suggestions buying full set of resistors for an amp build

[Gingertube]

All good suggestions BUT a warning.
Many of the 1/2 Watt Resistors are rated at 350V. For tube amp build/repairs I just keep a stock 2W Metal film (500V rated). My local bits store is happy to make me up a set of 10 of every value every year or so. If I don't have the value I need and have to make it up from 1/2 watt resistors I'll put 2 near equivalent value (near 1/2 what you want) resistors in series so I don't get over voltage failures. This applies mostly to anode load resistors.
Cheers,
Ian

[bob p]

I wish we had bits stores here. They're a thing of the past. Our last hope for this sort of thing was Radio Shack and they're gone now. Around here I'd have to drive an hour to find a store that has components on the shelf, and the selection would be poor. So I'm stuck using mail order.

> Many of the 1/2 Watt Resistors are rated at 350V.

Notice that some of the resistors that I linked to in my previous post are only rated at 250V, which is why I said to read the spec sheet. But I don't see that as a problem when they aren't ever going to be exposed to HF pulsed signals, over-voltage or power supply surges.

In the typical guitar amp a 68k resistor is normally seen at the input of the amp, where signal voltage is measured in millivolts; the 1k5 resistor only sees a couple of volts when used for cathode bias, or maybe 50V when used as a grid stop; the 1M resistor typically doesn't see more than 110V in an LTPI. For those apps I think 250V is OK, and if noise is a problem you can go higher. But for prototyping I don't like to use expensive resistors. I'll save those for the final build.

I'd certainly want to use higher voltage/wattage ratings in those locations that are subject to turn-on surge, like plate loads and PSU decoupling. For plate loads 2W / 750V would be a better rating, though those are harder to find. I like the idea of using two 500V resistors in series for plate loads.

For me, it works to buy parts that meet the spec that I'll use them in, rather than going over-spec on everything. But I see the wisdom in doing that, as it streamlines your parts inventory and makes decisions much simpler later on. but it costs a lot more to do that.

FYI: Aiken's site has a good page on this topic:

Technical Q&A

[R.G.]

It seems like the OP's problem may be intermixed objectives.

It's necessary to a functioning amplifier that the resistors have the right technical specs: resistance value, tolerance, voltage rating and wattage rating.

It's financially responsible to buy the best deal on these parts; that then forces you to guess whether you will use more of the same value in the future or only this one purchase. Resistors are cheap - they're nearly free, at a penny or two apiece when bought in bulk.

Buying one or two at a time means you're not buying the the resistor so much as buying the distributor's time and money to speculatively order what you might or might not ever buy, the labor in picking your resistor out of the bin and putting it in your package, and paying for the funding for the distributor to keep track of inventory stock levels and run the business. Buying one resistor means you're paying essentially for the cost of stocking, packing and shipping, plus the overhead costs of the business. If the resistor were free, you'd still have to pay the overhead costs.

The deal per resistor gets much better if you buy many at a time. You think you're buying more resistors and paying for them. You're actually simply getting more resistors for about the same overhead. The price is only trivially less than the single resistor price, for good reason - you're still buying nearly-free resistors and paying overhead. So the question for resistors not subject to some other criteria is really whether you ever think you'll need more. I only buy in 100-200 at a time, and have done so for a couple of decades. But then I use a lot of resistors.

If you put some special criteria on top of the technical specs, things get expensive. Resistors are only "nearly free" if you're using the types that are most used. In the 1950s and 1960s, that meant carbon comp, and they were not nearly free because production was not so automated.Today, production is in the process of moving out of leaded parts and into SMD. But the most common leaded parts are 1/4W carbon film or metal film. Get too far away from that material, 1% to 5% tolerance, or 1/4W and the price starts going up dramatically. Higher power, tighter tolerance, different resitsance materials all run the price per resistor up. The overhead costs per order remain about the same.

Adding on top of that a requirement about appearance makes this into the situation we have here. It's unusual for any one distributor to stock all the values of resistors from one manufacturer. I'm still appreciative if not surprised that Mouser can offer more than one brand of resistor in a range of values. But it's entirely possible that you will not get the best price per resistor at the same time you get the technical specifications AND all within one brand and model line AND from the same distributor.

Having lived through the rise of computers from something that was only barely possible (think glass enclosed rooms, raised floors and special air conditioning) to something that is inside nearly everything, I find it amazing that the internet should not only make everything so easy to find and get, but also convince us that whatever we can think up to want OUGHT TO be available instantly, in exactly the style and price we want. It's astounding, really.

[mikepukmel]

Thanks everyone, great discussion. Its definitely not appearance. Its specs (and the only thing I know to look for at this point is the schematics plus comments on various amp/electronics sites, plus construction. Some of these newer resistors are very small, have very thin leads and don't reach all the way across these old boards. OK its a guitar amp (the thing Im working on), so nothing critical going on inside, but it does have to have some kind of rough service ability. Take the amp out of the house at 68 degrees F, throw it in the trunk of the car at -12 degreesF, drive down some bumpy roads, take it back out throw it on top of a speaker cabinet, and get it roasting hot in a small practice area, and lost of vibration that, say, a TV set or DVD player (Im showing my age here) doesn't need to survive. So, automotive type service resistors, with specs that match the amp, seem to be good. So I make a list of the resistors for a XYZ build, find these: https://www.digikey.com/product-deta...9CT-ND/7351969 but they are not available in 270K ohms. Not like they all have to be color and shape matched, but it would be nice to have a simple order list, and be done with it.

Your comments are all great, understood, not critical or even important that they have to be all the "same" type/model etc. And the suggestion above about Xicon is great, they have lots of values available. And if I buy from Mouser/Digikey/Allied etc, I know what the heck Im getting. I had purchased a few odd resistor values from a few of the amp sites, will never do again. The resistors had hair thin leads, were so short they would not even come close to making it all the way across the boards (the boards that THEY sell) to the turrets or eyelets. and I have no spec sheet, so I honestly have no idea what it is.

I never got so far in purchase quantity to get a price break. I only thought, well if this amp X takes 2 of these, then I will buy 10 since most all of the amps I looked at building use this spec resistor. And Im paying shipping extortion, so ... I got 10. I always go back and think I should have bought 20, but I think I probably won't build 10 more amps, so ... probably OK.

[Chuck H]

Well... I would not say that the leads on the Xicon's I buy are thick or long. Since I don't build on prefabricated boards I have the "luxury" of designing around the component dimensions. But I haven't had any failures, like leads busting off at the ends or brittle leads just fatiguing and breaking. I build on eyelets and I raise the resistor body above the board surface (maybe 1/16" ish) so it floats on it's leads. I do this believing it may mitigate physical vibration so I won't need to "hunt down whatever is making that rattling sound". But I think it mat also provide some strain relief as things heat and cool. I figure if the resistor is tightly in place and soldered with no spare lead then there's a potential for stress if anything flexes. And FWIW it's a good idea to raise resistors that are prone to heat up much higher. A practice I've noticed even some known manufacturers don't always do when they should. Heat prone resistors do better with more free air and lead lead length for heat dissipation. Besides, burned resistors on the board cause that annoying char that can be a problem and looks horrid. I order so seldom that I don't usually write down the exact resistor model each time, but I've been using the Xicon's off and on for maybe 25 years and never had a failure I can blame on the resistors themselves. At 2.4 cents each (ordered 10 at a time for the .5W) they ARE virtually free. This price is available with a purchase of ONLY 10 and it's available to everyone. But then there's shipping. Since I prototype a lot or build custom orders I'm often paying more for shipping than I am for the resistors.

[mikepukmel]

... I've been using the Xicon's off and on for maybe 25 years and never had a failure I can blame on the resistors ...

Wow, that is saying a lot! Pretty good track record for Xicon's.

[eschertron]

At 2.4 cents each (ordered 10 at a time for the .5W) they ARE virtually free. This price is available with a purchase of ONLY 10 and it's available to everyone. But then there's shipping. Since I prototype a lot or build custom orders I'm often paying more for shipping than I am for the resistors.

Before following this thread, my last order from Mouser the price break was OK between singles and lots of ten, but better between lots of ten and lots of 100. So I purchased 100ea of the most common values, and 'just enough' (between 1 and 5, say) of the other values I needed. Mouser changed their price structure since then, so I'm glad I got wind of this thread. Just ordered more resistors in the last few days, a variety of values that should be found in about any tube amp, in multiples of ten. What a great deal! For a hobbyist such as myself, it's practically a lifetime supply. I still order the power resistors and other "not essentially free" components as I go, so Mouser will still get some business from me moving forward.

I do have the remnants of a RS 1/2W resistor assortment, from maybe 30 years ago. The commonly-used values are long gone, the one's I'll never use are - predictably - still here.

[bob p]

I don't like to buy in quantities of 10, especially when a quantity of 100 usually can be had at a similar price. the only time i'll buy resistors in small quantities is if they are both expensive and a special application or value that i don't plan on needing again.

back in the days of the old Ampage site I made a ledger that listed the BOM for every classic Fender amp, from Tweed to SF, from Champ to Super Twin. I also included some classic non-Fender amps as well as a few stompbox projects I was interested in. Once I had completed the list I ordered everything that I needed in bulk.

the result of doing that is that I've only placed additional orders for resistors maybe one or two times in the past 10 years. and when I do that, it's primarily because I'm ordering something else, and I just buy more resistors because I like to back up the truck to restock when I find them on-sale.

As far as power resistors go, I stock them too. It's not as if there are lots of different values that need to be stocked. i keep basic screen resistor values in stock for 6L6, EL34, EL84, 6V6 and 6550 -- that way I'm never forced into special ordering them, I just order replacements when I order something else. For PSU isolation resistors I also keep standard values in-stock so that I don't have to special order them either.

I guess I'm a compulsive parts hoarder. I have a pile of Hammond blank chassis on the shelf and I even keep a wide variety of iron in stock so that I can fix just about any classic Marshall or Fender amp on the spot without having to reorder parts. As much as I hate to admit it, I even have several boxes of new speakers piled-up in a corner.

If I need an oddball resistor value, I'm not likely to place an order just to get it. More likely I'll just use a combination of series or parallel resistors to reach that value.

[Enzo]

I stocked 5 watters. 1k, 470 ohm, and a bunch of 0.22, 0.33, 0.47 ohm. Now and then something else popped up.

[bob p]

I suppose that you were stocking ballast resistors because you fixed a lot of SS gear.

[Enzo]

Yes, of course, I almost added that note, in fact.

If all you do is tube amp work, there is very little need for fractional ohm resistors.


I did stock 1 ohm resistors for power tube cathodes, but not 5 watts for that.

But I agree it is good to stock common screen resistor types. I have a large enough selection I can cover typical cathode bias resistors as well, but there is no set value for that.