA, 500v max, DCR = 79.92ohms
So i'll try and aim for close to this.
 
Just done my first wind and got the following:

E25 lams - 3500 turns with 0.25mm guage wire, DCR = 178 ohm, Inductance = 15.6 H

this is with the bobbin pretty full, i'd say about 95% full, so could aim for much less and see if i can get the 3H mark, then try and wind one in the middle.
Can then test them and see how they sound in this JTM 45 or the Friedman BE-50 that im building.
 
Spencerleehorton said:
Just done my first wind and got the following:

E25 lams - 3500 turns with 0.25mm guage wire, DCR = 178 ohm, Inductance = 15.6 H

this is with the bobbin pretty full, i'd say about 95% full, so could aim for much less and see if i can get the 3H mark, then try and wind one in the middle.
Can then test them and see how they sound in this JTM 45 or the Friedman BE-50 that im building.
Have you gapped the core? Did you measure the inductance with DC?
 
I was thinking about inductor design software too .
An iron cored choke calculator would be a great idea .


Inductor calc here is a bit simplistic but it does allow you to add the core perm, all the other variables you know already, might be interesting to plumb in some figures and see how it lines up with the real word coil with and without core .

https://www.allaboutcircuits.com/tools/coil-inductance-calculator/

permeabillity for air is 1



 
Tubetec said:
https://www.allaboutcircuits.com/tools/coil-inductance-calculator/
This calc uses only one permeability, so one has to figure out the equivalent perm when teh core is constituted of different materials, which is the case with gapped cores.The global perm of a butt-stacked core is lower than one that is stacked 1x1. How much is more quesswork than science.
Since the OP has added a gap of uncertain thickness, only adequate measurements will give the final word.
 
Yeah real shame there is very little help out there for winding and calculating inductors.
That calculator is for round core, be nice to have one where you can put in all three dimensions.
I'll await a reply from CJ as he is normally pretty informative on these.
 
Old school Marshall's normally have about 150ma current for the B+, or so I've been made aware, each 12ax7 is 2mA so 3 x 12ax7 = 6ma.
19mA for the output txf centre tap
125mA for the 2 x EL34 output tubes

Have I got that right?
 
Spencerleehorton said:
Old school Marshall's normally have about 150ma current for the B+, or so I've been made aware, each 12ax7 is 2mA so 3 x 12ax7 = 6ma.
Correct. Actually it's a little less, but it doesn't matter.

19mA for the output txf centre tap
? The current going to the output xfmr center tap is the sum of the anode currents of the EL34's.

125mA for the 2 x EL34 output tubes
Actually, if you count the sum of the anodes and G2 currents at idle, it's somewhat less. Typically 40mA par anode and about 10mA per G2; for a total of 60mA.
The total current may reach 150+mA with the amp overdriven, however you don't really need B+ to be that clean at this point.
 
So are you saying that if I use for instance 0.25mm gauge which can take 860ma of current that i should be using a gauge of wire that can take much less?
Would like to know a bit more about what you have said, as allowing for lower current, your saying that the current will only be 150ma.
I'm confused on this!!
 
Spencerleehorton said:
So are you saying that if I use for instance 0.25mm gauge which can take 860ma of current that i should be using a gauge of wire that can take much less?
Not that you should, but that you may.
Now my consideration was that the inductance of the choke (and resulting filtering of ripple) decreases as the current increases. The decreased filtering is gonna be drowned by the audio signal. That means that what makes the amp quiet is the inductance at low current, i.e. at idle.
Now I don't know where the 860mA figure comes from. 0.25mm OD results in 0.049mm², so I would say the max current is less than 0.5A.
But this is not the only factor; what is the resulting DCR? The Joule effect may be the dominant limiting factor.

The max current should be about 150mA in normal operation, but in case of a shorted tube, you must have some kind of safety margin.
 

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