Hello Ramon,
First, thank you for the compliments. You are quite right about sellers like Pelican and Goodfellow, they buy wire and resell it in small quantities for a significant markup, they fill a niche for buyers who don’t need a lot of wire or can’t meet minimum purchase. They really should supply the wire data for the wire they sell since all wire manufacturers provide that information on the spools, most resistor manufacturers would not accept such unlabeled spools. MWS is another source which does not provide wire to specific TCR as I have talked with them and they won’t provide anything beyond the stated ‘normal’ range of TCR, such as ±20 PPM/°C for Evanohm. They may have changed that policy since I last talked to them but I kind of doubt it. When I asked them for a given TCR, all the salesmen will say it’s within ±20PPM/°C. There aren’t that many ‘original’ wire sources around and they know it so they can be selective in their customers and have no interest in small orders. It may be that is it simply too expensive for them to handle small orders but that doesn’t mean they shouldn’t at least reply to inquiries. More than likely, if you’re using an individual’s e-mail, they won’t reply, a business e-mail might at least get a burp out of them.
In general, a PWW resistor house can make any value of resistor if they have the proper wire/bobbin size available, some will charge a premium for nonstandard values but that is more marketing policy than real cost involved. I will make any resistor value if I have the wire and bobbin size available in inventory and there is no premium for oddball values. The main limitation I have is limited inventory of wire and bobbin sizes available in stock, replacement costs are high so it is difficult to increase the number of wire sizes I have. Yes I have had to no bid some resistors due to inventory limitations. Unless it is a rather large order that can support the purchase of inventory to do it, I have to turn it down. Another limitation is very high values, since the huge metal price increases since 2000, wire cost has nearly tripled and that makes resistors up around >500K more expensive. I’ve generally only made high values to repair ESI decades or instruments that need PWWs because factory replacements are even more expensive.
On the one hand, I don’t have any problems purchasing wire, that said, I’m still subject to their minimum order and I’ll also note that because they make the wire to my specifications, it raises the price from the standard stuff. The price of the wire depends on the wire size, larger sizes are lower cost, finer wire more expensive. Each purchase is quoted at the time and sometimes, they’ll change the quote more than once before all is said and done. How much wire is on a given spool also varies by the size (you’ll normally see the weight given on the spool), most of the time a given spool is in the hundreds of dollars each or higher.
I know this is going to sound self-serving but….while I encourage experimentation, if you really need <5PPM/°C TCRs for a project, you’re much better off in the long run to just buy reputable resistors. As proven in actual industry, it took decades to get TCRs down below 10PPM/°C with reasonable yields. It isn’t a case of somebody just tripping over a solution, it requires considerable knowledge of all of the factors that goes into a low TCR and stable resistor and most resistor houses are still struggling with many of those factors, the customer just never is privy to all that goes on behind closed doors and manufacturers don’t like their faults being put on display very much for some unknown reason (tongue in cheek).
One of things I see here is that a given parameter such as TCR is latched onto and is pursued, unfortunately unless all of the given parameters of construction are correct, the wire’s TCR is the least of your worries. I can guarantee that even if you have wire with 0PPM TCR, the end result is not going to be even close to that. There is a laundry list of parameters that will affect the finished TCR of a resistor and stability.
Experimenting will help you learn how things work and what causes this effect or that effect or even more than one effect but you don’t need very low TCR wire to do that.
Best regards,
Edwin