Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: TenderTendon on February 17, 2023, 10:09:26 pm
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I searched the forum and couldn't find any discussion regarding this. I could have been using the wrong search terms though. I was wondering how the DC resistance of an inductor impacts which one we choose to use in our HV power supplies. Is lower resistance always better? For example: I'm working off a schematic that calls for a 10H-200ma choke and the voltages of the PS nodes are not shown. If I had the option to buy one choke with 150 ohms resistance and another with 75 ohms resistance (both 10H-200ma), which would be preferred? I understand that the 75 ohm choke will be larger, heavier and likely more expensive, but if we ignore those attributes and had to choose based on DC resistance alone, is there a clear winner? If it matters, the output section is designed to remain clean, with a tight low end. Thanks...
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I'm basically wondering if the DC resistance of the choke affects the quality of the node (ripple), or does it simply affect the voltage of that node. If the lower resistance choke only results in a higher voltage, I imagine this could be counterproductive and adjustments would have to be made to screen resistors and dropping resistors of following filter nodes?
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chokes effect AC much more than DC, hence their typical use to filter the AC out of DC.
do the ohms law thing to determine voltage drop.
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… For example: I'm working off a schematic that calls for a 10H-200ma choke ...
Is it a secret? :icon_biggrin:
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If you have a vague idea what ripple reduction you need or want and know your current you can than check how big and how expensive your choke would have to bee.
Smaller wire diameter -> less current -> more DC resistance -> smaller choke and vice versa.
More turns -> bigger inductance -> more DC resistance -> larger choke.
If the resulting DC drop is too high, your choke is too small, sizewise and you need more space and money.
Core material might matter to some but if you find a choke and know the size and the DC resistance you can use the hammond choke datasheet to tell the resistance very precisely.
Vice versa 10H 200mA would be the size of a 20W output transformer, approximately, i didn't pull the hammond datasheet. Or four times the size of the typicall Fender 100R 5H choke.
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75R would have to be twice the size, maybe you are happy with 150R?
I am definately oversimplyfing. Anyway, i attached the datasheet.
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Your 200ma requirement implies that this is for a choke-filtered power supply where the entire amp's DC passes through that choke. A schematic would allow forum members to respond with more accuracy and experience.
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My tought is that what really is important is the current that can cross the choke without problems
about Henries, the more the best
so DC resistance on a choke is only a collateral effect, it is mentioned but you didn't look for a choke referring to DC resistance
always you ask for a certain value in Henry and a specified allowed current
Some people are afraid if they see that the project they want to build requires a certain H choke with a certain current allowed
and they have under hand a choke that is correctly rated in allowable current but is greater in H, high H are only a benefit
they reduce the ripple in a better way than the lower H values
so, why bother about DC resistance and H value (if this last is higher than what is required) ?
Franco
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… but you didn't look for a choke referring to DC resistance …
I do, it’s an important characteristic for the HT supply in eg AC30, early JTM45, where too low resistance value will stress the output valves when overdriven, increase the power output / reduce compression, change the dynamic response.
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… For example: I'm working off a schematic that calls for a 10H-200ma choke ...
Is it a secret? :icon_biggrin:
No, not a secret. It just wasn't the focus of this question. The schematic is attached below. I was just interested in the general rule regarding the DC resistance of a choke. I have read several articles that say an ideal choke would have zero resistance. I assume that to mean that the choke with the lowest resistance will always be the best choice if size, weight and cost are not a concern. Is there a point where a lower resistance choke would actually perform worse in the circuit than one with a higher resistance? Is there point where an XXX ohm choke works so good, that pursuing another with a lower resistance is pointless?
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… but you didn't look for a choke referring to DC resistance …
too low resistance value will stress the output valves when overdriven, increase the power output / reduce compression, change the dynamic response.
This is the type of answer I am looking for. I'm trying to understand if there are situations where a choke with a lower resistance would be worse choice than a higher one, or if lower is always better. Again, if size weight and cost are not concern.
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Your 200ma requirement implies that this is for a choke-filtered power supply where the entire amp's DC passes through that choke. A schematic would allow forum members to respond with more accuracy and experience.
No, it is a capacitor input supply. A schematic is attached a couple posts up...
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I do, it’s an important characteristic for the HT supply in eg AC30, early JTM45, where too low resistance value will stress the output valves when overdriven, increase the power output / reduce compression, change the dynamic response.
OK, right, add an in-series resistor and don't worry about the choke
it is a capacitor input supply
Nothing changes, capacitor or choke input, 200mA is surely the required sopportable current because all the amp is feeded via that choke, not only G2 as in some circuits
Franco
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The way I like to do it is to first find out what the MA requirements are for the choke in the amp I am building. Then I purchase several chokes, about 3 of them. Each of these chokes has a different DC resistive value. I play each choke through the amp and choose the one I feel sounds best in the amp. Its amazing how the difference in ohms value for each choke can change the sound and feel of an amp. I know this is very tedious and definitely not required but I love to hear the difference in compression that each of the chokes provides.
Cheers
Guy
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Did you ever tried adding an in-series resistor ?
A low value resistance will add a bit of SAG to your PS and may be this is the feature you're looking for
Or the difference in che choke you buy are also in H and not only in DC resistance ?
Franco
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Nothing changes, capacitor or choke input, 200mA is surely the required sopportable current because all the amp is feeded via that choke, not only G2 as in some circuits
Not true. The output tube plates are not fed by the choke. I seriously doubt a 200mA choke is needed in this case.
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Ciao Steve
I didn't look to the schematic :BangHead: :BangHead: :BangHead:
If the choke didn't feed the Power Tubes also I doubt about the necessity for a 200mA choke
or better .... it is not required, I see very difficult (impossible) a preamp require such a big current
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EDIT: Looking to the schematic I see, no B+1 thru the choke, only preamp and G2 :worthy1: :worthy1: :worthy1:
Franco
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I think the introduction of compression, sag, squish, dynamics, etc. into this subject has complicated things. The schematic has also complicated things, as my questions aren't specifically about the amp I'm building. I'm just trying to wrap my head around what role the resistance of a choke plays on the quality of the DC produced, all other thing being equal. Let's assume this is an audiophile hifi amp and we're looking for the cleanest DC bus, right up to the limit of the valves and transformers. Hell, let's assume it's not an amplifier at all and we're just looking to supply clean DC into a load.
Things I'm curious about:
Will the lower resistance choke provide less ripple, as the following capacitors are recharging quicker?
Will the lower resistance choke simply provide less voltage drop, with no other advantages?
Should the resistance of the choke have an optimum relationship to the amount of capacitance following it?
Is it possible that a choke can have too low of a resistance for a specific capacitance and charge capacitors too fast?
If a 10H/200ma choke with 150 ohms resistance does an adequate job in a filter, would there be ANY advantage or disadvantage to replacing it with a 75 ohm choke? Again, size, weight and cost out of the question. I'm just interested in the quality of the resultant DC voltage provided.
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An ideal choke would have zero dc resistance.
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An ideal choke would have zero dc resistance.
OK, thanks. I am going take that as: An ideal choke would have zero DC resistance, so the closer we can get to ideal, the better.
Moving on... A couple people have commented that the amplifier shown in the schematic would not require a 200ma choke in that position. According to that schematic, what is the maximum current that choke would be required to pass? (estimated, of course)
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If a 10H/200ma choke with 150 ohms resistance does an adequate job in a filter, would there be ANY advantage or disadvantage to replacing it with a 75 ohm choke? Again, size, weight and cost out of the question. I'm just interested in the quality of the resultant DC voltage provided.
If the current draw is constant, eg class A hifi, then there’s no benefit.
If it varies, then the lower resistance choke will provide better regulation, ie voltage stays more constant / less sag.
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Consider 5mA for triode (really usual they can be 1mA per triode, some configuration and tube 10mA)
And add a bit more for safety
Franco
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Consider 5mA for triode (really usual they can be 1mA per triode, some configuration and tube 10mA)
And add a bit more for safety
Franco
What about power tube screens? Don't they draw current? If so, should we calculate at full power?
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Plus the screen supply.
A Marshall Super Lead 100 choke will work. (The extra half dozen 12AX7 triodes won’t add up to much extra current)
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If a 10H/200ma choke with 150 ohms resistance does an adequate job in a filter, would there be ANY advantage or disadvantage to replacing it with a 75 ohm choke? Again, size, weight and cost out of the question. I'm just interested in the quality of the resultant DC voltage provided.
If the current draw is constant, eg class A hifi, then there’s no benefit.
If it varies, then the lower resistance choke will provide better regularity, ie voltage stays more constant / less sag.
Thanks for this excellent answer. That helps me understand a lot! :thumbsup:
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Did you ever tried adding an in-series resistor ?
A low value resistance will add a bit of SAG to your PS and may be this is the feature you're looking for
Or the difference in che choke you buy are also in H and not only in DC resistance ?
Franco
Ciao Franco. Yes the difference in the chokes I buy for each of my builds also have slightly different H values ( usually in the 5H - 12H range. I have tried resistors before but I noticed a choke of equal resistance always sounds a little nicer. I don't post here a lot but you can find me on ampgarage with the same handle.
Cheers
Guy (Gaetano)
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Ciao Gaetano
Things will change a bit also because choke has an effect on maintaining the voltage, they try to give a bit of stabilization, so different values has different performance from this side
Farò caso al tuo Nik e così rintraccero' i tuoi post su ampgarage
Franco
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Ciao Gaetano
Things will change a bit also because choke has an effect on maintaining the voltage, they try to give a bit of stabilization, so different values has different performance from this side
BTW, I was meaning to add an in-series resistor with the choke to increase a poor resistance (if more SAG was expected)
Farò caso al tuo Nik e così rintraccero' i tuoi post su ampgarage
Franco
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Am I throttling my amp?
I used the Hammond organ choke on my AB763 single channel from Hoffman's schematic. I'll post my schematic below. Donor chassis was a Hammond AO-43.
Hammond Mfg. data sheet shows a 20H choke @ 30mA and gives its resistance as 1026 ohms. The Fender replacement choke than Hammond Mfg. suggests is a 5H @ 50mA choke but doesn't give a resistance.
The amp seems to work fine, but I'm wondering if I made a mistake? My choke is supplying 2 6V6s, 2 12AX7s, and 2 12AT7s. It's 20H and I measured resistance at 475 ohms but I don't know how much current it can source, if that's the right term.
Is there a way for me to figure if it's supplying enough current?
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Hammond Mfg. data sheet shows a 20H choke @ 30mA and gives its resistance as 1026 ohms.
Which choke are you talking about? Which circuit?
I used the Hammond organ choke. The amp seems to work fine, but I'm wondering if I made a mistake? Mine is supplying 2 6V6s, 2 12AX7s, and 2 12AT7s. It's 20H and I measured resistance at 475 ohms.
Is there a way for me to figure if it's supplying enough current? Am I throttling my amp?
That 14H choke that was on the AO-43 chassis was not being used in the power supply. It was serving as the plate load for the reverb driver V7. However, it will work very well in a 2x6V6 AB763 power supply as shown in your schematic. I used that same 14H choke in my 6V6Plexi power supply.
The choke does not supply the current. That's the job of the PT.
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Which choke are you talking about? Which circuit?
It's the 194A, 4H @ 50mA, suggested as a DR/Vibrolux replacement.
That 14H choke that was on the AO-43 chassis was not being used in the power supply. It was serving as the plate load for the reverb driver V7.
Yes, thanks for the correction.
However, it will work very well in a 2x6V6 AB763 power supply as shown in your schematic. I used that same 14H choke in my 6V6Plexi power supply.
Yes, that's why I used it in mine and the amp seems to work fine.
The choke does not supply the current. That's the job of the PT.
I must be phrasing it wrong. My thinking was that all of the amp's B+ current has to flow from the PT through the choke, right? So I guessed that the choke's current rating would be a limiting factor. Sounds like I'm wrong in that but I don't understand why.
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My thinking was that all of the amp's B+ current has to flow from the PT through the choke, right?
your schematic has TAP A before the choke, so all the BIG current will "miss" the choke. The inductor will only "see" about a tenth of the PS current
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Shooter,
Yup, that explains it. Quick and clean. It's obvious in hindsight but I'm still new enough at this to miss obvious explanations. Now this post and the whole discussion make a lot more sense to me.
Thanks!
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The DC resistance of the 194A
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TL;DR - Understand what voltage-change causes in other parts of the amp; then you will see the choke DCR is small enough to be "insignificant" in most applications.
... I am going take that as: An ideal choke would have zero DC resistance, so the closer we can get to ideal, the better. ...
"Better" for what? We need to consider what impact will occur for a given resistance.
... It's the 194A, 4H @ 50mA, suggested as a DR/Vibrolux replacement. ...
As Esquirefreak points out, the Hammond 194A choke (https://www.hammfg.com/files/parts/pdf/194A.pdf) that carries Screen + Preamp current has a DC Resistance of 167Ω. What happens when all of the rated 50mA is pulled through the choke?
167Ω x 0.05A = 8.35 volts.
We would then expect our screen voltage to drop 8v due to the choke's DCR. What impact does that 8v drop create?
The graph at the top of age 6 of the 6V6GTA data sheet (https://frank.pocnet.net/sheets/135/6/6V6GTA.pdf) shows (max) plate current for changing screen voltage. Looking at the curves "Ec2 = 250v" and "Ec2 = 150v" at about 75v on the plate, we see plate current falls 50mA for this 100v decrease of screen voltage.
50mA x 8v / 100v = 4mA of plate current reduction.
Now if our Deluxe Reverb delivered 162mA peak plate current through 1650Ω to output 21.7 watts, then reducing that to 158mA delivers: (0.158A x 0.158A x 1650Ω) / 2 = 20.6 watts. That's a barely-audible reduction of output power.
Meanwhile, our Deluxe Reverb (https://el34world.com/charts/Schematics/files/Fender/Fender_deluxe_reverb_ab763.pdf) has series screen resistors of 470Ω, so those alone will cause almost twice as much screen voltage drop (and resulting power output compression). So the choke's DCR is less significant than the screen resistors we employ to protect the tubes.
And the small-ish filter caps will cause the supply voltage to drop when we go from idle to max-power in the Deluxe Reverb. So they will also contribute to sag, and perhaps more than either the choke or the screen resistors (mainly because those screen resistors are intentionally sized to be as small as possible, while still delivering some protective effect).
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Thank you to everyone who chimed in to answer my add-on question about Henries vs. Ohms. I learned more in this one post than I have in an hour's reading of Blencowe. These explanations helped connect the theory I sort of know to the practical applications of electronic theory which is what I really want to learn. Hands-on knowledge like this is exactly why I hang out here.
:bravo1:
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...an hour's reading of Blencowe.
I think I have said this before: Blencowe (all the non-dummy writers) expect you to already grok Voltage, Current, and Ohms Law before class starts.
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I think I have said this before: Blencowe (all the non-dummy writers) expect you to already grok Voltage, Current, and Ohms Law before class starts.
Too true. I started off cold, having a good understanding of basic physics and having build some transistor circuits. But that was it. I've bought probably eight "intro" books and only L. Jurick's really started from zero, and he never really gets into much depth. NEETs isn't bad but not audio-focused. Most books explore a topic and then wrap it up with some variation of "and of course it will be seen that..." Not helpful. Also, I am fairly comfortable with Ohm's Law and DC after three amps. But no comfort level with AC signals at all, especially impedance. I just connect them the way the schematic says, hope for the best. I seem destined to pick up everything piecemeal by just building stuff and asking questions as I go along.
I really envy you guys that had access to a formal education in this stuff.
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I had no real formal teaching.
AC impedance is just resistance that changes with frequency. In most audio we try to NOT have changes with frequency (flat response). So much DC theory copies into AC theory with only slight difference.
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... NEETs isn't bad but not audio-focused. ...
Electronics is not "audio-focused" because it's broader than (just) audio. And what you really need to make sense of audio's interactions is all the basic foundational stuff.
I really envy you guys that had access to a formal education in this stuff.
My "formal education" consisted of 6 months of the Navy teaching me "not-audio electronics" because your career might take you in any direction. I found NEETS to be invaluable in presenting the basic-basics in an easily readable format.
The rest of my formal education was about 2 decades of being corrected by PRR, with some occasional book-reading on the side.
It's almost embarrassing how I didn't understand some concepts for about 10 years because I was applying too much magic to them; then audio/guitar stuff is dead-simple most of the time. The "magic" comes in the interaction of many simple things.
If you were to attend the Most-Perfect Class on Tube Guitar Amps, you would be bored the first 3 or 4 semesters being taught all the foundational stuff you need to know inside-out to catch all the implications in the classes on the fun audio stuff.
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If you were to attend the Most-Perfect Class on Tube Guitar Amps, you would be bored the first 3 or 4 semesters
:laugh:
probably the #1 reason I found Military schools beats college, the military schools you had to beat out 1/2 the class in order to move on to the next class. boring or not, you want Tech-school, you did ALL the home-work, twice, then one more time just to be sure. :icon_biggrin:
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Thanks to all,
I'll give NEETS another look. After two years' it might click a little bit more that it did at first.
Actuarially speaking, not likely I'll have another ten years to build amps so maybe I'm in a bit of a hurry. It's good to know that lacking a formal education in electronics is more common here than I thought. I guess I'm on the right track, building more amps and asking more questions. I am grateful to have a community like this to learn from.
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I am grateful to have a community like this to learn from.
I am too.
Just to get back to the OP's topic, Sluckey mentioned above that he did not think a 200mA rated choke would be necessary in this (Solo 100) circuit. The way I have always though about this is to add up the max screen current for the output tubes, add in a some small number of mA for the preamp tubes, then increase by 5-10% or so for margin of safety, and call it a day. Is this approach reasonable?
So, by the above method, screen current is 140mA max, add in 10mA for preamp, add in another 15-20mA for safety, and you get a conservative requirement of 165mA. Problem is, of course, that within the easily available Hammond line of chokes, you can get a 10H choke at 150mA, or one at 200mA. These are not small -- the 150mA one weighs in at 4.25lb, the 200mA at 5.5lb.
If the amp were for me -- I'm not out on the road gigging much -- I'd make the choice of the smaller choke. If this were for a stage amp -- and of course this is exactly what the Solo 100 is (given 6L6GCx4) -- you pick the larger rated choke.