RCA BC-2B

[dustbro]

Urban myth. They may be noisier, though.

I dunno. I built a 2 channel mic pre with everything being equal except - one with metal film resistors and one with carbon comp. They definitely sounded and reacted differently.

 

[emrr]

Not a myth to my ears.  I've restored low feedback amps like these with both types, and it's audible.  I'm not talking two preamps, but multiples of the same type. 

 

[DaveP]

I would need some scientific evidence before I believed in carbon comp v. metal film.  After all it is just electrons shunting across atoms in the end.  If someone was to show a HF roll-off from carbon comps then I could see that they affected the sound.  Also if they were to show some kind of delay down to some mS then I could see that affecting the sound.  As it is, the increase in noise negates any benefits to my mind.

The sound of different capacitors was eventually put down to losses due to the piezo-electric effect and the rigidity of capacitors was increased accordingly as a result of this work.

best
DaveP

 

[gary o]

Maybe this could be a good project to test out the resistor sound with so few resistors they could be swapped out quickly for a A & B test..... or maybe the sound difference maybe less as so few resistors to hear so maybe not a good idea ...... after all ha

 

[lassoharp]

The sound of different capacitors was eventually put down to losses due to the piezo-electric effect and the rigidity of capacitors was increased accordingly as a result of this work.

Could it be a similar situation with CC vs MF?  MF more revealing of something in the envelope that's already there, because it's a better resistor in that regard?  As a rough analogy, I'm thinking of how some cheap speakers are flattering and good ones make material sound worse because they are more accurate and reveal flaws.

Only other thing I can think of are distortion artifacts of some sort.  Different core materials in transformers seem to noticeably sound different.  Can a single resistor create any measurable distortion?  (vs what it would be in straight wire)

 

[emrr]

Build 4 of something like BA-2, 2 with carbon, 2 with metal film, and you'll be able to pick the type by ear easily.  This I have done to satisfy my curiosity.  I like qualities of each, yet I'm not losing sleep over it.  In the end I always go metal film in antique preamp rebuilds (full tear down), considering what humidity does to noise in carbon comp over decades of time.  If the preamp sounds fine with the carbon, and it's all in spec, I leave it alone. 

 

[buildafriend]

Ill make this one with metal film resistors for a lower s/n ratio and to save money. The capacitors will be orange drop http://www.mouser.com/ProductDetail/Vishay-Sprague/225P22494YD3/?qs=GkwyZfp1uEEVPepWgSUyy7pI1Jt759DRBaet4eFdw2g%3d

 

[soapfoot]

I would need some scientific evidence before I believed in carbon comp v. metal film.  After all it is just electrons shunting across atoms in the end.  If someone was to show a HF roll-off from carbon comps then I could see that they affected the sound.  Also if they were to show some kind of delay down to some mS then I could see that affecting the sound.  As it is, the increase in noise negates any benefits to my mind.

The sound of different capacitors was eventually put down to losses due to the piezo-electric effect and the rigidity of capacitors was increased accordingly as a result of this work.

best
DaveP

[Only other thing I can think of are distortion artifacts of some sort.  Different core materials in transformers seem to noticeably sound different.  Can a single resistor create any measurable distortion?  (vs what it would be in straight wire)

You guys may enjoy this link. Old enough to be an internet cave painting by now, but often cited.

http://www.geofex.com/article_folders/carbon_comp/carboncomp.htm

 

[DaveP]

Thanks Soapfoot,

That's the first sensible explanation I've heard; as plate resistors they could give 2nd harmonic distortion, due to a poor temperature co-efficient.  At last a scientific explanation!

I would still prefer to use MF and bias the tube for 2H distortion if that was my bag.

Anyway, now we know. 8)

best
DaveP

 

[lassoharp]

That's some interesting info.  It also makes one rethink on what to make of the readings you get when checking old CCs in vintage gear.  Suggesting maybe cases of where reading R with meter may not be telling whole story.  I will make note to check R based on voltage drops vs static readings for HV positions.  Thanks.

 

[gary o]

After reading that article couple years back I built a 2nd point to point LA2A using old carbon resistors same transformers & tubes ....the carbon just sounded somehow nicer, Im not clever enough to understand the technical paper work but I know what my ears like......After I took out all MF resistors from my DIY BA6A after recording a vocal verse n chorus of a song quickly soldered in carbons & again it just has a sweeter sound, I dont care about a little extra noise....

Im wondering if the effect is as audible in a pre with few resistors tho.......I have a DIY BA2A with carbons might have to do as Doug says build a second with metal films

 

[lassoharp]

We also have Carbon Film & Metal Oxide to fit in there somewhere.  I generally use one of those types for all of my builds.

 

[gary o]

After reading that interesting article again Im wondering if BC2B isnt good choice for hearing the carbon resistor effect as it has few resistors and feed back built in......the BA2A has no feedback but looks like only two resistors worth having carbon comps, I think thats what I did with mine , I will pop the lid off and see whats there..... I wonder is a push pull pre more fun for carbon resistor play....

 

[emrr]

You could hear it with either of these. 

 

[gary o]

Would carbon comps in the power supply circuit effect the sound......

 

[abbey road d enfer]

Would carbon comps in the power supply circuit effect the sound......

According to the saying "everything is in the signal path, except maybe the pilot light", yes.
But how?  Considering resistors in PSU are generally inserted between smoothing caps, they could produce a "negative sag", since an increase in current reduces the resistance, then the voltage drop will be less than if the res was constant. Now the effect would be infinitesimal (although measurable); in addition, these resistors are generally powering class A stages, where the current is more or less constant.
In a pentode push-pull amp, though, the screen res may have a microscopic effect.

 

[emrr]

I wouldn't bother there.  If you were after obvious PSU effects, I think there's already way too much capacitance in a BA-2. 

 

[buildafriend]

I'm trying to select an output xfrmr and a power xfrmr

I have ended up here for the output but then i got stuck..: http://cinemag.biz/output/output.html

as for my power xfrmr I have ended up here and feel confident: http://www.alliedelec.com/search/productdetail.aspx?SKU=70008994

http://www.alliedelec.com/images/products/datasheets/bm/Allied_Transformers/70008994.pdf

 

[buildafriend]

Hm, in this new schemo it says to use a 12AV7 on the input but the original had 2 RCA 1620's. Could I trouble one of you guys by asking why this is like this?

Edit.. I think I might see why. the 12av7 is a twin triode where the 1620s are single.

now what route is best.. Hmmm

http://www.ebay.com/itm/RCA-1620-METAL-TUBE-TESTS-GREAT-/261054635518?pt=Vintage_Electronics_R2&hash=item3cc8119dfe#ht_1912wt_943

 

[emrr]

Don't do it!  1620's cost $10 from any reputable tube dealer.  Don't pay a penny more, avoid the ebay hucksters and call a real dealer.