EEVblog Electronics Community Forum
Electronics => RF, Microwave, Ham Radio => Topic started by: coppercone2 on August 24, 2022, 01:33:43 am
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I am curious about this. The 461A amplifier I spared for switch addition has this method, a wire of maybe 3 turns wound around a carbon resistor.
What is the point of this and how is this designed? Would a modern design still use it, or does it have something to do with lack of proper inductor materials? Does this have anything to do with the resistor material acting differently in the magnetic field, or would it work equally well if it was in parallel with the resistor?
Not much information about this, other then its a form of de-tuning.
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On the plate supply?
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Like this for example?
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It forms a damped (low Q) L.
Maybe as shown by Joe, it
was commonly used in anode of triodes to damp parasitics of higher frequency than the wanted frequency, caused by lead stray inductance resonating with inter-electrode capacitance. Being connected coaxially inside the inductor, the resistor is more effective at damping compared to a separate resistor which would have its own lead inductance.
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Not sure if this helps as I have no idea what you are looking at but this is from ARRL handbook.
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well this is a VHF transistor LNA but it looks very similar. I don't see which one is in the manual schematic so I would have to get the box out of storage to look to see which circuit it is on, but IIRC it might not have been listed in the schematic because when I was checking it I thought I found a shorted resistor (since at the time I knew the schematic value) until I noticed the coil around it. I wonder why they would put it in a solid state amp. I guess I have to get the box open again and take a look so I can identify which resistor they put it over (that box is a 5 stage stagger tuned VHF amp that uses differently tuned LC coils from 1KHz - 160MHz with a max output of ~7dbm)
But anyway, there is no interaction between the magnetic material and the carbon composition material, its just there for ideal placement reasons?
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This has been discussed in this forum before. Resisters were often cheaper than forms for coils so some manufacturers simply used a high value resistor as the body for winding a coil onto. The give away is that it was usually used with in DC, or low frequency AC, signal paths and the impedance of the resistor is MUCH greater than the coil impedance so the resistance had no significant effect on the operation of the circuit.
A true story: When I was in college one of my profs brought in a wing tip light out of his Cessna aircraft that wasn't working. The light had a coil and resistor combo like that and no one could understand why the light wasn't working since there was continuity through the resistor/coil. But I measured it and the resistance through the combo was about 10,000 Ohms IIRC so needless to say the lamp simply wasn't getting enough current to operate. I checked the coil closely and found a burnt spot in the wire. I carefully unwound the coil until I reached the burnt spot and I then soldered the ends back together again and wound the wire back onto the coil. I checked the DC resistance and it was only about 1 Ohm. We reconnected everything and checked it and it worked like a champ. He reinstalled it in his aircraft and never had any further trouble with it. I should add that the light assembly also had a small DC motor in it that rotated a reflector around the lamp so that it made the light appear to blink even though the lamp was actually on all of the time. The coil was in series with the lamp and motor to help block any AC noise from feeding back into the aircraft electrical system.
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thats a little weird that they would allow so much confusion to occur to save very very small amounts of money on low production equipment (in general it seems that it is confusing to anyone trouble shooting the circuit) and that for some reason I think a composition resistor makes for a worse bobbin for the coil, since I think it can short out, or are these resistors short circuit proof?
https://groups.google.com/g/sci.electronics.basics/c/PPL_AXC6ydU?pli=1
it says they can bake themselves into a low resistance
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thats a little weird that they would allow so much confusion to occur to save very very small amounts of money on low production equipment (in general it seems that it is confusing to anyone trouble shooting the circuit) and that for some reason I think a composition resistor makes for a worse bobbin for the coil, since I think it can short out, or are these resistors short circuit proof?
https://groups.google.com/g/sci.electronics.basics/c/PPL_AXC6ydU?pli=1
it says they can bake themselves into a low resistance
Over a long time, carbon composition resistors tend to increase in value, due to heating & other causes.
If grossly overloaded, any resistor can go short circuit, but going low isn't the default just from having a "hard life".
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still compared to no parts being there its a bit... improper.. surprised it got popular
its like one of those... hey you know we have some many of these lying around and the shape is kinda similar...
Ok I feel like the problem is that the thing you replace it with should be non-electrical whatever it is. Something really not in the circuit, not dependant on reasonable parameters (i.e. not what happens frequencies greatly outside of bandwidth. Anything else seems bootleg when the part is not supposed to be part of the circuit. It almost seems that ANY resistor (or component) used there, even for creative cheapness, should not be applicable to circuit diagrams without crazy stuff like electrometers and material parameter analyzers. Like a lego brick would make more sense to me then a resistor (but assuming it was a high temperature lego brick), with the point being that a lego brick intuitively has nothing to do with electronic circuits.
I was really hoping that they would have a partial electrical reason. Not that what you said is what is happening in that schematic (it may be a intentionally damped inductor like Joe Q said), but I just really don't like it for some reason. If it was in mega ohms it would start to look like some generically applicable widget I guess, at least in my mind, but when its like Kohms that seems dodgy catagorically (it starts to remind me of a damp piece of wood being used in a circuit for isolation reasons (shitty breadboard))
Like as a building material its like saying ima put up a interior wall using old stereos ..... or like someone just stuffing old couch cushions and mattresses in the wall and covering it with sheet rock as insulation (like actually doing it and selling it to someone as home contracting not just some temp survive in wood cabin for a week before rescue). I mean I guess those things seem like they might work but damnnn .. like if you tried to ratify that in some kind of code it would sound like some survival manual psycho shit like the page about destroying equipment on ww2 navy gear (pickaxes are ok and dynamite is good too!)
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It forms a damped (low Q) L.
Maybe as shown by Joe, it
was commonly used in anode of triodes to damp parasitics of higher frequency than the wanted frequency, caused by lead stray inductance resonating with inter-electrode capacitance. Being connected coaxially inside the inductor, the resistor is more effective at damping compared to a separate resistor which would have its own lead inductance.
This is the correct answer.
The resistor is a medium value and dissipates or dampens any potential VHF parasitic oscillations. At the much lower operating frequency the inductor has a low reactance and effectively shunts the resistor.
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It forms a damped (low Q) L.
Maybe as shown by Joe, it
was commonly used in anode of triodes to damp parasitics of higher frequency than the wanted frequency, caused by lead stray inductance resonating with inter-electrode capacitance. Being connected coaxially inside the inductor, the resistor is more effective at damping compared to a separate resistor which would have its own lead inductance.
This is the correct answer.
The resistor is a medium value and dissipates or dampens any potential VHF parasitic oscillations. At the much lower operating frequency the inductor has a low reactance and effectively shunts the resistor.
and, as shown in a previous post, it was common in a number of homemade (but commercial too) HF tube amplifiers, I've used it several times with 6kd6, 813, pl519... based amps
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A few oldies...
[attach=1]
The two on the left and the one on the bottom were recovered by tube b&w TV sets. The two on the top right (sorry one is damaged ;) ) were commercial units for hobbists/repair, bought in the late seventies. One of them is covered with vax, that was quite common.
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I am curious about this. The 461A amplifier I spared for switch addition has this method, a wire of maybe 3 turns wound around a carbon resistor.
What is the point of this and how is this designed? Would a modern design still use it, or does it have something to do with lack of proper inductor materials? Does this have anything to do with the resistor material acting differently in the magnetic field, or would it work equally well if it was in parallel with the resistor?
Not much information about this, other then its a form of de-tuning.
It looks to be a simple parallel RL network that is used to define the amount of negative feedback vs frequency in each amplifier stage .
At low frequency the inductor has very low reactance so the resistor is effectively shorted. This means that the other 180R resistor in series with the RL network provides quite heavy negative feedback. This will tend to reduce the excess gain of the stage at lower frequencies.
At higher frequencies the parallel RL network will look like a resistance in series with some inductive reactance and this will tend to reduce the negative feedback. So this maintains gain at higher frequencies and the aim would be to get equal gain across LF through 150MHz.
My guess is that you have to tweak each inductor to get a flat frequency response in the 5 stage amplifier. Each one of these RL networks will be optimised for a particular part of the LF through 150MHz bandwidth of the amplifier. So you would have to know which one to tweak to flatten a particular part of the frequency range. It should be possible to get a really flat response.
Without this tailored negative feedback the amplifier would have lots more gain at low frequencies compared to the gain up at VHF.
I suspect this 'old school' coil over resistor construction method was used out of convenience and it also saves space. It would be unsuitable for today's fully automated assembly systems using SMD though.
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if you mill a slot under the part it might be possible to add a coil to a SMD part
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+1 for damped inductor.
I have also seen this in series with audio amplifier outputs, where it allow you to control the impedance seen by the amplifier in the ultrasonic range. This is done because loudspeaker/cable combinations have an impedance in this range that only your favorite deity knows and you don't want your amp oscillating at 100 kHz or 1 MHz or some such frequency.
John
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Maybe I'm the only one who is looking at the actual HP 461A circuit so I've attached it below if it helps focus on the real reason the parallel RL network is there.
Each amplifier stage uses resistive negative feedback with an emitter resistor and there is also resistive feedback between collector and base using the RL network in series with a 180R resistor.
Both forms of feedback help with stabilising the gain and input impedance over a wide bandwidth. The reason the inductor is there is to help flatten the frequency response across 1kHz to 150MHz as I described in my previous post. The inductor will help flatten the gain at lower frequencies and it may also help to slightly boost gain up at the higher frequencies. Each coil in each parallel RL network can be adjusted to help to achieve a flat frequency response across 1kHz to 150MHz.
This application of the parallel RL network in the HP 461A isn't quite the same as the other suggestions about fitting the parallel RL network as a parasitic suppressor between the output of the amplifier and any external load.
In the case of the HP 461A amplifier this RL network is part of the negative feedback and the primary reason it is there is to help flatten the frequency response of the overall 5 stage amplifier.
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Maybe I'm the only one who is looking at the actual HP 461A circuit so I've attached it below if it helps focus on the real reason the parallel RL network is there.
Each amplifier stage uses resistive negative feedback with an emitter resistor and there is also resistive feedback between collector and base using the RL network in series with a 180R resistor.
Apparently you are correct! I'm curious though, those inductors are shown as variable. How can that be if they are wound on resistor bodies? Do they just squeeze the windings together as needed during calibration?
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I'm not sure how they are adjusted. I would expect HP to configure the coil separately so it could be that this isn't the RL network in question but I couldn't see another one in the service manual. Maybe an extra RL network was added to this amplifier?
It does seem strange that the final amplifier stage output feeds direct from an emitter follower with no series resistance. Maybe an extra RL network got added here and it isn't on the circuit diagram?
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No it’s not the stage inductors it’s only labeled as a resistor on the schematic. I can take a look today since this seems popular to find the schematic value. There is only one of these iirc and it’s not the adjustable ones. I think the final stage is what I’m talking about. I only saw one of those combo resistors. The adjustable ones are covered in wax like tuned rod inductors
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OK thanks. I've seen an RL network like this used as a parasitic suppressor as others have already suggested, or as RL networks in power supply feeds (to minimise voltage drop at DC) or as part of an RF feedback network as I described earlier.
I'd expect to see some deliberately added ESR at the amplifier output because it uses a direct connection to an emitter follower as the output. If a fairly fast BJT was used here I'd expect it to produce some negative resistance up towards UHF. Slower parts are going to be OK (in terms of stability) without any added ESR. However, you did say the part was already on the schematic marked as a resistor so I guess it can't be fitted here unless there is a revised schematic available that shows a series resistor at the amplifier output connector?
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Agree with fourfathom,
Its the original high impedance grandfather of the modern low impedance ferrite bead.
Its a lossy series impedance added to damp out nasty ringing and resonances, even oscillation !
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Another ancient technique using these curios components you don't see today, were shunt peaking coils in wideband amplifier applications such as video amplifiers and oscilloscope vertical amplifiers..
Back in the tube era, to lift up a sagging high frequency response, it was common to place an inductor in series with the anode load resistor.
The rising load impedance lifted the high frequency gain. But sometimes the gain might rise so high up near a resonance, instability might result.
The small inductor then may have a damping resistor placed in parallel to tame the beast. So you might see a few turns wound on a resistor body. Cheap and effective.
Sometimes this component was selected on test during manufacture, and the schematic might show an adjustable inductor, although in practice the part was selected by trial and error, not tweaked.
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I think its the L1 near the 50 ohm input resistor. It has a HP part number i can't read. I originally must have read the schematic backwards (the board is weird because the cable goes to the back rather then have the input be right at the front, IMO the input should be in the front and the output should snake back to the front since it already has the amplification noise). So HP does not provide a resistor value, I thought it was a high value resistor near the other side of the PCB. But it does look like a coil over a resistor, but its only about 3 turns. It's varnished very heavily.
god this scan must have been fed through the machine 1000 times
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It is used as R+L in parallel circuit. But sometimes high resistance resistors can be used just as a coil frame
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If the inductor is L1 then this is shown in the fuzzy schematic below. The input BJT is an emitter follower (highish input impedance) so some shunt resistance is used to set the input impedance to be approximately 50 ohms. If you work out the equivalent resistance of all the input resistors it will be close to 50 ohms. i.e. there's 1800R, 59R and (499+55R) at the input.
However, up at VHF the emitter follower will have an input capacitance of a few pF and this will spoil the input match up at VHF. I think L1 is there in series with the 59 ohm resistor to compensate for the input capacitance of the BJT and therefore L1 will probably be adjusted to minimise the input VSWR up at VHF.
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I've encountered them in Tektronix 7A26 plugins. They are used as RLC filters in DC supply lines. There are 4 shown in the schematic: 3.2uH on a 10 ohm carbon composition resistor.
The reason I looked into this is that the tantalum to ground fails short. The coil then cooks the 10 ohm resistor releasing an amazing amount of smoke. Had that happen on more than one 7A26. I bought a bunch of 10 ohm carbon composition resistors and a spool of fine wire and rewound my own. After replacing the failed tantalums, as well as all other tantalums that hadn't failed yet. The voltage rating of the originals was rather low. I think I used 35 volt replacements.
I took the closeup photo so that I could count the turns on an intact coil. 47 for those playing along at home.
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I have a impedance analyzer now, maybe I will desolder it and measure it for fun to determine the L1 values (that parts list is just too far gone). I just need a fixture. The best I could do right now is put it in a BNC to female banana adapter (screw it under binding posts). Not sure how high up that will go, but I need to fix one of the bands in the impedance analyzer (60-110MHz is broken, waiting on PCB). I measure a mica capacitor OK using the banana jack adapter). I don't think its the worst fixture in the world.. but it should make working on this amp alot more fun in the future, because I see specifically what I am replacing the resistors with (i.e. compare parasitics of new parts to old drifted stuff that is out of spec)
with modern equipment I find usually I can fit a much higher voltage rating capacitor in the same spot, even with tantalums, and still have a size reduction.
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Be careful if you change the 60uF caps (eg C14 C16) as these are probably solid tant caps and I would expect these caps to have low ESR and low package inductance and they will perform well over a huge RF bandwidth. A modern solid tant might be even better but you would need to prove this before swapping any caps in the RF amp section.
The input emitter follower will have a high input shunt resistance and probably has a few pF input (shunt) capacitance and this Cp will probably be quite constant over a huge bandwidth. So to cancel this (5pF?) capacitance and to obtain a good 50 ohm input match to the amplifier you would ideally need to fit a 50 ohm shunt resistor in parallel with a (negative) shunt capacitor of -5pF across the input of the amplifier. One way to achieve both objectives is to put a small inductor (L1) in series with a resistor of just over 50 ohms (R9 59 ohms).
The parallel equivalent of this network is going to be very similar to a -5pF cap in parallel with a resistance of about 50 ohms.
For example, 18nH (L1) in series with 59 ohms (R9) looks similar to -5pF in parallel with a 60 ohms resistor and this holds true across LF and up into VHF.
60 ohms is a bit higher than 50 ohms but note there is also an 1800 ohm resistor in parallel (R10) and there is also (499+55 = 554 ohms) in parallel.
60 || 1800 || 554 = 52.6 ohms. This is very close to 50 ohms and there will also be some parallel resistance from the BJT. This will have been designed by HP to get really close to 50 ohms input impedance.
Therefore, the value of L1 will be chosen to deliver the correct negative shunt capacitance that offsets the input shunt capacitance of the emitter follower. Note that these capacitances are shunt capacitances Cp in parallel with shunt resistance Rp and not series capacitance Cs in series with resistance Rs.
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I think those tantalums should be fine no? I thought they basically have no wear/age mechanism and they fail because power supply designers did not limit them well, or carbon comps that limit them degrade to low impedance that do not limit them enough?
Like if C2 was tantalum that might be a concern, but C14? The current levels in this thing seem really low, it does not use a SMPSU.
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Yes, I'd hope they would be fine if they are solid tants. If they are failing it will be fairly obvious because the response of the amplifier will change.
Do you really want to mess with L1? I'd be tempted to leave it in place and just play with a 56 ohm resistor in series with (say) a 16nH inductor and (for a learning exercise) see if you can prove that this produces -5pF in parallel with about 56 ohms on your jig/analyser. If it doesn't then maybe your jig could need to be optimised in some way.
I'm pretty sure the amp has been designed this way in order to compensate/offset the input capacitance of the emitter follower and to obtain a good input match for a 50 ohms source. My guess is that the engineer at HP probably experimented with several NPN and PNP emitter follower transistor types before settling on the one used in the final design. The one that is easiest to compensate would probably be the one that was chosen for the HP 461A amplifier.
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you think it might break from desoldering or something? I guess messing with something that varnished is a concern. The problem I have is that I need to make something like a lever detent switch to press the button on the attenuator if I ever intend to get this amplifier working again, which is a shop project more then anything
(I don't like the mechanism of the attenuator trigger, not one bit, i find it insulting), but yeah I guess I can try to make that circuit myself now that I can measure stuff in VHF with the kind of numbers you are giving me since its just curiosity.
Those long resistors like to crack BTW, there was a cracked one in the attenuator, I guess heating those parts up for no reason is a bad idea..
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I've never seen inside the HP461A so I don't really know how fragile these parts are. However, see below for a simulation to show that 56 ohms in series with just under 16nH is quite similar to 57 ohms in parallel with -5pF. The -5pF capacitor turns pink in the simulator if it is a negative capacitor.
You can see why HP would have adopted this compensation approach using a 59 ohm resistor in series with a small inductance. Of course I don't know if the input capacitance of the emitter follower is 5pF. It could be 3pF or it could be 7pF or something different. However, there will be a series RL network that can compensate it reasonably well.
The value of 15.6nH in the simulation below is quite critical. If I change it to 18.6nH it changes the shunt reactance by 20%. I guess that's another reason to leave L1 well alone for now at least.
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yes, when I tried to make a dead bug oscillator (low UHF) I noticed that those little coils (which I tried to wind around drill bit blanks as a former, and no it did not work) were very sensitive (some low values in the nH range were needed by the schematic, if it is infact a coil that is soldered over a dummy resistor, then I can see how the solder joint connecting the resistor getting desoldered and repositioning the coil could be a problem
https://www.analog.com/en/analog-dialogue/articles/does-the-assembly-orientation-of-an-smps-inductor-affect-emissions.html (https://www.analog.com/en/analog-dialogue/articles/does-the-assembly-orientation-of-an-smps-inductor-affect-emissions.html)
better not mess with it
https://article.murata.com/en-global/article/basic-facts-about-inductors-lesson-6 (https://article.murata.com/en-global/article/basic-facts-about-inductors-lesson-6)
The analysis you did of the circuit to show the sensitivity to component values is a very interesting technique, I guess it should be obvious that its a 20% change, but when units are that small its easy for that to get over looked, I definitely remember like 2-3 nH shifts occurring from basically nothing at all with those tiny coils I wound. Given how much care it looks like HP went to in order to stabilize the input impedance its not worth messing with it
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Yes, I agree it's probably best to leave L1 alone for now. In case anyone finds this stuff interesting I've rescaled the plot below to show capacitance and it shows how 56R in series with 15.7nH can synthesise a (negative) -5pF shunt capacitance that would both cancel the 5pF of an emitter follower and it would also terminate it quite well with a shunt Rp of about 50 ohms.
I think what you see in the final HP461A circuit is the result of the work of a keen engineer who probably spent quite a bit of time swapping out different emitter follower transistor types and would also have found ways to arrange the compensation network to try and squeeze out the best match to 50 ohms across 1kHz through 150MHz. I suspect it could have been quite rewarding to improve the match because this improves whatever input impedance graph is shown in the specifications for this amplifier. A feather in the cap of the engineer!
So the final solution for R9 and L1 could be wound or arranged in a special way to get the best possible match up at VHF. This might make it a bit strange to look at. I can't comment much more because I've not seen R9 and L1 myself.
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thinking about my previous UHF circuit attempt, I think that the 10-20 nF inductor would fit that size on the circuit ( i played with the coil designer program alot) for that thickness. I tried to make mine thinner but at that width I bet its exactly that. I wanted to go smaller diameter then what HP has in there for my prototype and I think that I ended up with a turns count I did not like manufacturing.
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The resistor body is a convenient and inexpensive coil form for low values of inductance, and the resistance can be lowered to apply a controlled amount of dampening.
Tektronix did this in the 1960s, 1970s, and 1980s for CLC decoupling.
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Are these resistors always carbon composition? Once you think about another resistor kind, it seems like you have a coil over a coil and that seems weird. I wanted to do some experiments but the first thing I realized is that all my resistors are some sort of coil structure. That is starting to remind me of a transformer
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Are these resistors always carbon composition? Once you think about another resistor kind, it seems like you have a coil over a coil and that seems weird. I wanted to do some experiments but the first thing I realized is that all my resistors are some sort of coil structure. That is starting to remind me of a transformer
All or almost all of the examples that I have seen use a carbon composition resistor, but the self inductance of a spiral cut film resistor would be insignificant compared to the inductor.
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In most applications of a helix-cut film resistor, the high resistance of the resistive structure should dominate over the inductive reactance. In fact, I often see that the internal capacitance (turn-to-turn of the helix) is more important than the inductance with axial-lead resistors. In old-style carbon composition resistors with high ohmic value, the internal capacitance through the talc grains used to increase the bulk resistivity causes the measured real part (resistance) of the impedance to fall at high frequencies (talc dielectric shorting out carbon grains).
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you know it might also be interesting to wind some inductors over different tubes and see what happens when they are concentric and also physically parallel. I might as well
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The practical aspect is that it is much easier to wind a coil over the perfect cylinder formed by the molded case over a carbon composition element than the "dog bone" shape of a ceramic rod with end caps.
In the example shown below, the 4 big ones are 3.2 microhenries and 10 ohms and feed 10 microfarad solid tantalum capacitors. Tektronix wound them themselves.
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Has anyone seen a inductor wound over a capacitor?
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I have seen coils wound around piston trimmer capacitors to make a tunable circuit.
Note that the coil won't be happy if a closed metal cylinder is inside it, which looks like a shorted turn.
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Has anyone seen a inductor wound over a capacitor?
I've seen "traps" (parallel L/C tuned circuits used to isolate multi-band antenna sections) made from a coil of coax, shorted at one end and the center-conductor open at the other, wound on a plastic pipe form. It's physically short enough that the distributed transmission-line effects are minimal; the L is the coiled coax shield, and the C is the internal center-conductor-to-shield capacitance.
Does that count?
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Has anyone seen a inductor wound over a capacitor?
I've seen "traps" (parallel L/C tuned circuits used to isolate multi-band antenna sections) made from a coil of coax, shorted at one end and the center-conductor open at the other, wound on a plastic pipe form. It's physically short enough that the distributed transmission-line effects are minimal; the L is the coiled coax shield, and the C is the internal center-conductor-to-shield capacitance.
Does that count?
i guess, I was wondering what is the likely hood of coming upon something like a foil rolled up capacitor with a coil around it though. Not sure if a tight spiral counts as a shorted turn.
Maybe I will try it and see what happens before and after later tonight, before that magnet wire dies of old age
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Well I wound like 30 turns of fine wire, maybe more, around a 22nF axial foil capacitor, I think polystyrene (the clear one you can see the spiral through)
When hooked up to the meter it said 20nF at 1Mhz with 0.2 ohms and capacitance started reading negative around 8MHz
When I soldered the coil over it, at 1MHz it said 4nF + 10 ohms, and at 5MHz it read similar to the unmodified capacitor (the ohms dropped back down to 0.3 and capacitance rose), but today is not a day to write things down on a note pad and I don't remember the numbers. But that is like a notch of some kind right, since 4nF and 10 ohms conducts less then 20nF and 0.2 ohms and it looks like the textbook notch filter of LC parallel in series with a load, so by happen stance it looks like I made some kind of low frequency notch filter by winding 30awg kynar wire over a capacitor.
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I would like to read the servicemanual of the HP 461A (pdf).
Is there anybody who can post a link or send me a copy?
Thanks.
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A few oldies...
(Attachment Link)
The two on the left and the one on the bottom were recovered by tube b&w TV sets. The two on the top right (sorry one is damaged ;) ) were commercial units for hobbists/repair, bought in the late seventies. One of them is covered with vax, that was quite common.
Those are not wound on resistors. They are wound on ferrite rod cores that have axial wire leads bonded to them. Completly different component.
Winding on a resistor is as others have said generally used to give a low Q (at self resonance) coil. The resisor has to be low inductance. This is easy with carbon composition but they are hard to find. Modern lossy ferrite "EMC" beads and sleeves have displaced this "trick".
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just for clarification the schematic of the part says L because I was reading it backwards when I Made the thread... however it does look like someone just wrapped a bunch of wire over a resistor and varnished it.