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Thread: Replacement Caps

  1. #1
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    Replacement Caps

    Hello there,
    So I have this vintage Canadian amp here, no schematic, and I'm replacing a number of caps that have drifted way high in terms of capacitance measurement. Photos below. I'm not sure what to replace these with. For starters, both the red ones and the gray ones have power ratings of 600 WV. I assume WV means "watt-volt". For my purposes can I just read that as 600 V?

    The gray ones appear to be electrolytic but have a relatively low capacitance (.05uF) and no clear indication of polarity, unless the silver on one end indicates negative and the black on the other end indicates positive. I usually order caps from Digikey and the lowest value electrolytic I can even search for is 0.1uF. They are between mains power (on/off switch, fuse) and chassis ground.

    The red ones have even lower values (.005uF to .25uF) and have a black stripe on one end to indicate polarity. But does that mean they are necessarily electrolytic? Here I run into the same problem of being unsure whether it's even possible to source electrolytic caps with such low values.

    Can anyone advise on suitable replacements?

    img_1442.jpgimg_1443.jpg

  2. #2
    Supporting Member mozz's Avatar
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    Mouser should have those, maybe .05 is called 50 nanofarads 50nf. WV should be working voltage.
    Last edited by mozz; 12-01-2017 at 10:30 PM.

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    Senior Member Enzo's Avatar
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    Yes, WV - working voltage.

    The gray ones are not electrolytics. Note they are connected between ground and the incoming 120vAC. They are mains rated film caps.

    The red ones are also film caps and have no polarity. The black stripe indicates "outer foil". Outer foil really doesn;t matter to you, though I am sure we can start an argument over that.

    0.05uf is 50nf, but Mouser will be happy using 0.05uf.

    0.1uf is a common standard value. 0.05 is a rather dated value. There are still 0.05 in the catalog, but you will find more hits using 0.047, which is a standard value.
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    Quote Originally Posted by Enzo View Post
    Yes, WV - working voltage.

    The gray ones are not electrolytics. Note they are connected between ground and the incoming 120vAC. They are mains rated film caps.

    The red ones are also film caps and have no polarity. The black stripe indicates "outer foil". Outer foil really doesn;t matter to you, though I am sure we can start an argument over that.

    0.05uf is 50nf, but Mouser will be happy using 0.05uf.

    0.1uf is a common standard value. 0.05 is a rather dated value. There are still 0.05 in the catalog, but you will find more hits using 0.047, which is a standard value.
    Ok, thanks Mozz and Enzo, that helps a lot to know that they are not actually polarized electrolytic caps. I thought they might be because the gray ones kind of look like electrolytics and the red ones SEEMED to have a polarity marking, I wasn't aware of the "outer foil" previously. Also the fact that all of them measured quite high, I thought that might be an indication of dried out electrolytics. Good to know for now and future reference. Thanks for clarifying the meaning of "WV" as well.

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    Older caps do measure high and they had pretty slack tolerances from new. Any cap attached to the mains is worth replacing with a modern (safer) class cap. Signal and other non-electrolytic caps I leave alone unless they're leaking DC or show outward signs of deterioration.

  6. #6
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    If the caps are going to mains, they should likely be removed with a 3-prong AC conversion anyway.
    I'm hoping this is not a 'widow maker' type amp. What is the tube complement?
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    Supporting Member Jazz P Bass's Avatar
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    "both the red ones and the gray ones "

    The grey capacitors should be removed.
    One clearly goes to the mains fuse holder & the other appears to go to a 'polarity reversal' switch.

    Need a better pic of the red caps.
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    Quote Originally Posted by Mick Bailey View Post
    Older caps do measure high and they had pretty slack tolerances from new. Signal and other non-electrolytic caps I leave alone unless they're leaking DC or show outward signs of deterioration.
    When I say high, I mean quite high. For example the one going from the fuse to ground is labeled .05uF and measures .5uF. There are 4 of those red caps in the amp and measure as follows:

    1. spec: .25uF act: 1 uF
    2. spec: .033uF act: .09uF
    3. spec: .1uF act: .68uF
    4. spec: .005 act: .037uF

    You wouldn't change caps measuring that far out spec as a matter of course?

    Quote Originally Posted by Mick Bailey View Post
    Any cap attached to the mains is worth replacing with a modern (safer) class cap.
    So a modern cap is safer by virtue of modern safety standards or do I need to look for specific safety ratings?

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    Quote Originally Posted by g1 View Post
    If the caps are going to mains, they should likely be removed with a 3-prong AC conversion anyway.
    I'm hoping this is not a 'widow maker' type amp. What is the tube complement?
    Yes, I am planning to install a 3 prong cord. So those two caps are not needed? what was their original purpose? Not a widow maker, it has a power transformer. Tubes are 5Y3, 6AT6, 12AU7, 12AX7, 2 x 6BQ5 (EL84).

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    Quote Originally Posted by Jazz P Bass View Post
    ... the other appears to go to a 'polarity reversal' switch.
    For what it's worth, it goes to the on/off switch.

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    The caps intended for mains use are Class X (line-to-line) and Class Y (line-to-ground). Most modern power supplies use them for EMI/RFI filtering and switching noise suppression. In one sense 'death caps' secretly made a comeback with SMPS supplies, though those classification of caps fail safe. Even some recent non-SMPS amps have a filter section comprising one or more common-mode inductors and a number of safety-rated noise suppression caps. More common perhaps in consumer audio gear where immunity from noise is as important as reducing noise fed back into the mains.

    I always remove death caps and install a 3-prong lead. Here in the UK they had no functional use and usually were not installed at manufacture, but I see plenty of imported US gear that still has them in place. Where there are other suppression caps (across switch contacts, for example) I replace these with a Class X cap.

    Otherwise, as far as cap values go, unless the value has particular significance I leave it alone. There are very few places in a tube amp where the cap value has any impact. Otherwise I'd be doing wholesale cap replacements in almost every 50-odd year old amp that comes my way. Particularly some AC30s where not a single component reads anywhere near what it's supposed to be.

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    Senior Member Enzo's Avatar
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    Are you measuring these caps while they are still wired into the circuit? That will sure alter readings.

    ANything is possible, but I have a hard time believing a film cap has gone up to seven times its original value.
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    Quote Originally Posted by Enzo View Post
    Are you measuring these caps while they are still wired into the circuit? That will sure alter readings.

    ANything is possible, but I have a hard time believing a film cap has gone up to seven times its original value.
    Out of circuit.

    img_1444.jpg img_1445.jpg
    Last edited by bobloblaws; 12-02-2017 at 04:06 AM.

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    Quote Originally Posted by Mick Bailey View Post
    The caps intended for mains use are Class X (line-to-line) and Class Y (line-to-ground). Most modern power supplies use them for EMI/RFI filtering and switching noise suppression. In one sense 'death caps' secretly made a comeback with SMPS supplies, though those classification of caps fail safe. Even some recent non-SMPS amps have a filter section comprising one or more common-mode inductors and a number of safety-rated noise suppression caps. More common perhaps in consumer audio gear where immunity from noise is as important as reducing noise fed back into the mains.

    I always remove death caps and install a 3-prong lead. Here in the UK they had no functional use and usually were not installed at manufacture, but I see plenty of imported US gear that still has them in place. Where there are other suppression caps (across switch contacts, for example) I replace these with a Class X cap.
    OK, so if I understand you correctly I can just remove those two caps (and I SHOULD remove them anyway since they are death caps) because grounding the mains will (more or less) eliminate noise that the caps were installed to suppress.

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    The original death cap was intended to provide an AC chassis ground reference for a mains plug that could be inserted either way round. It was usually wired to a switch that would flip one end of the cap to either leg of the mains. Some amps had them hard wired and if the amp buzzed you unplugged it from the wall and inserted the plug the other way round.

    If the cap was referencing the live side of the supply and shorted out, the chassis would be live. In any case if a death cap leaks you can get a floating voltage on the chassis. Even if it doesn't leak, there will still be a residual voltage because of the reactance at 60hz.

    With a 3-prong supply these are redundant, and in any case not a good idea nowadays. I'd remove them.
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  16. #16
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    Quote Originally Posted by bobloblaws View Post
    When I say high, I mean quite high. For example the one going from the fuse to ground is labeled .05uF and measures .5uF. There are 4 of those red caps in the amp and measure as follows:

    1. spec: .25uF act: 1 uF
    2. spec: .033uF act: .09uF
    3. spec: .1uF act: .68uF
    4. spec: .005 act: .037uF

    You wouldn't change caps measuring that far out spec as a matter of course?



    So a modern cap is safer by virtue of modern safety standards or do I need to look for specific safety ratings?
    Enzo is right, there is no way a film cap can go 700% high in capacitance, not even 50%, no physical way surface increases that much or film thickness decreses by the same amount, only way to increase capacitance.
    Then how come your meter reads that high?
    Ah ... but your meter is not measuring actual capacitance
    What they actually do is apply some AC, typically 1kHz squarewave at one end of the cap, load the other end with a suitable value resistor, and measure how much AC do they find at the other side.
    Of course, the higher the AC goes through , the higher the capacuitance, and viceversa, so they scale that AC MEASUREMENT by some convenient factor, show it on a screen (or with an analog needle/scale display) and label it as capacitance.
    Not a Lab measurement by any means, but good enough for casual Service Tech needs.

    Problem is, if cap is very lossy, insulation is degraded, etc., it will pass more current than expected which on a poor capacitance meter (whatīs bundled as an extra inside a general purpose multimeter) it will be wrongly read as higher capacitance.

    Two similar "flawed measurement" cases:

    * germanium transistors show very high Hfe , DC current gain.
    Truth is they are passing way more current than expected, not because of high gain but simply because they are very lossy.

    * poor meters trying to read ripple: not true AC meters (expensive) but cheap ones (which can only read DC) with a diode stuck in series and readings multiplied by 2.25 X ... good enough to measure mains or transformer taps but read any DC present as "monster AC".

    Both cases: flawed measurement techniques showing the flaws.
    Juan Manuel Fahey

  17. #17
    Senior Member Enzo's Avatar
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    I have been using meters to read voltage and what have you for over 60 years now, and still now and then I overlook things. I remember once reading a resistor and getting zero ohms. I thought how odd, I never have seen a shorted resistor. Then two more in quick succession.... Oh, meter set for current.

    But far more common is when my 9v battery is down to like 5 volts, but the meter still functions. So I believe it. But the readings can be way off, and it takes a moment to sink in I need a new battery.

    I don't know that it happened to you, but check the meter battery, that can influence readings for sure.


    Juan, I never thought about it deeply, but in my head I had the meter using a different scheme. What I had thought was the cap function puts out a DC current into the test cap, and watches the slope of the charging. Bigger caps charge slower. And ther is some internal algorithm to convert to microfarads.
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    Quote Originally Posted by J M Fahey View Post
    Enzo is right, there is no way a film cap can go 700% high in capacitance, not even 50%, no physical way surface increases that much or film thickness decreses by the same amount, only way to increase capacitance.
    Then how come your meter reads that high?
    Ah ... but your meter is not measuring actual capacitance
    What they actually do is apply some AC, typically 1kHz squarewave at one end of the cap, load the other end with a suitable value resistor, and measure how much AC do they find at the other side.
    Of course, the higher the AC goes through , the higher the capacuitance, and viceversa, so they scale that AC MEASUREMENT by some convenient factor, show it on a screen (or with an analog needle/scale display) and label it as capacitance.
    Not a Lab measurement by any means, but good enough for casual Service Tech needs.

    Problem is, if cap is very lossy, insulation is degraded, etc., it will pass more current than expected which on a poor capacitance meter (whatīs bundled as an extra inside a general purpose multimeter) it will be wrongly read as higher capacitance.

    Two similar "flawed measurement" cases:

    * germanium transistors show very high Hfe , DC current gain.
    Truth is they are passing way more current than expected, not because of high gain but simply because they are very lossy.

    * poor meters trying to read ripple: not true AC meters (expensive) but cheap ones (which can only read DC) with a diode stuck in series and readings multiplied by 2.25 X ... good enough to measure mains or transformer taps but read any DC present as "monster AC".

    Both cases: flawed measurement techniques showing the flaws.
    Well that is good to know, even though it has shaken my faith. Thanks, Jaun.

  19. #19
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    Quote Originally Posted by Enzo View Post
    Juan, I never thought about it deeply, but in my head I had the meter using a different scheme. What I had thought was the cap function puts out a DC current into the test cap, and watches the slope of the charging. Bigger caps charge slower. And ther is some internal algorithm to convert to microfarads.
    Maybe a Fluke does that, and it pressupposes having a $1 stamp sized microprocessor thingie doing the job, think a PIC or similar), so maybe even mid priced meters do it that way today, but that in the last 10 years or less; while the squarewave oscillator (which might well cost 10/20 cent in bulk) could be any CMOS or even TTL gate or couple transistors oscillating plus a diode and a resistor trick has been used since the 70īs .

    And Iīm cheating , I didnīt guess it on my own but actually read old Meter User Manuals explaining it ... when such manuals were new that is

    FWIW even my original Central 200H meter (proudly Made in Japan) and bought new in 1969 could measure Capacitance

    Technique, explained in the User Manual, was to connect suspect capacitor in series with some transformer high voltage tap , measure voltage to ground both straight to tap and through cap (which would be a lower value) and with both check a handy graph printed on the manual, you could get cap value within 20%, close enough for servicing.

    But ... but ... holding a cap against a high voltage tap while fumbling with your meter?

    Well, I guess no Lawyers were allowed to write Tech Manuals way back then.
    Juan Manuel Fahey

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    Old Timer J M Fahey's Avatar
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    Iīm copypasting this PM because it may be useful to somebody else:
    Older AC meter

    Do you have an example of an older AC meter that would qualify for this statement? Hp 400 series voltmeters? or would a Simpson 260 qualify?

    * poor meters trying to read ripple: not true AC meters (expensive) but cheap ones (which can only read DC) with a diode stuck in series and readings multiplied by 2.25 X ... good enough to measure mains or transformer taps but read any DC present as "monster AC".

    Thanks,
    Nosaj
    The statement is 110% true in modern digital $10 meters, which is what 99.9% users are using (including me who save the good ones for nobler purposes) ; real old PRO meters such as HP or Simpson were definitely not in my radar screen (why would I assume they were used unless specifically stated) and might or might not be compensated for that, user manual should be read.

    The middle range ones I used were NOT compensated for that use, and there was a specific input designed to measure Audio (AC) superimposed with strong DC, be it a tube plate or cathode or a transistor collector, no split supply Op Amps way back then either which usually have zero or a few mV DC at the output; such special input was often labelled "Output" to utter confuse Noobs who expected something to come out of it, while true meaning was: "jack to measure Audio Output" of a gain stage.

    And what it did was to add a typically .1uF 400V cap in series with the "regular" AC measuring terminal.

    My beloved Central 200H, most popular cheap multimeter in 1969, on many counts suoperior to current cheap multimeters (and a few mid range ones:

    1) notice the prominent "Output" terminal, which allows it to read AC mixed with strong DC:


    and the reason why it can: a large mean black .1uF x 400V cap:


    2) notice it proudly states it can measure Capacitance, right on the Spec sheet:

    2 wide scales: 10uuF (10pF in oldspeak) to .001uF and .001uF to 1uF ... what else can you find inside a (1969) Radio, TV or Amplifier?

    But ... but ... what about electrolytics?
    Electrolytics? (twist your nose as if you were smelling dog shit) ... who wants to measure those?
    "Everybody" knows they are -50% +100% rated, all you want to know is whether they are open or not.

    3) the icing on the cake: it can measure, straight from factory and without any extra test probe, up to 2500V DC

    Definitely no Lawyers writing that user manual or designing front panel.
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    Supporting Member Chuck H's Avatar
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    My Fluke measures up to 5uf I think. I've used it to check larger electrolytic cap values by placing a 4.7uf cap in series with the test and then doing some math. A little off topic for the actual requirements, but it's a good cheat in a similar vein so I thought to include it.
    Last edited by Chuck H; 12-02-2017 at 09:27 PM.
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    Senior Member Enzo's Avatar
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    Not to take us further away but...

    Are we discussing two things? One could feed a signal through a cap and calculate capacitance from it. I was thinking of meters that could measure capacitance by themselves. I don't recall any meters that spit AC out of themselves into a test cap. Yes the technique using an external signal genny with your meter has been around a long time. But the meter doesn't do it on its own.
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    Quote Originally Posted by Mick Bailey View Post
    Otherwise, as far as cap values go, unless the value has particular significance I leave it alone.
    Mick, can you expound a bit on that? How would I determine which caps have particular significance and which don't? I imagine it's down to experience and knowledge for the most part, but I'd like to be set on the path toward said experience and knowledge

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    I'm not Mick, and he may have other thoughts on the matter, but...

    In general, something like a filter cap might have a nominal value of 20uf. In the old classic days of Fender, it was common for filter caps to have a tolerance of -20%/+80%. That means the thing could measure anywhere from 16uf to 36uf and be good and within spec. The value is not critical in a filter cap. Today caps have a much tighter tolerance, but the fact remains it isn't critical.

    Cathode bypass caps are a close second in many cases. That 25uf cathode cap goes down to way below guitar freqs, so if it measures 20uf instead of 25uf, who cares.

    In the tone stack, the vales are chosen for the frequencies they represent. If you change them much, then your EQ bands will shift around (I mean more than they already do normally). So they are much more significant.

    Something like a bright switch is also more significant. If you wander far off the 100pf that is in the specs, it affects how much "brightness" is applied, subtle sparkle ir shrieking shrill. Same with coupling caps between stages, they affect the highs and lows coming through - the tone.

    Hard and fast rules? No, but those are general tendencies. And it is unlikely a 100pf ceramic cap will measure 300pf or 40pf anyway. Same with film caps. That 0.022uf isn't probably going to be way off. It might get real leaky or short, but that is a different matter.
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