Seeking a source for affordable non-inductive 1,2,3,4,& 5 Watt Resistors - Page 1
Forum section: electronics / rf-microwave

author: postfive
author_rank: Contributor
timestamp: August 31, 2024, 10:41:14 pm
content: I'm building a broadband RF amplifier to cover up to about 30 Mhz. A design criteria is to have as little tuning or peaking at any frequency within the range, so I want to avoid introducing inductance with any of the components. This includes short leads and special care in selecting the resistors. Since this is tube-based, I will need higher-wattage through-hole style dropping resistors for the plate B+ and cathode circuits, from 1 to 5 watts depending on the location in the circuit. Clearly, wirewound types are ruled out. I have true 1/2 watt Allen Bradley style carbon composition resistors to satisfy this requirement in portions of the circuit, but I am having a lot of difficulty locating a source for reasonably priced higher-wattage true carbon comp resistors that have zero inductance. I have searched all the large standard parts houses (DigiKey, Mauser, Newark etc.) and they either have them in ridiculous bulk quantities at even more ridiculous prices, or they only have a very few selected values - not the ones I need (4.7K 1 Watt, 10K 5 Watt, 100K 2 Watt). I've searched specialty sites like RF Parts but there again, no dice. I have purchased some "carbon composition" resistors from Newark and AliExpress, and found in both cases that they were actually manufactured with a spiral-applied resistive film layer on a central insulating core and they measured depending on the value in micro Henries to milli Henries. Questions: 1) Does anyone know of a source of reasonably priced Allen Bradley style true non-inductive carbon composition resistors in these wattages? 2) I've seen on the web in a couple places stating that if the resistance is around 500K or higher, then the minimal inductance measured can be ignored as far as impacting frequency response. Your thoughts?

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: September 01, 2024, 12:51:02 am
content: Try these people:https://www.surplussales.com/They specialize in tube components and there are other components they have that you may need also . They do have non inductive resistors. These are the types used in HF amps by lots of people.The other sources I have used are closing.....Fair Radio and Sphere Research.. Unfortunate.

author: ahbushnell
author_rank: Frequent Contributor
timestamp: September 01, 2024, 06:21:09 pm
content: What inductance are you looking for. If your 4.7k resistor has 1 uH of inductance which is high. The corner frequency is 4.7k/(2*pi*1 uH) which is 748 MHz.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: September 02, 2024, 07:28:20 am
content: Could 2512 SMD resistors work?You might have to do some bodging to make them through hole (and figure out how to heatsink them) but they should just be a film of resistive material.I'd also be a lot more worried about stray capacitance, generally the impedance actually goes down instead of up at higher frequencies (for large resistor values).

author: MisterHeadache
author_rank: Regular Contributor
timestamp: September 03, 2024, 03:18:20 pm
content: Regarding NOS / surplus sales of carbon comp resistors - buyer beware! I've bought them from two different sources over the past couple of years (including the one listed above) and it was hit or miss on the values. Most were still within 10% but I had a 33k 2W that measured 44k - into the bin it went.Quote from: EggertEnjoyer123 on September 02, 2024, 07:28:20 amVery true. I've also been very interested in this topic, see this thread:https://www.eevblog.com/forum/projects/how-to-measure-the-inductance-of-a-through-hole-resistor/. I'm still poking around in my lab measuring various resistors, will be updating results to that thread soon.

author: jonpaul
author_rank: Super Contributor
timestamp: September 04, 2024, 06:32:35 am
content: have boxes of genuine Allen Bradly 1/4, 1/2, 1, 2 W axial 5..10%We used many in paralle to make higher power loads, terms, etc.IF you can wait a few months, PM me with a list,I might have what you need.Jon

author: Zoli
author_rank: Frequent Contributor
timestamp: September 05, 2024, 05:32:49 am
content: Check LCSC thru-hole resistors selection; there is an non-inductive option.

author: factory
author_rank: Super Contributor
timestamp: September 07, 2024, 08:12:36 pm
content: Quote from: MisterHeadache on September 03, 2024, 03:18:20 pmApparently they need slow baking to remove moisture, then resealing, see the comments about treatment here;https://www.hificollective.co.uk/components/allen_bradley_resistors.htmlI didn't bother with that in a recent repair, they were in non-critical circuits, that worked fine with slightly out of tolerance parts.Any carbon comp resistors from the well known suppliers (e.g. RS, Mouser, Farnell etc) are end of line, no one is making them (that I know of), I found this when I ordered some from RS last year, they couldn't complete the back order.David

author: postfive
author_rank: Contributor
timestamp: September 11, 2024, 05:10:34 am
content: Thanks all, I went to the Surplus Sales site but they did not have the values I needed, either singly or equivalent in parallel combinations. JonPaul; thanks for checking; I still need 1 4.7K 1 Watt, 1 10K 2 Watt, 1 10K 5 Watt, 2 100K 2 Watt. Higher wattages up to 5 Watts are acceptable (larger than that and it becomes a space management issue in the chassis). I ordered some from Newark that were billed as being non-inductive in their datasheet, but when measured they had measurable microHenries of inductance with two separate calibrated meters. $30 down the drain for nothing. I've been thinking of taking drastic steps and maybe cannibalizing an old radio chassis so am getting fairly desperate; today's modern manufacturing methods are turning out components that have hidden flaws or limitations. Another good example of that is modern poly or film capacitors where the old-style waxies or poly had the shield lead marked, but modern caps are not marked at all. Most of the time that is not a problem if it is connected to a PC board because lead lengths are short and often there is a ground plane. But if you are on an older tube design with point to point wiring, having the shield end of the cap attached to the higher voltage side of the circuit can induce noise or hum and radiate a signal in unwanted sections of the radio. Thank goodness there are ways to detect which end is the shield, such as Mr. Carlson's Lab SIFT circuit.

author: Uky
author_rank: Regular Contributor
timestamp: September 11, 2024, 06:04:37 pm
content: Regarding AB carbon resistors: As far as I know, they went out of production many years ago.I have a small amount of different values BUT: They have all aged by drifting upwards in resistance value.Some > 10% outside their stated tolerance.This both for NOS and those found in old instruments from the 70's.You may try metal film axial resistors on a ceramic body. Eg. DigiKey stock (or used to stock them)but they do not come cheap.

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: September 12, 2024, 08:07:02 pm
content: What about metal oxide film resistors? They are used in switching power supplies and other applications where low stray inductance is important, they can tolerate high pulse overloads and they are available in power dissipations up to several watts.BTW my Yaesu FC-707 antenna coupler uses 20 1k/5W metal oxide film resistors in parallel as dummy load. The slight stray inductance is compensated with a 10pF mica capacitor.Greetings,Rainer DG1SMD

author: postfive
author_rank: Contributor
timestamp: September 13, 2024, 03:46:21 am
content: Kleiner suggested using metal oxide film resistors. I have tried several from multiple reputable sources (DigiKey, Newark, Mauser) and all had measurable inductance. What is the formula to use to determine the amount of capacitance needed for a given value in Henries to counteract the inductive effects?Yes, I agree that old AB style resistors will drift out of tolerance with time. I have tons of 1/2 watt resistors that I collected going back to the 60's; I find that most of the time they are way off. I have not tried baking them in an oven to see if they would change - interesting concept there.Regarding SMD resistors - is there a chart that shows size versus wattage? EggertEnjoyer123 suggested 2512 size - what wattage are those?

author: kleiner Rainer
author_rank: Regular Contributor
timestamp: September 13, 2024, 09:57:35 am
content: I would avoid SMD resistors in a tube based RF amplifier. One of the points to keep in mind is that the trimming of the resistors during manufacuring by laser cutting results in a weak spot along the resistance layer, unless you specify "pulse proof". I had a case where this failure mode led to early failures of circuit boards. The other point to keep in mind is the low maximum operating voltage - you dont get much distance between the electrodes of an SMD resistor (and the pads on the circuit board are even closer).If you check out old ARRL handbooks, you will find lots of well made examples of tube based RF amps and many useful informations about correct ways to select and place the components. My main worry would not be stray inductance in the power resistors, but the wiring between components - you need clearance due to high voltage and heat. The unavoidable distance between the main components will give you lots of stray inductance.Next problem: the anode choke. Do you plan to restrict the frequency range to the ham bands, or do you want a true broadband amplifier? If the latter, be prepared to switch the anode chokes depending on frequency. You dont want neither series nor parallel resonance of the choke near your operating frequency.Recommended reading:https://www.w8ji.com/Amplifiers.htmW8JI is the designer behind Ameritron PAs.Greetings,Rainer DG1SMD