carbon comp plate resistors

[iknowjohnny]

I've always wanted to try this but i never remember to order any when i order other things, and I hate to pay $6 shipping for a few resistors. But i just ordered some things from mouser and i remembered this time. I have 3 ax7's in my amp thats a marshall 800 style amp but with a higher gain pre, otherwise very similar. I ordered 6, one for each plate R and one extra. (CF needs no resistor)

the question is, should i use them in all cases of are there places that would be better left with the carbon films that are there now? i also forgot to make on a 82k for the PI, but i've used 2 100k's before and it doesn't seem to matter anyways. It's always said the PI is never really balanced anyways and even if it were they tend to sound better unbalanced. So not sure whether that matters, but would you replace both with the 100k CC's or leave the 82k there and put the CC on the other side or what?

Stupid questions, i know. But i figured with all the knowledge here i'd prefer to hear anything i didn't know or think of before i solder them in.

[Cliff Schecht]

Carbon comps have a voltage coefficient of resistance meaning that as the voltage changes, so does the resistance. This effect is only really noticeable if you push the carbon comps towards their voltage limits, but it is a noticeable increase in distortion. The idea is to only use them in places where the signal swing is very significant and the signal to noise ratio is already high. So with that said, it makes the most sense to use CC plate resistors after the first stage, maybe after the second stage depending on if you dig the sound that is added. Put 82k and 100k CC plate resistors on the PI.

[Alexo]

It's commonly recommended to use them on the plates of the pi, as Cliff suggests, as they are said to generate the most even order distortion when they are pushed. You don't get much benefit by putting them on V1, other than increasing your chances of having a hissy amp. The CF stage might benefit from them too.

My only qualm with the common wisdom of putting them on the pi is that any distortion they generate will then be common-mode, so all that even order harmonic distortion (what little there is) will be symmetrical and therefore odd-order.

[iknowjohnny]

hmmm.... from what you guys are telling me it sounds like theres really not many good places for them other than maybe the second stage. I was led to believe they'd be good on all plates in the pre. Not what i'm used to reading, but i guess i'll just throw em in the parts drawer or maybe put a pair on V2 and leave it at that.

[JamesHealey]

Absolute rubbish put them on every plate load and evaluate the tone. All my own designs are like this and noise is never an issue!

[iknowjohnny]

OK, LOL! Boy, you never get a consensus on any question about amps do you ! James, i'll give it a go, then remove the ones on V1 if need be, then on the PI if need be. By the way, you say you use them on every build and every preamp plate. So what's your opinion of thier tonal benefits?

[Richie]

What about current? when they warm up or get hot they also change value.
Their are many long debates over using them. So you will hear alot of different suggestions as to what or if they change things.
You could probably do a search and see people have removed them from their old vintage amps,replaced them with something else and , and then say the amp lost it magic tone. Put them back in and the magic tone was back.If they hiss or pop or make noise, you have a bad one.replace it with one that doesn't.
Yes they can and do cause problems in some amps. pops crackles, hiss white noise,sometimes they drift in value etc. Replace them with a good CC resistor, the noise and problem is gone. Now other things too, some will say use this brand or that,some are better made than others. if your using NOS,or more current made ones.
It all depends on what you like.
Some may never hear the difference. Some say they can. Many of the old amps people call the holy grail of tone use CC resistors.Although alot of those are using old iron and caps too..Which all of that can add up. Do as Mr healey said and try them.
Let us know if you can tell a difference. Many times i don't agree with everything i read. I've worked on amps for many years,had amps from the 40s in here,and the resistors are not making any noises,causing problems and measure within their tolerence. Amp quiet as a mouse. Then i've had later amps,that did have problems, that needed some replaced. So you never know.

[KT66]

The coefficient of resistance in modern carbon comps is lower than that of vintage ones, so if you really want to get that mojo ( and the COR is the reason for it, which the general consensus seems to be that it is ) then you might want to pick up some vintage ones to try. If you do that though, make sure to order extra - the last time I wanted to try that I ordered a few 100K Allen Bradleys from an Ebay seller and all of them made more noise than a bowl of rice crispies.

[Richie]

I wanted to try that I ordered a few 100K Allen Bradleys from an Ebay seller and all of them made more noise than a bowl of rice crispies.

I think like in amps, sometimes it depends on where they were stored. many amps that have been left in basements that can have moisture,seems have more problems. I think this can be the same for the resistors. I also like to remove any outside buildup that might have occured over the years. Makes for a better solder connection too. One could use some small alligator clips, tac those in an amp, then go through many to see which ones are not noisey or have problems.

[iknowjohnny]

Well, it's subtle but it does seem like a little improvement in the distortion. Like it's a bit richer and smoother, hard to say exactly. But any improvement is good, especially cheap improvements. Didn't put one on the 3rd stage yet tho but i will. That R is on the tube and a bit hard to get to, but know that i know i like them one will go there too. No extra noise at all either, tho it's always been a very quiet amp to begin with.

[iknowjohnny]

Ok, now this is a bit odd. I figured i'd post the results then solder then resistors in more permanently and while i'm at it put one on the 3rd stage like i said i would in my last post. I'll be damned if that didn't change the tone more than the other 4 combined. I'm not even sure how to explain the tone, but it's more vintage sounding and has that sort of Townsend like thing going on where you play a chord and it's very balanced and every string is heard clearly. And even richer still. It's too late to really give it a workout, but i think i really like this.

It also brings up a question...if it makes the biggest difference on a given stage, does that mean that is the stage that is contributing the most OD? If not then why? The fact it's got a 820R cathode vs the 1.5k's on the others and therefore higher voltage?

[Richie]

See, since you tried it yourself, you have your on thoughts on if it changed the tone,for better or worse. Most will say they add exactly what you are hearing.

As for the different values,OD etc. the signal gets amplified through the circut.
I know it may sound crazy,but i have a friend that he was modding a wah,and he changed one resistor,and said it sounded much better.
Same value different type. he even though it was crazy, so he tried other resistors, and it would loose the sound he heard. put the one resistor back, the sound or tone came back. he was on a mission to find out what type or brand resistor is was.
Now you see why some like to use CC resistors in amps.
Many have done the same thing and like the change.
Some tried putting high dollar or exotic brand metal film or CF resistors, and report that the amp lost the sound it had.Then go back and put the CC back in,and the sound comes back to what it was before. I also think some things just can't be explained as to why this or that happens. Caps are another thing along the same lines.
it sometimes depends on the circut itself,what will sound good,or pleasing to the ear. Does someone dragging their fingernails across a chalkboard make you cringe? Some it may, some may not. Sound can have a different reaction on people. Thats why we have amp forums to discuss things like this.
And why you experiment to find out for yourself.

[JD0x0]

I realize this thread is old, but I didn't want to start a new one for this question. What would you consider to be the minimum voltage to use CC resistors with? The first triode of V2 on my amp is only supposed to have roughly 150V on it. That seems low to take advantage of a CC. I have a MF in there now.

Also, V1 is hovering around ~225V on both triodes and has CC on their now. Should I leave them if I'm not having noise issues? I think I have some spare MF's on hand that I could swap them with. The consensus seems to be no CC on V1, but if I'm not experiencing noise issues, is there an issue? What about an advantage?
Honestly, I could leave the amp as is, but I like to tinker, like many people here, I'm sure.

[Structo]

Honestly, I wouldn't waste your time.

Unless you just like to mess around with the amp and hear what various components sound like.

Most of the time we try to eliminate noise while keeping desirable overdrive distortion.

I don't want sizzling or popcorn or any of that type of noise.

The thing about the vintage amps is their parts are all out of wack from
use and age.
So putting a brand spankin new Bradley CC of the correct value may not result in the tone that was there.

Vintage CC resistors are known to absorb moisture over the years.

Try using them on the plates and cathodes.

[Reeltarded]

I like them, I also like CF and MF and MO and I like amplifiers.