help with resistor size

[Chuck H]

Isn't it interesting that the larger resistors are the noisy ones.?. Intuitively I would have thought the smaller resistors would have been noisier. In fact that's why I didn't use the ones I have, though I admit to not researching the specs. The small size being discordant in appearance on an eyelet board was the deal breaker. But my thinking was that such a small resistor must be capable of more heat within their smaller dissipating surface area, but more heat means more hiss, right? I'm sure the devil is in the details.

[Enzo]

Intuitively I would have thought the smaller resistors would have been noisier.

That presumes all the resistors are of similar type and construction. If the Chinese large ones were crappy knock-offs, versus high quality small sized ones, it is not surprising.
You can't take one spec out of context and extrapolate from it.

[bob p]

Isn't it interesting that the larger resistors are the noisy ones.?

I honestly don't know why the MultiComp are so bad, but they are. Could it be size? Material? Construction technique? Is there just a teeny little resistor hiding in a great big package? I honestly don't know.

They're an inexpensive Chinese product, cheap in every respect. I was not able to determine why they're so noisy -- I tried looking up their specs and the spec sheet was just horrible. Not much information. I got the impression that they weren't giving out some of their resistor specs because they didn't want people to know how crappy the product really is.

I won't buy MultiComp branded generic passives unless I don't have any other choice. I prefer to stick to the brand name products unless I just can't find what I'm looking for.

[FourT6and2]

For metal films, I really like:

Vishay CCF60: 1W, 500V, 1%
Vishay/Beyschlag MBE0414: 1W, 500V, 1%
PRP 9372: 1W, 500V, 1%

The Beyschlags are really nice. So are the PRP. The Vishay are smaller; nice for cramped locations.

[mikepukmel]

For metal films, I really like:

Vishay CCF60: 1W, 500V, 1%
Vishay/Beyschlag MBE0414: 1W, 500V, 1%
PRP 9372: 1W, 500V, 1%

The Beyschlags are really nice. So are the PRP. The Vishay are smaller; nice for cramped locations.

Thanks FourT6and2, I ended up buying some of these (I think some of each), could not get all of the values for 1 and 2 watts in the same model unfortunately. Do you have a good supplier? I tried digikey, allied, and mouser. All great suppliers but they don't carry all of the values for all models we need for amp building.

[mikepukmel]

Watch out doesn't mean emminent danger. We should always wear a seat belt driving the car, but if you don't, the car won't blow up or refuse to start.

70C is very hot.

If you learn nothing else theoretical in electronics, learn Ohm's Law. it is not complex, the formula is simple, the three variations of that formula derive from one another. it simply states the relationship between voltage, current and resistance in a circuit. it is something you can count on, and something I use every day. It isn't some corny school book thing, it is a real practical useful bit of science. I have been soldering for 60 years plus, and I have a calculator next to me at all times to do the arithmetic, and I use Ohm's Law every single day I am in the shop.

Thanks Enzo!!!! There were two questions, one that I could not get completely answered picking through blogs and data sheets. The first question, was wattage. its printed on the schematics and plenty of discussions about it. The second question is about how well a particular construction resistor handles voltage spikes. Both from spikes that make it through the line into the amp, and the major one, from contact bounce and the blast that occurs when you flip the "standby" switch to "on". Back in the day, I had a project to measure spikes in various places in very simple circuits, that had a bunch of spst on off switches in various places, with a 'scope, and it was surprising. We didn't have the best scopes, but aagghh, some really tall, narrow spikes did get sent through the circuits.

Anyway, read #1 was about spike handling for carbon comp's. Seem pretty good. And read #2 was about spike handling for metal film/metal oxide, seem to be 'much worse' but heck I don't have any reference point yet. I did read up on various places like Aiken Amps, and seems like the builders builders put higher wattage resistors in line with that standby switch, not for their higher wattage handling per se but as a proxy for better voltage spike handling.

Hence me fumbling around to put slightly bigger (higher wattage handling) resistors in line with the path that spikes would traverse the circuit when the standby switch is flipped on. I know you don't *need* a 1 or 2 watt in these places (from your calculations, thanks! and reading about the circuit), but more for longevity due to better spike handling.

Enter these tiny, mini-me, micro, nano resistors. Now Im at a dilemma. The reading says that the builder gurus put higher wattage resistors in certain places for better spike handling. Will these teeny resistors still have the same spike handling capability? I don't know. And no idea how to find out. The data sheets have some minimal data, but I did find other resistor types that specifically state "high voltage spike handling".

Geez, this electronics thing is deep!


You guys have the experience, alll I can do is read at this point, and cross my fingers.

[mikepukmel]

That presumes all the resistors are of similar type and construction. If the Chinese large ones were crappy knock-offs, versus high quality small sized ones, it is not surprising.
You can't take one spec out of context and extrapolate from it.

Of all the things that keep me up at night, I think that bothers me the most: So I go to, say, Digikey, and start ordering resistors. I soon discover that I can not pick a particular type (e.g. Vishay CCF60) and just pick all the wattage and resistance), so I start picking through the list each time I pick a voltage, resistance and wattage. Some resistors are only available in huge lots, so I have to pick a different brand/model. I end up with a whole bunch of different brands/types.

Since I don't know particular brands to stay away from, Im worried that I'll get a clunker in a critical position in the amp, it will fail, and I will get an unwanted light show right in the middle of a led zeppelin III cover.

I have read a few horror stories from people who bought those bulk Chinese (250 values, 5000 resistors for 19.99) on Ebay. I would hope that big places Digikey, Allied and Mouser don't stock the brands that blow up in your face. But I don't know.

[mikepukmel]

I had my own reservations the first time that I came across the modern tiny metal film resistors:

thread: http://music-electronics-forum.com/t21277/

I ordered an abundant supply of passives from Newark several years ago, and ended up with a boatload of the even smaller SFR16S series in the DIN 0204 size case. I have no reservations about using them.

I've used them without any problems in the hottest location in a Fender amp: 1k5/500mW stretched right across the tube socket. Heat stability was a major concern, as heat is the worst enemy of the grid and screen resistors that get stretched across tube sockets. One of the nicest things about the physical package of these tiny resistors that they don't take up much real estate when you're working in a tight space. The DIN 0204 size is so small that the little MF grid resistors just disappear behind the MO screen resistors.

I agree -- such tiny resistors might look out of place on a 5E3 board, but that's more of a matter of our expectations than anything else. I don't hesitate to use them because they perform well in-circuit and that's what really matters.

Thanks Bob, this is *great* news. Really hard to find posts from people who used these in a tube amp circuit and has some experience with longevity. These 1960's Fender style cases are really awful things from a physics point of view, looking at them from the inside. Not a vent or a fan, very thin height wise, tubes mounted UNDER The chassis, surrounded on the top and sides with nice thick heat insulating wood covered with heat insulating tolex.

But, we have lots of friends and such who have had these amps, 54 year old amps, have been played for decades and run just fine, only had to have a few crumbling resistors and a few electrolytic caps replaced. So, the *original* components fit the design just fine.

Its great to hear that you were able to use these teeny tiny resistors and they work fine. (which is what Enzo wrote above!!)

[mikepukmel]

Isn't it interesting that the larger resistors are the noisy ones.?. Intuitively I would have thought the smaller resistors would have been noisier. In fact that's why I didn't use the ones I have, though I admit to not researching the specs. The small size being discordant in appearance on an eyelet board was the deal breaker. But my thinking was that such a small resistor must be capable of more heat within their smaller dissipating surface area, but more heat means more hiss, right? I'm sure the devil is in the details.

Im no electronics guy, (obviously!!!!) my background is physics, so with that pound of salt: the bigger the resistor (physical size) the more material the signal is going through, so with same-same construction comparison, you'd expect more noise and distortion on a physically larger resistor.

[mikepukmel]

Just my opinion:
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You mentioned 220k and 1.5k. That looks to me like a plate and cathode resistor for a preamp. Hopefully my math is correct here. 1ma is a typical tube current. 1ma through a 220k resistor means 220v dropped across it. SO a 400v B+ would leave 180v on the plate. 1ma and 220v means dissipation is under a quarter watt. Since the same current flows through the cathode resistor, a 1ma current through 1.5k dissipates only a milliwatt or two. That is why those parts in Fender amps were 1/2 watt for decades.

Enzo, if you used the 1/2 watt carbon comp's, they lasted for decades, some are 55 years old, and still run fine.

But we don't have any, say, 10 or 20 year data on what happens when we put the new, high tech material resistors in these kinds of applications. I guess when the old ones absorbed moisture, and fell apart, they didn't fail gracefully. But how will the new ones degrade over the years?

[Enzo]

What?? Film resistors have been around for ages. Look at say a Peavey Classic 30, came out in like 1994. All film resistors. If 20-25 years isn't enough, I don;t know what is. COmposition resistors drift value with age, much more than others.

Handles voltage spikes? I have never found resistors to be high up the list of failures. Sure, they burn up all the time, but not usually their fault. A shorted transistor or power tube is going to burn up resistors. I have a whole bunch of nice film resistors in my collection, I got them 25 years ago or so. I just measured a ton of them during inventory, they were all dead nuts on value. I tossed out a lot of old composition resistors as the values were way out of whack.

[FourT6and2]

Thanks FourT6and2, I ended up buying some of these (I think some of each), could not get all of the values for 1 and 2 watts in the same model unfortunately. Do you have a good supplier? I tried digikey, allied, and mouser. All great suppliers but they don't carry all of the values for all models we need for amp building.

Mouser has the CCF60s. MBE0414 are impossible to find in the US. But I'm set up with a biz account with a few distributors in Europe and Asia so I can get them. The PRP can be found at Parts Connexion. If you want the Beyschlags, send me a PM. Maybe we can work something out.

[Dave H]

the bigger the resistor (physical size) the more material the signal is going through, so with same-same construction comparison, you'd expect more noise and distortion on a physically larger resistor.

Looking at it another way, at the same place in the circuit either a large resistor or a small resistor will be running at the same power but the larger resistor is able dissipate the heat better so it runs cooler and a cooler resistor has less thermal noise.

[PeanutNore]

I design my own PCBs for most of my builds, and I use those 6.5mm Vishay 1 watt metal films in everything. I love them. They work exactly as advertised and I've never had any issues. It makes it much simpler to design a board because I can use the same resistor footprint whether I'm going to use a 1/4 watt carbon film, 1/2 watt KOA Speer metal film, or 1 watt Vishay metal film - they're all 6 to 6.5mm long.

I do keep 9-10mm 1 watt metal oxide resistors on hand, though, to use when I need something with a longer lead for terminal strips, or in B+ dropping resistors where they will truly have 450+ volts across them while the preamp filter caps charge.

[FourT6and2]

I design my own PCBs for most of my builds, and I use those 6.5mm Vishay 1 watt metal films in everything. I love them. They work exactly as advertised and I've never had any issues. It makes it much simpler to design a board because I can use the same resistor footprint whether I'm going to use a 1/4 watt carbon film, 1/2 watt KOA Speer metal film, or 1 watt Vishay metal film - they're all 6 to 6.5mm long.

I do keep 9-10mm 1 watt metal oxide resistors on hand, though, to use when I need something with a longer lead for terminal strips, or in B+ dropping resistors where they will truly have 450+ volts across them while the preamp filter caps charge.

Gonna need a part number. Vishay makes 100 different type of resistors. "6.5mm Vishay" doesn't really narrow it down lol.