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    Resistor Type and One Other Question

    LeGrace
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    Post by LeGrace on Wed Aug 14, 2019 9:44 am

    Opened up my second M125 to install AB board into it and noticed that the two of the four 100 ohm resistors are charred black! Values test OK but they sure don't look healthy! When I search resistors quite a few different types come up. Can I use metal film or do I need to stick to carbon film? Amazon has some metal film ones with Prime shipping option.

    Another question for anyone using a GZ37 rectifier tube. Have you altered the value of the cap immediately after the rectifier socket? In the M125 it connects to the 40 uF leg of the quad cap but I'm being advised by a couple of credible sources that this value should be changed to 4 uF when substituting in a GZ37, going by the GZ37 data sheet. 40 is fine for a 5AR4. Except I haven't had very good luck with 5AR4 plus I have a fair amount invested in GZ37.
    cci1492
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    Post by cci1492 on Wed Aug 14, 2019 11:18 am

    LeGrace, I'm using a GZ33 I think similar to the 37 and haven't changed anything. Doing the yearly checkup/cleaning on my M125s and all resistors look okay (at least on the first amp, haven't open #2 yet).
    LeGrace
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    Post by LeGrace on Wed Aug 14, 2019 1:56 pm

    Thanks for the replies. The unanswered question for me is what caused these resistors to get burnt like this in the first place? I mean they look like they've been baked in an oven for too long. Is this the likely result of a singular event (ie prior red plate), or could it be a cumulative thing? Anyway I'm going to tighten up my sockets, replace the resistors with new metal film ones, and hope for the best.
    tubes4hifi
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    Post by tubes4hifi on Wed Aug 14, 2019 2:24 pm

    for most audio use you'll want to use metal film resistors, but for resistors connected to grids of tubes,
    the advice is often to use carbon film to prevent any possible oscillation. Do not use carbon comp resistors for anything,
    but the carbon films are fine, and Takman and some others even sell audiophile grade which are excellent.
    Also, for most resistors larger than 1 watt I also spec metal oxide resistors.
    peterh
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    Post by peterh on Wed Aug 14, 2019 3:30 pm

    carbon comp resistors are non inductive, thus they keep their properties way up in frequency. Thats 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.

    Right tool for the task at hand !!
    Bob Latino
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    Post by Bob Latino on Wed Aug 14, 2019 4:49 pm

    I would not use carbon composition resistors in any tube audio gear. With time, carbon composition resistors will usually stray their value UP. After a number of years the values will usually be higher than they were the when the amp was built. Carbon FILM resistors are fine to use in any modern tube amp as are metal film and metal oxide resistors. If you have a vintage two channel stereo tube amp that is 40+ years old and has carbon composition resistors, most likely the resistor sets will NOT match closely from channel to channel after 40 years. This will lead to channel imbalance and degraded sound quality.

    Bob
    PeterCapo
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    Post by PeterCapo on Wed Aug 14, 2019 6:54 pm

    So little love for carbon comp?  I know.  It's the same with some folks over at AK where I have seen it discussed.

    I agree that metal film resistors are generally better, and I hope to find time to build a new PC-5 for my PAS using all metal films.  I used metal film and metal oxide for my dynakitparts.com Stereo 70 in 2013.  But, carbon comp still has a following and, I think, are still viable for audio.  

    As far as decades old carbon comp go, somewhere on the Internet there are procedures for baking them out.  Then, just measure them and see if they are in-tolerance.  I'll bet they will be, unless they have been subjected to some sort of unusual physical, electrical or chemical stress.  But, simply desoldering old ones can throw them out of tolerance, probably due to moisture or perhaps other contaminants they have absorbed.  If there is a way to bake them before applying any desoldering heat, there might be a better chance that they will stay in-tolerance.

    My PAS has a 1961 date stamp on the transformer and the original carbon comp resistors were all in-tolerance with only one on the edge of tolerance.  The voltages line up just fine with the chart in the manual, for both channels.  I think you can get a warmer, richer sound from carbon comp, if that's what you like.  Brand new, current production carbon comps are still being made for anyone who might like to give them a try.
    LeGrace
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    Post by LeGrace on Wed Aug 14, 2019 7:03 pm

    I just learned a lesson, never to be forgotten. With amp powered I put my finger on one of the non charred resistors, it was cool to the touch. Then I put my finger on one of the charred ones to see if it was getting hot. I ended up on the other side of the room with smoke coming out of my hair. A slight exaggeration, but not by a lot, I got a wicked shock. I will never ever physically touch anything inside these amps with the power on ever again!!!!!
    PeterCapo
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    Post by PeterCapo on Wed Aug 14, 2019 7:07 pm

    Glad you are still here to tell the tale.

    There is actually more to safe work practices around these things than simply not poking around inside with your finger. I do hope you will conduct some independent research online.
    LeGrace
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    Post by LeGrace on Wed Aug 14, 2019 7:13 pm

    PeterCapo wrote:Glad you are still here to tell the tale.

    Thanks, I am emotional right now about how close I might have come to electrocuting myself. I feel like a prime candidate for a Darwin award.
    PeterCapo
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    Post by PeterCapo on Wed Aug 14, 2019 7:19 pm

    Here's some material for you to look through: https://www.aikenamps.com/index.php/safety-tips-for-working-on-tube-amplifiers

    Don't feel too badly.  Not all that long ago I was being less careful than I should have been and almost got nailed.  Caught it in time, though.

    We should probably always include in these threads some reference to safer work practices.
    HarryY
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    Post by HarryY on Wed Aug 14, 2019 7:28 pm

    500 Volts DC is pretty healthy and worth being very careful around.

    High Voltage is to be respected, it can definitely ruin your day.
    PeterCapo
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    Post by PeterCapo on Thu Aug 15, 2019 11:24 am

    @Peter W. can you provide some links to readily available, current production isolation devices to which you refer?

    cci1492
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    Post by cci1492 on Thu Aug 15, 2019 11:50 am

    As a kid I used to go around the neighborhood on garbage nights to collect TV sets and radios. I learned the same way about reaching in a live tv set and then again on one that wasn't plugged in trying to get a picture tube out. Something you never forget!
    deepee99
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    Post by deepee99 on Thu Aug 15, 2019 12:18 pm

    Peterw,
    Is not a Variac an isolation transformer? Or do you need two separate windings to make an isolation xformer?
    deepee99
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    Post by deepee99 on Thu Aug 15, 2019 12:31 pm

    Gotcher. Variac regrettably has fallen into the vernacular, as have kleenex and ketchup which are also brand names.
    LeGrace
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    Post by LeGrace on Sat Aug 17, 2019 2:24 pm

    Replaced the two blackened resistors. Despite the severe charring amazingly they both still test within a couple ohms of the specified value. Undoubtedly why everything was still sounding fine. Although I can now see why I got the shock. Outer casing is partially vaporized away, exposing the internal circuitry.
    deepee99
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    Post by deepee99 on Sat Aug 17, 2019 2:31 pm

    LeGrace wrote:Replaced the two blackened resistors. Despite the severe charring amazingly they both still test within a couple ohms of the specified value. Undoubtedly why everything was still sounding fine. Although I can now see why I got the shock. Outer casing is partially vaporized away, exposing the internal circuitry.  
    LeGrace, I'm only guessing that you're luck deficient here, but if that's the case, boost a pair of lead-asbestos gloves from your local fire station or firefighter. Or better yet, wear a straightjacket when you're working on powered-up gear Laughing
    peterh
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    Post by peterh on Sat Aug 17, 2019 2:44 pm

    LeGrace wrote:I just learned a lesson, never to be forgotten. With amp powered I put my finger on one of the non charred resistors, it was cool to the touch. Then I put my finger on one of the charred ones to see if it was getting hot. I ended up on the other side of the room with smoke coming out of my hair. A slight exaggeration, but not by a lot, I got a wicked shock. I will never ever physically touch anything inside these amps with the power on ever again!!!!!
    A man has to take some voltages !
    The important thing is that the current is limited ( good isolation) and does not pass the chest area, in addition that no muscles will increase the grip ( poking with finger is ok, grabbing with the hand is not ok)



    deepee99
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    Post by deepee99 on Sat Aug 17, 2019 3:29 pm

    peterh wrote:
    LeGrace wrote:I just learned a lesson, never to be forgotten. With amp powered I put my finger on one of the non charred resistors, it was cool to the touch. Then I put my finger on one of the charred ones to see if it was getting hot. I ended up on the other side of the room with smoke coming out of my hair. A slight exaggeration, but not by a lot, I got a wicked shock. I will never ever physically touch anything inside these amps with the power on ever again!!!!!
    A man has to take some voltages !  
    The important thing is that the current is limited ( good isolation) and does not pass the chest area, in addition that no muscles will increase the grip ( poking with finger is ok, grabbing with the hand is not ok)



    In reality, I was always taught to keep one hand in the back pocket of one's trousers if the gizmo was plugged in. This one-handed approach pretty much prevents you from doing anything beyond wielding the red meter-probe one-handedly. You don't need to move the black (ground) one around so secure it to a good ground first. The are times when you need both hands for both probes (e.g. to measure a resistor or locate a short) but the beauty of that is that the majority of those tests do not require any juice from the amp to measure their values.
    Jewelry is an absolute no-no. Wedding rings and wrist-watches are excellent conductors, plus if you drop a power amp on your ring the tip of the finger will soon develop gangrene.
    gktamps
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    Post by gktamps on Mon Aug 19, 2019 12:40 pm

    And conversely, in guitar amplifier design and construction, selection of carbon comps as plate resistors is often a design choice because of the shot noise and Johnson noise they contribute in that and other locations; this noise is one of the components of a vintage tone.

    It seems the two largest concerns with CC resistors is noise when it isn't wanted in a circuit, and drift, usually from moisture absorption.
    peterh
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    Post by peterh on Mon Aug 19, 2019 12:46 pm

    gktamps wrote:And conversely, in guitar amplifier design and construction, selection of carbon comps as plate resistors is often a design choice because of the shot noise and Johnson noise they contribute in that and other locations; this noise is one of the components of a vintage tone.

    It seems the two largest concerns with CC resistors is noise when it isn't wanted in a circuit, and drift, usually from moisture absorption.
    None of these concerns are applicable when used as stoppers.

    Also; the scary stories about the drifting CC resistors are aggregated. One should not be scared from them when used with care and good judgement. Avoiding high values plate resistors is one such area where other types are better.
    PeterCapo
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    Post by PeterCapo on Mon Aug 19, 2019 1:03 pm

    Tend to agree with @peterh

    From a strictly technical viewpoint, metal film is probably more stable and rugged under some circumstances, but carbon comp is still generally fine for audio, I would think. IIRC, there is a way in which carbon comp actually outperforms metal film - I think it is in voltage rating or voltage surges/spikes, or something like that. Again, not necessarily something that you'd have to worry about in a tube amp, but it still leaves carbon comp as viable for hi-fi.
    gktamps
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    Post by gktamps on Mon Aug 19, 2019 1:05 pm

    peterh wrote:None of these concerns are applicable when used as stoppers.

    Also; the scary stories about the drifting CC resistors are aggregated. One should not be scared from them when used with care and good judgement. Avoiding high values plate resistors is one such area where other types are better.

    I didn't suggest they were. I gave an example of using them purposely for the effect as plate resistors in guitar amps, just to add some context.

    I also didn't suggest fearing anything, let alone resistors. However, I have thousands of CC resistors that have drifted 20-50% in value. Perhaps they could be baked, but I've decided not to use them because of this. On the other hand, I've got properly stored CC resistors that are still pretty close in tolerance.
    PeterCapo
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    Post by PeterCapo on Mon Aug 19, 2019 1:09 pm

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

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