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    Resistor Type and One Other Question

    PeterCapo
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    Post by PeterCapo on Mon Aug 19, 2019 1:16 pm

    Fair enough.

    But I still used the nearly sixty year old, original carbon comp resistors when I refurbished my PAS, and it really sounds remarkably good.
    gktamps
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    Post by gktamps on Mon Aug 19, 2019 1:29 pm

    I have long found the info at www.resistorguide.com to be helpful - check the links Peter W. provided.

    Haven't read this document thoroughly, but it provides a table for baking resistors.

    http://www.rcdcomponents.com/rcd/technical.htm

    Click on Resistor FAQ's.

    Other info on baking I've found online have been anecdotal without much in the way of references to testing.
    PeterCapo
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    Post by PeterCapo on Tue Aug 20, 2019 9:06 am

    Again, I don't dispute the technical differences between different resistor types, as would be manifest and quantified in an R&D laboratory, for instance.  Numbers don't lie - not even with carbon comp resistors.  Meaning, it's worth remembering that classic audio gear originally achieved very good distortion and frequency response figures using carbon comps, for example.
    Bob Latino
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    Post by Bob Latino on Tue Aug 20, 2019 9:37 am

    PeterCapo wrote:Again, I don't dispute the technical differences between different resistor types, as would be manifest and quantified in an R&D laboratory, for instance.  Numbers don't lie - not even with carbon comp resistors.  Meaning, it's worth remembering that classic audio gear originally achieved very good distortion and frequency response figures using carbon comps, for example.

    This is quite true but some carbon composition resistors from the "vintage days" were found after many years to sometimes drift in their value (usually upward). If you have two channels on an amp, it is possible after many years that some of the resistors don't match very well from channel to channel. Older carbon composition can sometimes absorb moisture.

    Check out the Google link below about carbon composition resistor value drift.

    Carbon composition resistor value drift

    Bob

    PeterCapo
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    Post by PeterCapo on Tue Aug 20, 2019 9:44 am

    Yes indeed, and I have brought this point up myself in some of my postings here and there.  Hence, bake-out procedures may be worth trying.  Bake-out or not, all one need do with very old carbon comp resistors is to measure them with a meter to see if they are still within tolerance.  And, carbon comps are still being made today, so brand new ones are available, if desired.
    HiGHFLYiN9
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    Post by HiGHFLYiN9 on Wed Aug 28, 2019 11:04 pm

    I feel like the most important part of the thread is being glossed over. Having resistors burn out is a safety issue and it means there is something amiss with the circuit, potentially the wiring is incorrect, or the value of the parts used do not have enough headroom. It could have something to do with a solid state rectifier putting more stress on the circuit if you are using one. I recommend gathering some values with your multimeter (using proper safety precautions) and share them with Bob/Tubes4HiFi to ensure everything is working at the proper spec. Using a variac is a good safety precaution as others have mentioned. Bob may recommend using higher wattage resistors as a precaution.

    As far as resistor types, everyone has their religion opinion, which is perfectly fine. Most carbon comps drift over the decades (very significantly as I've measured in 60s era gear), but many people (including venerable audio engineers like Pete Millett) use them as grid resistors. They are noisy and technically not good in the signal path, but they have a sweet/warm sound. If you can afford tantalum resistors in the signal path, they are widely considered the best. At $4 to $10 each, it's a bit much for most people. I personally like carbon film for signal and vishay metal film for everything else. For wirewound, Mills non-inductive generally sounds better in audio circuits than something like like metal oxide or sandcast. If you're not particular, you can use vishay metal film for just about everything and it'll work well.

    Best thing to do is just experiment with different resistor types and come up with your own opinion Very Happy
    PeterCapo
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    Post by PeterCapo on Wed Aug 28, 2019 11:38 pm

    HiGHFLYiN9 wrote: ... Most carbon comps drift over the decades (very significantly as I've measured in 60s era gear), but many people (including venerable audio engineers like Pete Millett) use them as grid resistors. They are noisy and technically not good in the signal path, but they have a sweet/warm sound. ...

    Current is low in grid resistors, so noise shouldn't be an issue there.  Even so, designs from the 1950s and 1960s achieved some excellent S/N figures using nearly all carbon composition resistors.  Chief Engineer Kara Chaffee of deHavilland Electric Amplifier Company makes some use of carbon composition resistors and PIO capacitors in her current production amplifier(s).  I understand that Shindo Laboratory amplifiers use NOS parts from years ago.  Wouldn't be surprised if there are others.  Not saying I think carbon composition is the only way to go, but I don't think concerns about them should become disproportionate, either.

    HiGHFLYiN9 wrote: ... Best thing to do is just experiment with different resistor types and come up with your own opinion Very Happy  

    Good idea.
    Bob Latino
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    Post by Bob Latino on Thu Aug 29, 2019 8:53 am

    Check out the Youtube video below on carbon composition resistors. They probably could be used in some high voltage applications but IMHO, they have no place in tube audio where tolerances have to be kept close.

    Bob


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    1973shovel

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    Post by 1973shovel on Thu Aug 29, 2019 11:16 am

    PeterCapo wrote:I remember seeing bake-out procedures in a couple of places online, but I cannot remember where.  Do you know where to find them?

    Here's where I posted the carbon composition resistor baking procedure, taken from an old Allen-Bradley data sheet. (If anyone wants the complete A-B data sheet, feel free to email me). Moisture seems to be the culprit, so I store in-tolerance carbon comps in small, lidded containers, with a small desiccant pack, like the ones which come inserted in a bottle of vitamins, for example.

    https://db.audioasylum.com/mhtml/m.html?forum=tubediy&n=265947&highlight=bake+1973shovel&search_url=%2Fcgi%2Fsearch.mpl

    As to the usefulness of carbon comp resistors, I'm in agreement with Pete Millett, Eli Duttman, PeterCapo, and peterh, with his following quote:

    carbon comp resistors are non inductive, thus they keep their properties way up in frequency. That's why they are the best for grid and screen stoppers. And as such they do not add noise and their exact values are of no concern. wrote:

    The "sound" of different types of resistors aside, carbon comps are the optimum choice for stopper positions, and as peterh also noted, their exact value isn't critical. Start with them in tolerance, and even if they do drift over the years, your electrolytic caps are probably shot by that time anyway, so check and replace (if necessary) the carbon comp stopper resistors when you're replacing the caps. Simple enough.
    PeterCapo
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    Post by PeterCapo on Thu Aug 29, 2019 12:23 pm

    Bob Latino wrote:Check out the Youtube video below on carbon composition resistors. They probably could be used in some high voltage applications but IMHO, they have no place in tube audio where tolerances have to be kept close.

    Bob



    None of this guarantees that carbon composition resistors, old or new, will be so out of tolerance that they are inappropriate for audio.  We don't know how old the resistors in the video are.  The ones he measured look kind of like samples I've seen for sale online that could date as far back as the 1930s - longish bodies and broader color code bands.  And, if they were not stored properly, there’s all the more opportunity for moisture ingress.

    He didn't say anything about baking them out, which is a point that you can’t just ignore.  I'd like to give it a try some day for myself and see what happens.  Also, NOS resistors for sale online are sometimes seen in plastic bags with desiccant packs to keep them dry - a knowledgeable seller should do this.  Knowledgeable DIYers should also do this.

    Some current production CC are comparable to mil-specs, and some places sell NOS mil-spec CC.  Have a look through this manufacturer's datasheet for current production Ohmite carbon composition resistors, which acknowledges and offers advice regarding their idiosyncrasies: https://www.ohmite.com/assets/docs/res_od_of_oa.pdf

    For instance, this Ohmite datasheet includes, along with other advice, the following statement: "Generally, any moisture absorbed during storage will be 'baked out' during the soldering operation. If the product is stored properly the resistance shift during the soldering operation will be minimal, less than 2% or 3%."  This is workable for audio.  Remember also that in the original Dynacos, they were able to cull 1% matched sets of CC resistors for the phase splitter and bias resistors on the PCB.

    To repeat, I am not saying that carbon comp is the only way to go, and I am not denying their idiosyncratic properties for which you’d want to be on the lookout.  But, let's not get so lopsided in our thinking that we regard worst case scenarios as the unavoidable norm. If we viewed coupling capacitors the same way, then we'd also have to heed those who feel that decades old PIO capacitors are always a very poor choice.

    We change-out electrolytics and other old parts because over a long period of time they no longer perform like they did when they were new.  If it would even be necessary, what's wrong with doing the same with CC resistors?  Scroll down through the datasheet for the Ohmite CC resistors.  Like a lot of things in this hobby, one could work through the issues.
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    1973shovel

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    Post by 1973shovel on Thu Aug 29, 2019 1:47 pm

    I just finished watching the video where he shows carbon films and metal films have an increase in inductance over the carbon comps. He says "You're probably going to be using chip resistors, so you wouldn't have to worry about it". We know we're not using chip resistors in our tube amps.

    I also found it rather ironic that the guy's name is "The Radio Mechanic", yet he never mentions that it's wrong to indiscriminately swap out carbon comps for metal films (for example) in RF circuits, if they were originally built using carbon comps. More than a few Dynaco FM-3s have been screwed up by these substitutions. One article I have on parts substitution says, "Pure carbon composition resistors decrease in value much faster than film types in RF circuits, therefore if you replace carbon composition with a film type, in some RF circuitry , there may be a change in circuit operation." If you find an old tube radio, for example, don't just change all the resistors in the RF sections to carbon film or metal film if it was originally designed with carbon composition resistors. The engineers knew the characteristics of carbon comps in an RF (radio frequency) circuit and took that into account when they designed it.

    I happily use carbon film and metal film resistors in my amps, except in the grid and screen stopper positions, where I insist on carbon composition.
    HarryY
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    Post by HarryY on Thu Aug 29, 2019 8:31 pm

    I have a whole bunch of unused carbon comp resistors.

    I should gatherer them up and sell them on ebay.

    cci1492
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    Post by cci1492 on Thu Aug 29, 2019 8:33 pm

    HarryY wrote:I have a whole bunch of unused carbon comp resistors.

    I should gatherer them up and sell them on ebay.


    bake'em first Very Happy
    deepee99
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    Post by deepee99 on Fri Aug 30, 2019 3:13 am

    cci1492 wrote:
    HarryY wrote:I have a whole bunch of unused carbon comp resistors.

    I should gatherer them up and sell them on ebay.


    bake'em first Very Happy
    And say these are of the rare, polarized one-directional design, and don't "blossom" until about 5K hours.
    HarryY
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    Post by HarryY on Sat Aug 31, 2019 12:43 am

    deepee99 wrote:
    cci1492 wrote:
    HarryY wrote:I have a whole bunch of unused carbon comp resistors.

    I should gatherer them up and sell them on ebay


    bake'em first Very Happy
    And say these are of the rare, polarized one-directional design, and don't "blossom" until about 5K hours.

    Plus they have been stored in a temperature controlled environment.

    I will occasionally use one in a test circuit if I don't have a better one on hand.
    But mostly never used them due to their size.


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