Small Metal Film Resistors in Amp Builds Question

[Ed_Chambley]

I have a lot of .5 watt metal film resistors.  They are the small ones that look like ones in pedals or on home electronic boards.  I have never seen a voltage rating for a resistor.  Would these be fine to use in an amp build?  Other the wattage and ohms, are there other rating one needs to be aware of?

[Willabe]

I have never seen a voltage rating for a resistor. Other the wattage and ohms, are there other rating one needs to be aware of?

Yes voltage.

Some web/on line catalog sites list the voltage limit on their sites or give a link to spec sheets from the manufacture.


                           Brad      

[FYL]

I have never seen a voltage rating for a resistor.

All are rated in their datasheets. Most small MF Rs are OK for 200 or 250 V across them - your B+ may be much higher, what's important is what the resistor sees.

Would these be fine to use in an amp build?

Small MFs are perfect as grid leaks, loading Rs or small signal cathode loads. Plate loads are another story, I vastly prefer to use over-rated Rs there, typically 1 W for a stage with a max dissipation of 200 mW and 2 W above. The Rs will last longer, run cooler and noise will be lower.

Other the wattage and ohms, are there other rating one needs to be aware of?

Max voltage, max power, resistance and tolerances are the main factors.

Some types of carbon-based Rs have a negative temperature coefficient - their actual resistance decreases with temperature, metal films and MOXen show a positive one - R increases with T.

Carbon Rs drift significantly under load, most stabilize after "burn in".

Noise should also be considered for input stages and critical applications. Broadly similar Rs using the same technology can show vastly different results here, some are noisy, others quiet as a mouse.

Inductance can be a problem with some wirewounds. Capacitance exists but isn't significant in audio applications.

[Ed_Chambley]

Thanks for the info.   I did a little reading, well a lot, and found out some very interesting things.  I have always just used carbon comps and some metal film I got from Doug and Radio Shack Carbon Films.  Anyway, it is good to know over-sizing resistors reduce noise.  I looked at the Resistors on Mouser and got a handle on the proper voltage.  Funnny thing tho, those photos of the trainwreck that someone posted from the ebay sell looks like Ken Fisher used Carbon Film on the board and metal film in the B+.  At least that is what they looked like.  That is the first time I have seen the inside of a wreck and was very surprised that the heater wires were very loosely wound.  Both Transformers on the same side and no choke.  Looks very simple.  I guess there is something to be learned from Ingred.

[Willabe]

I guess there is something to be learned from Ingred.

Maybe, maybe not.    

Problem is (and HBP's said this in another thread) most of use have never played through one or have been in the same room with someone playing through it, so how do we even know what it sounds like. We don't.

Most here have played through any number of FMV amps, etc. and have fixed/modded/hand built them and many others, so we do have a pretty good idea on how they sound and how physical layout and lead dress will effect their sound/tone _and_ noise level.  

With that being said I was surprised to see, at least from that amp Fisher built, not only were the heater wires very loosely wound and with only a couple of twists for each pair, any other pairs of wires that could/should have been twisted (as pairs) were not. I had heard that he did put PT/OT right next to each other as some amp companys did through the years but this is not always best or even workable as far as noise.

IMO, he charged a pretty hefty price for an amp with this part of lead dress like it was. Cuts down on build time. I guess he didn't believe in it much.      

In a couple of Gerald Weber books he added Fishers "Trainwreck Papers" from an Angela's catalog and all his articles from VGM, where IMO, he was a part of kick starting the vintage build craze. I bet if I re-read those he talked about it being best to use twister pairs where you can.

 
                            Brad        

[Ed_Chambley]

You are correct that legend has an upper hand.  I have a point tho.  My cousin asked one time if Hank Williams would not have died at 29 would he have been as big of a star as he was.  Of course he would have.  Brad Paisley can play anything he wants and has great knowledge of amps.  That is why Tony Bruno does work for him.  His Trainwreck is not for show and I have heard him plugged straight into it.  And while we know the fingers have a whole lot to do with tone, plug ole Brad into a Crate 15r and he will sound good and flat.  Things of legend do not become legend because they are not special in some way except for the pet rock.

[HotBluePlates]

We probably oughta keep this stuff in another thread instead of hijacking, but...

I too was surprised by the overall lack of twisting of the heater wires when I saw those pictures. However, I then noted how there was a lot of empty space available, and Ken routed the heater wires well away from everything else. Where they did come near other wiring, they cross largely at/near right angles. I got the impression the few twists there were more about keeping the heater wiring in place than much anything else.

I'll not venture into the carbon comp or not debate here. I'll say I used Dale metal films in my latest build, generally with an eye to lowered noise.

... Brad Paisley can play anything he wants and has great knowledge of amps.  ...  His Trainwreck is not for show and I have heard him plugged straight into it.  ...

Brad has a 'Wreck? That's pretty cool. I was present in the shop when he first picked up a Dr Z (to my knowledge anyway), and yes, he'll sound generally great through anything.

[Ed_Chambley]

We probably oughta keep this stuff in another thread instead of hijacking, but...

I too was surprised by the overall lack of twisting of the heater wires when I saw those pictures. However, I then noted how there was a lot of empty space available, and Ken routed the heater wires well away from everything else. Where they did come near other wiring, they cross largely at/near right angles. I got the impression the few twists there were more about keeping the heater wiring in place than much anything else.

I'll not venture into the carbon comp or not debate here. I'll say I used Dale metal films in my latest build, generally with an eye to lowered noise.

... Brad Paisley can play anything he wants and has great knowledge of amps.  ...  His Trainwreck is not for show and I have heard him plugged straight into it.  ...

Brad has a 'Wreck? That's pretty cool. I was present in the shop when he first picked up a Dr Z (to my knowledge anyway), and yes, he'll sound generally great through anything.

I don't know if it is entirely hijacking.  I was trying to learn a thing about straying from standard components.  Resistors.  Funny thing you mentioning Dale resistors.  After FYL's post, I began to do a little research as I am building my first amp where I am making everything.  I got the chassis back from the chrome shop yesterday.  I used a layout provided to me and did a layout.  It sure is easy to drill holes on a flat piece of metal.  It took a lot of work and I am very happy with the results.  It is just like a fender princeton blackface chassis except with more holes on the back.  I ordered all Dale resistors and using 1 watts minimum.  They are very nice to work with as the leads are long and not quite as thick as Carbon Comps so it makes it easier to get multiple pieces in the turrets Doug has. 

Brad telling me to look at voltages was a big help as well.  I have heard and read if you use larger watt resistors they are quieter, but I have never had it explained so I don't know why.

It is difficult to build an amp and get exactly what you want.  There are always some give and take.  I appreciate the info as resistors are a HUGE part and will help me to achieve a more spanky sound.  Least I hoping.

[phsyconoodler]

Twisted heater wires are totally unnecessary.Parallel heater wires are just fine.I built an 18 watt TMB combo with 18 gauge lamp cord.No hum.
  Mesa(some models),Soldano,Tone King,are just some amps that you won't find a twisted heater wire in sight.
  It simply isn't necessary to twist heater wires.They just have to be parallel to each other if using a modern heater wiring method.Anyone see any twisted heater wires in Vintage Fender Tweed amps?Lots of them are dead quiet.
  I still twist mine cause it's easier in many cases.
In fact Soldano uses bare wire for heaters.No insulation to be found.Same as Mesa Big Block amps and lots of others.

[John]

Twisted heater wires are totally unnecessary.Parallel heater wires are just fine.I built an 18 watt TMB combo with 18 gauge lamp cord.No hum.
  Mesa(some models),Soldano,Tone King,are just some amps that you won't find a twisted heater wire in sight.
  It simply isn't necessary to twist heater wires.They just have to be parallel to each other if using a modern heater wiring method.Anyone see any twisted heater wires in Vintage Fender Tweed amps?Lots of them are dead quiet.
  I still twist mine cause it's easier in many cases.
In fact Soldano uses bare wire for heaters.No insulation to be found.Same as Mesa Big Block amps and lots of others.

That is interesting. I've read just a few places about using zip cord for heaters, but thought I was not understanding it right or something. You're the first one that put it so bluntly. What HBP said about the "lots of space" layout also coincides with K'OC saying that a more airy layout is always better, where possible.

Then again, purple and green wires just look so pretty twisted together. 

[Ed_Chambley]

Twisted heater wires are totally unnecessary.Parallel heater wires are just fine.I built an 18 watt TMB combo with 18 gauge lamp cord.No hum.
  Mesa(some models),Soldano,Tone King,are just some amps that you won't find a twisted heater wire in sight.
  It simply isn't necessary to twist heater wires.They just have to be parallel to each other if using a modern heater wiring method.Anyone see any twisted heater wires in Vintage Fender Tweed amps?Lots of them are dead quiet.
  I still twist mine cause it's easier in many cases.
In fact Soldano uses bare wire for heaters.No insulation to be found.Same as Mesa Big Block amps and lots of others.

Thanks, that is good to know. Some repairs I do will be much easier.  Just make sure they are parallel.  And all this time I have been putting wire in a drill and working like hell on 9 pin sockets.  I got a repair right now where I need to replace 3 tube sockets.  The amp was dropped.  If zip cord will work, speaker wire should work and it has a stripe to make it easy to keep common pin connections.

[FYL]

I have heard and read if you use larger watt resistors they are quieter, but I have never had it explained so I don't know why.

Thermal and contact noises are lower. Check http://www.aikenamps.com/ResistorNoise.htm for more info.

[mresistor]

OT   Concerning twisted filament wires; in the TW there is so much real-estate in the chassis around the sockets that it doesn't appear to have been a major concern for Ken. The filament wires are well away from the rest of the circuit and preamp.