Carbon Resistor Mojo...................

Just a thought Iv read lots a stuff on the resistor mojo I read of the proof that they produce the harmonics that our ears like all be it a tiny amounts & how they do this when high voltage is present.

Heres my silly thought...say we have a tube pre & we put carbon resistors in circuit where voltages are high...we should then have our sweetened sound its not a lot tho maybe even in our imagination.

Say the plate resitors are a 100K ......what if we had 2 x 50Ks in series....do we get more sweet harmonics ? or 2x 200K in paralell do we get more that way ? less ? no change ?

If we get more either way we could use ten carbon resistors in place of one, I spose we would also get more noise tho.....just a silly thought maybe il justb try in a see how it sounds I just made a little tube pre its laying on my bench now.

Couple more questions in a pre that has feedback the small amount of Mojo, is it lost if the circuit uses feedback ? What about in a push pull circuit is the Mojo canceled out ?

i assume you have read this article....by rg keen
http://www.geofex.com/Article_Folders/carbon_comp/carboncomp.htm

i used some carbon comps in my princeton reverb that i built from the ground up in the phase invertor section and it is dead quiet.
It has some good sweet sound, but i havent tried the same amp with carbon films or metal films in those positions, so i couldn't tell you
how it compares. I would avoid using them in sensitive areas for sure.

One day i was messing around and wanted to see how much resistors drift with heat so i took a heat gun and really got some 100k's going...
i think the metal film went to 102, the carbon film went to around 105, the carbon comp went around 112.
Now the metal film after about 5 minutes went right back to 100...the carbon film was like 101ish..still very close...the carbon comp went to around 105. I had alot of heat on these things and wanted to see if really torching them would make a permanent change in value...the carbon comp did indeed and will swing more.

I havent tried liquid nitrogen on them though...
It was just a playtest. I couldn't see these resistors getting this hot in a circuit with current going through them without blowing.

if you try out your tests, please post your results.
Some say resistors will not make that big of a tonal difference, just use metal film for reliability.
Others swear by the mojo...

I know by my ears that capacitors can have a huge effect between types in certain positions...but not much in others..
I suspect different resistor types are the same.

About eight years ago, I built two versions of my CLA-2A compressor (a Teletronix LA-2A clone-like-object) both identical except that one had carbon comps throughout and the other had metal films throughout, to test this very subject.  There was definitely an audible difference between the two units. I'd call the difference obvious, not subtle.  The unit with carbon comps had higher distortion and noise, and not really in a pleasing way.  I thought the carbon comps might shine on certain instruments like bass or lead guitar, but that really wasn't the case. The unit with metal films sounded better on everything. So, every unit I've built since then has used metal films exclusively.  Just one data point for your consideration.

---Joe

Cool thanks for yr thoughts & link ....I have read but its good the link is here other can also read.

love yr blowtorch test fun stuff  ;D

I DIYed a Rca BA6A limiter a while back used metal film resitors at the time.....sounded great...I read that link and other stuff about carbons...I then worked out which resistors to replace in the BA6A plugged in soldering iron recorded a verse & chorus of a song on my DAW, left all levels the same, quickly swapped all resistors,  10 I think... fired it back up recorded same vs chorus on other track...

I know its not scientific but to me it sounded better bit more hi end like an exiter type quality, dont no how to exlain the sound but better, I was pleased & left it that way.

The carbons I used were given to me , a box full some used before some about 1/2 inch long some about 3/4s I take it they different wattage dirty whit bodies about 5mm dia...most of the values were bit off....I measured them all on meter and matched pairs as the BA6A is pull pull circuit.

When I was a lad I used to hang around some waste ground near where I lived picking fruit....years before there were houses on that ground hence fruit trees...I still go there in the summers, its like woods now, I take my daughter there...I no where theres old junk buried like old TVs, Im gonna take the metal detector down there & dig up some carbon resistors haha lets see if the earthy goodness has given them extra Mojo   il let you know I.

I will post what i find after multiple carbon plate resistor experiment.

Totally agree about the capacitors....another member Emrr I think was telling me how capacitors in old days usually had larger value than meant to be & in recreating old circuits to use slightly larger values...theres been cases where old gear has been recapped & sounded thinner due to new on spec capacitors.

Hi Joe, thats interesting Iv done a similar thing built 2 LA2As as u say one carbon thru out other MF thruout...I plan a AB test trouble is I only have one set of transformers at the moment....to make matters worse the capacitors in the LA2s are different to.

How about my question about the feed back in circuits anyone......IF ..there is any good mojo can it be canceled out by feedback or circuit design only leaving us the extra noise....Im afraid I dont know too much about the theory, but is that possible ?

Now this is what I call an interesting thread.
I built 2 RCA BA2c preamps one with carbon comp one with metal film resistors. The metal film resistors sounded cleaner and "faster" but I preferred the sound of the carbon comp resistors because they have more colour in this circuit. I guess its a subjective issue.
Personally I love old 50s,60s and 70s sounds.
CC had slightly less top end but it had a more focused top end - are CC resistors slightly inductive???

Also on my mixing desk half of the top end boost cut pots are conductive plastic and half are old wirewound pots. Top end from the old wirewounds is much much nicer than the new conductive plastic pots and all my clients think the same. The CP pots are Bourns and were not cheap.

There is a lot to be said about the MOJO of old parts.

Cool thats funny I built a Ba2a one of my fav pres ....Is BA2A same as BA2c ? anyway there isnt many resistors in a BA2A so I put couple a switches in there switch between carbons or metals ...I found the same I like the carbons best as you say its subjective ...I wonder if some circuits suit carbon more & some metal film more maybe all the parts react with each other.....

I dont know if carbons are inductive....maybe someone could chime in..

I ve just swapped some 1uf modern poly caps from my DIY BA6A for old oil ones like in original and it sounds gueyer i like it...I built the thing with all modern parts & have swapped nearly all the parts now after listening tests its now mostly old parts....its never sounded bad & I dont know how a real one sounds sadly buts it sounds a lot differnt now after all the changes...Im trying to find out what gives it its sound,{i think its 6V6s} compared to two DIY LA2As ...yes one carbon comp other metal film, it has so much more top Im not even sure if its meant to be this way it sounds nice tho thats whats lead me to the carbon comps, I have a scope goin to learn how to use it & investgate some more..

carbon comps will drift more ( thats already been proven ) and they can introduce hiss especially in a guitar amp .. the biggest thing that would a change  circuit would be the caps

What I would do is build a triode gain stage using a 12ax7 like a part of a Fender preamp.  I would then find different 220K MATCHED plate resistors, CC, metal film, carbon film and power it with a higher B+ and adjust the operating points to what you want.  The higher B+ and plate resistor is to get closer to the voltage rating of the resistors with the same plate voltage.  A possible added bonus could be the higher plate R and higher B+ looking more like a CC to the plate and reducing the tube part of the distortion.

Then measure the distortion with different resistors and compare the measurements.

If you look in "The Art Of Electronics" there is a section on using resistors in high voltage sections of circuits.

Without getting into you guys sonic integrity discussion, my experiences are that metal film are just more reliable & stable.  I have built several valve compressors for studios point to point.  When I first started doing this I used to use a combination or carbon film & metal film.  The 2 failures I have had to repair were down to carbon film resistors going way out of spec & finally going OC.  Nobody has complained about the sound with 2 replacement metal film resistors after they have been changed.

Most of the vintage valve gear that I have repaired has needed resistor changes if they are carbon composite, because they start to increase in value, to the point where the amp starts to shut down. 

If the unit sounds toppy to me I just cut it with the eq on my desk !

I just re read that article posted by Mrphotodude the anwer to one of original questions was there I missed before ...that feedback in a circuit cancels out the resitor distortion.

Ive DIYed 11 different tube pres 4 different Vari Mu limiters & 2 La2as couple tube mics too, as i said before I have no originals to compare how things should sound I have various Sowter rca & oep audix transformers I try in all the units....evrything I build I audition components while its on the bench with crock clips....change a cap record a vocal on the DAW, change a resistor record again & so on If a circuit has feedback within it I try it without feedback I usually end up with switches to have feed back or no feedback, Iv just about come to the conclusion that I dont care what it says on paper I now plug listen solder, I usully find I like the non feedback amps that prob distort more maybe thats why I prefer the sound of the carbons maybe my ears are not up to spec, I no the carbons they more unreliable & bit noisyer, Im the only one who uses my gear so Im also the guy that has to fix it so I dont mind.

Hi Rob the difference in sound for me with my Ba6a & BA2A in my not that scientific test recordings was quite noticable & not something you could get to by adjusting yr EQ.... like the difference between two microphones say... so hard to explain a sound & of coarse its personal taste....weather it was the 2nd harmonic distortion as explained in that article above or because most carbon values were a tad higher, the BA has some feedback goin on that should cancel out the distortion ....I dont know.

Hi Gus was gonna ask how do carbons fair in mics? the old classics only had carbons didnt they?
You'l have to forgive me Im still learnig....but how would I go about measuring distortion ? I have a scope and synthersizer for square wave can i do it with that....I bet all the amps I like the sound of have most distortion

gary, do you think you could post an audio clip of that difference? I'm really curious...

Gary

Are you using Carbon film or carbon composite resistors ?

Hi living sounds I dont keep all my tests some are here some where who knows where, il see if I can knock something up with BA2A pre amp il have to change back to metal film its but quite easy to do.

Hi Rob carbon comp not film some 2nd hand quite a lot measure more than resitance they sposed to be...if in push pull circuit I match pairs.

From the article that you guys seem to be referencing:

"Realistic levels might be 200V across a 1/2W resistor, and a 75V signal swing. That would give you a 2.6% distortion - enough to be audible as sweetening."

Two problems:

1. He says "2.6% distortion," but what he really means is that the resistance value will change by 2.6%

2. Everyone I know uses 5% resistors to build tube amps....so how does a 2.6% change in resistance suddenly become mojo?

Sorry, but I call BS.

;D\
Don't call BS, please don't!
Instead, I would recommend some more reading.
2.6% the article referring to is dynamic change of resistance with voltage, whereas 5% you referring to is value tolerance, which, importantly, doesn't change with voltage...

that article is the only one i have come across about the myth of the cabon comp mojo which is why i posted it.
i generally prefer metal film for reliability and low noise. I have only used the carbon comps in a few amps in a few positions based on that article.
My princeton has a feedback cut switch and it does get a little noisy when i cut the feedback, but nothing to frown about.
I have since put in a 10k pot in series with the 2.7k stock feedback value so i can dial it in where i want.
I think as far as the feedback cutting out any harmonic distortion depends partly on where the feedback is taken from too.
Some designs have it taken from the first gain stage and others at the phase invertor. I'm sure it will cut out some of the "magic" if there in the process of cancelling distortion. I know the princeton is still harmonically rich when i crank it. There are whole lot more factors then just the comps in a few areas though. Gestalt theory-the whole is more then the sum of its parts.