Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: 1blueheron on January 30, 2022, 10:11:14 am
-
I am having a dilemma when it comes to purchasing a choke for power filtering. It was recommended I buy a replacement version of the Fender 125C1A. Different sellers seem to list different specs for their version of a 125C1A.
For instance, Hammond lists model 194B as their substitute model. It is 4H @ 90mA It does not list Ohms on spec sheet.
Mojotone lists MOJO777 as their version. It is 3.5H @100mA & 100ohms.
Is this just a +/- tolerance issue or is one going to be superior to the other?
I have a lot of inductors, is there a non-destructive way to tell if one that I have would be suitable for my application?
-
On Hammond products, you have to go to the specification sheet specific to the product. Hammond indicates DC resistance of 108 ohms.
https://www.hammfg.com/files/parts/pdf/194B.pdf (https://www.hammfg.com/files/parts/pdf/194B.pdf)
-
> is one going to be superior to the other?
No so you would notice.
-
My understanding is that a choke痴 measured inductance will tend to be significantly affected by the degree of DC being passed whilst the measurement is taking place.
Bear in mind that as DC increases much above the rated max, the magnetic circuit may start to saturate, which will cause the inductance to collapse (and lose the filtering effect).
So with you unknown chokes, you might want to try to watch out for that.
Maybe the Hammond would measure 3.5H at 100mA? :icon_biggrin:
-
I知 actually pretty curious about this myself.
4hy seems to be the standard for fender. 100 ohms.
What happens if I use a 3hy at 100ohms?
-
My sim is probably wrong, but I got just over 5VDC difference:
-
There seems a lot of PT resistance (162ohms)?
-
My sim is probably wrong, but I got just over 5VDC difference:
That痴 a cool sim.
Not to hijack the thread but hopefully the answer to this question will help us both understand the function of the choke and the reasoning behind its value.
I get that it smooths the dc for the screens.
My related question is, I知 building another 6G16 vibroverb. It requires the typical fender chokes listed above, somewhere in that range.
I plan on using EL34s in the output, and I have a triad C-12x on hand
6 Henries
160 mA
165 ohms
Any issues using this for the vibroverb with EL34s ?
If no issue, what if I plug in 6l6s ?
Still no issue?
-
Not to hijack the thread but hopefully the answer to this question will help us both understand the function of the choke and the reasoning behind its value.
I get that it smooths the dc for the screens.
My related question is, I知 building another 6G16 vibroverb. It requires the typical fender chokes listed above, somewhere in that range.
I plan on using EL34s in the output, and I have a triad C-12x on hand
6 Henries
160 mA
165 ohms
Any issues using this for the vibroverb with EL34s ?
If no issue, what if I plug in 6l6s ?
Still no issue?
my understanding of a choke is that it filters out ac ripple to an extent that a much higher value resistor would with much lower dc resistance measurement, so that you are not dropping too much voltage between plate and screen nodes
now for specifics: will that choke work for your amp? yes most likely.
how do i confirm in such instances?
quick and dirty way: compare to parts in similarly-powered amps (what comes to mind is 50w marshall chokes 5h 125ma)
you can also hit your tube datasheets and confirm that choke rating is greater than maximum power tube screen current along with 'a few' extra ma for the little bottles
-
you can also hit your tube datasheets and confirm that choke rating is greater than maximum power tube screen current along with 'a few' extra ma for the little bottles
Along those lines, this page helped me to get a better grasp on adding up current requirements along the chain, especially his diagram at the bottom of the page. He indicates 19mA for two EL34 screens, and 1mA per individual triode:
http://www.valvewizard.co.uk/smoothing.html
-
Along those lines, this page helped me to get a better grasp on adding up current requirements along the chain, especially his diagram at the bottom of the page. He indicates 19mA for two EL34 screens, and 1mA per individual triode:
http://www.valvewizard.co.uk/smoothing.html
this is also a good resource, although also includes plate current in the result: https://thesubjectmatter.com/calcptcurrent.html
studying a lot of math/physics in college, my philosophy is to learn the theory behind stuff and then find good calculators to use when actually needing the information (parallel resistor calculator and rc filter calculator get a ton of work when i'm sketching schematics/building/troubleshooting)
-
Heck yeah thanks guys, yeah I always wondered the best way to check screen current, I値l dig into that.
So the last part of the puzzle is why 4-5 henries?
Why not 2 henries or ten henries?
I just wonder how they go about calculating the necessary inductance of the choke.
Anyhow, 6 henries it is! This build is coming along rather nicely, got the bored all soldered up this evening.
With any luck, it値l be done next weekend.
Thanks friends.
-
So the last part of the puzzle is why 4-5 henries?
Why not 2 henries or ten henries?
I just wonder how they go about calculating the necessary inductance of the choke.
quick and dirty answer: copy someone else's work that is known to work
this link oughta shed some more light: https://www.aikenamps.com/index.php/chokes-explained
-
So the last part of the puzzle is why 4-5 henries?
Why not 2 henries or ten henries?
Quick Answer: 2 Henries might not filter as much as desired, 10 Henries would be more size/weight/cost (and be spending money past the point of diminishing returns).
6G16 Vibroverb has the 4H choke then a 16ｵF filter cap.
XC = 1 / (2xπxfxC) = 1 / (2xπx 120Hz x 0.000016 Farads) = ~83Ω
XL = 2 x π x f x L = 2 x π x 120Hz x 4H = ~3016Ω
Ripple Reduction: XC / (XC + XL) = 83Ω / (83Ω + 3016Ω) = 1 / 37.34
The sim example here (https://el34world.com/Forum/index.php?topic=28486.msg313715#msg313715) shows 16v of ripple applied to the choke & screen filter: 16v / 37.34 = 428mV ripple (tiny for a screen)
A 2H choke would have a reactance of 1508Ω, and result in a ripple of 835mV (small, but double that of the 4H choke).
A 10H choke would have a reactance of 7540Ω, and result in a ripple of 174mV. That's nearly 2.5 times smaller ripple, just as the choke has 2.5x more henries. But 400mV is already "small enough" and the 10H choke carries a space/weight/cost penalty.
Sometimes you will hear, "We don't make anything just 'good enough' we over-build!"
Remember this often also means the speaker under-engineered their product, wasting money on over-spec parts that no longer deliver a perceptible performance enhancement. The clever engineer knows where to reduce expense without reducing quality or performance.
-
> I got just over 5VDC difference:
You changed the current from 90 to 100. Are you really going to change the circuit 10%? Weren't you going to use one or the other in the same amp?
-
OK, I think between the feedback here and some googling, my initial question has been answered. Either would work fine and they are comparable.
It was recommended in a previous thread that I use the 125C1A in the little single ended 6AQ5 amp I am building. With a single 6AQ5, 2-12AX7/7025 and a 5879 if I will be pulling far less than either 90ma, or 100ma. If I understand correctly, inductance rises as current falls so a inductor rated at 4H at 90ma will provide more than 4H at the current I will be drawing.
Is this correct?
If I were to use a 125C3A (4H 50mA) rather than the 125C1A, I would likely still be well within the rated current draw but would not get the advantage of the additional inductance and might get slightly more hum as well as lose a bit of voltage due to slightly increased resistance?
-
If I were to use a 125C3A (4H 50mA) rather than the 125C1A, I would likely still be well within the rated current draw but would not get the advantage of the additional inductance and might get slightly more hum as well as lose a bit of voltage due to slightly increased resistance?
This depends on your schematic. If the 6AQ5 plate current will flow through the choke (as in my Lil Smoky (http://sluckeyamps.com/smoky/smoky.pdf)), then use the bigger 125C1A choke.
If the 6AQ5 plate current does ***NOT*** flow through the choke, then use the small 125C3A choke.
And if you don't know, then use the bigger 125C1A choke.
Don't make it any more complicated than that. :icon_biggrin:
-
It was recommended in a previous thread that I use the 125C1A in the little single ended 6AQ5 amp I am building.
a pi filter (CLC) before the plate node of a single-ended amp is one of the best mods you can do wrt noise (doubly so if you will be using a wider range speaker than the champs came with). i did this with my big boy vibrochamp (6l6g power tube) and it sounds great, only noise really is some hiss from organ salvaged carbon comp resistors i used for 'more mojo'
-
Got it. Thanks to all.
-
> I got just over 5VDC difference:
You changed the current from 90 to 100. Are you really going to change the circuit 10%? Weren't you going to use one or the other in the same amp?
I knew I goofed somewhere. :icon_biggrin: With the same 100mA load they're within less than a volt. The Min/Max differences are about 240mV to 260mV with the 4h Hammond having the slight edge.
-
> within less than a volt.
0.8V exactly. 8 ohms difference at 100mA makes 800mV. There's no significant AC here.
Why is the "MEAN" column not showing? That and DIFF tell most of what you want to know.
-
Thanks PRR.
I hope I'm not derailing this thread. At least these simulations suggest that there's probably no noticeable difference between the two.
-
No worries about the derail. I'm always cool with more science :icon_biggrin: My primary question was answered to the point that I could get a choke ordered and the project back on track. Also saved me a few bucks.
Would still love to know a non-destructive means of determining inductance on a coil. I realize there are a lot of variables such as core type etc. so it might not be a cut and dry math formula that can be derived with a known impedance and current.
I have boxes of them so it grieves me to buy more wondering if I already have the same thing or better.
-
here's one method, it's a little early to get through the fog, so reader be ware
A Simple Method to Measure Unknown Inductors (dos4ever.com) (https://www.dos4ever.com/inductor/inductor.html)
-
OK, so I have the instructions, now if I only had a function generator and an O-scope.... So much for my money saving proposition...
-
That's over-complicated and not real suitable for power filter chokes.
Feed 60Hz 3.15V through choke and 1.5k resistor in series. If voltage across the resistor is 2.22VAC then choke is 4H. Can you see how this works? Can you find the 3dB resistor for other choke values?
-
Since I'm reading this thread, I have to ask...I assume it matters what size the capacitors are with a specific inductance in a C-L-C power supply filter?
-
this won't get you a good grade but;
the C-L-C is primarily tasked with making the rectified DC, filtered DC, making it constant DC vs wobbly DC.
the math is frequency dependent, or filters better or worse, depending on frequency. (the bigger the cap, the lower the filtering frequency)
time is also a component, it takes time to charge large value caps.
-
...I assume it matters what size the capacitors are with a specific inductance in a C-L-C power supply filter?
https://worldradiohistory.com/UK/Wireless-World/40s/Wireless-World-1949-11.pdf
-
if you start on page 418, by the time you work through the math and finish up on page 420, well, you should be MORE than ready :icon_biggrin:
-
...I assume it matters what size the capacitors are with a specific inductance in a C-L-C power supply filter?
Ripple is multiple frequencies. If you try to "tune out" one frequency, others are enhanced.
The usual practice in audio is just to make all filter parts "large".
There is a commercial optimum depending on relative costs of chokes and caps. In the 1930s large caps were expensive so we had large chokes. By 1950 caps were so cheap that chokes withered and vanished.