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  • Echolette M40 , how to get rid of parasitic oscillation

    Hi everybody,

    i start a new thread here which is a kind of a continuity with the previous one that was title "troubleshooting a M40 with loud hum"(for beginers)

    - thanks to members of this site i managed to remove this hum, it seems that it was due to faulty caps causing a DC leakage (and especially the 0.1uf like C23.)
    -All the electrolytic caps are new exept the four 2nf caps behind the front pannel hooked to the input vol pots.

    - Now the amp is working and have a descent sound, but there is this noise, white noise/parasitic oscillation that i can control with the main volume "lautstärke"
    it's very loud, i can also control the tone of this noise with the trebble&bass pots..
    it kind of sounds like a vintage radio when you're searching stations..

    - Do somebody have some tips to get rid of this parasitic oscilliation noise?

    - i checked the value of a lot of resistor in it and they seem ok
    i already changed the 20k plate load resistor of Rö1&2
    -i hope it is "just" a loud parasitic noise that's not gonna damage the amp..

    - i soldered the 1Meg resistor between the ground & the tip, on the input jack, it has reduced the noise a bit
    but i have lost the crunch..

    in the attached pics this my work so far ( it's not neat, the filtering caps are temporary set)

    cheers Nick

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  • #2
    On a preamp i made, i had a noisy carbon plate resistor on a preamp valve, replacing that with metal film did the trick & the hiss was gone
    Your pics show a lot of old carbon resistors.
    If you pull each preamp valve in turn, see if the hiss is gone when one is removed, if so, one of the plate resistors on that tube may be noisy
    While you are at it, just change all the preamp plate resistors to metal film

    Comment


    • #3
      Hi Mozwell,

      thanks for the tip!, the M40 seems specific, it has five 12AX7 and the four inputs are connected to only two of them,
      i guess there is two more for the second stage where the output+tape recorder in are connected, and the last for phase inverter.
      so when i pull whatever 3th,4th and 5th the unit is dead quiet.
      i already replaced the plate resistors for the two first valves where the 4 inputs are connected,
      last night i changed the 50k plate resist. of valve 5, and now i don't have new resistors in stock for valve 3&4
      i've got a back up unit so i know it's not great but i will take the resistors from there while i order new ones.

      we'll see how it goes..

      cheers

      Comment


      • #4
        Hi again Notea,

        Connect the back lug on all the input sockets you have fitted to the ground wire at the front of the jack.
        When nothing is plugged in , the input to all the tubes/valves should be grounded.
        Noise will also be reduced when bottom cover is fitted.
        Make sure all metal to metal grounding if any - is good.... i.e. where a ground may be carried by chassis bolts
        holding two surfaces together. Pull apart and polish with emery paper..(fine grit sandpaper).
        Make sure base/bottom cover also connects well electrically to chassis.
        Plug in guitar and test.
        Hard to estimate but what is the proportion of noise to guitar signal?
        Is there some way we can hear the noise? i.e. 20 sec audio clip?
        Parasitic oscillation is high pitched squealing well roughly speaking!
        White noise is common from hi gain amps.
        Is "the noise" like a FM tuner off the station or a tv on an unused channel (not many of them these days)?
        A roar of hiss like an audio cloud ? or squealing like when two distant stations interfere with each other
        on medium wave or short wave ?

        Not sure where the original Din sockets got their ground.
        The jack should share the same ground point as the tubes ground.

        Apart from the noise what does the guitar sound like... OK um distorted/clean/weak/strong/pathetic ?

        Comment


        • #5
          Hi Oc!

          Nice to see you again!

          -on pic 1 this is how i connected the input jack sockets as you said (i hope i understood well!)

          -on pic 2 this is how i wired the output jack sockets:
          for the 6 ohms output, i wired as so; the 2 hot wires on the same lug the ground in series with the 15ohms, we can't see on the pic but i removed the unused lugs of this socket in case it could touch something from the top,
          this is correct or should i have kept the 4 lugs and connect them like i did for the 15ohms output?
          On the same pic, the black arrow shows the contact between the PT and chassis, i don't remember if this should be isolated from it with a cardboard joint do you know?.

          -on pic 3 we see the ground spots, i cleaned the first one when i replace the HT socket, and the second one where the OT is mounted to the chassis, there should be a perfect contact here isn'it?

          and now in the audio file attached this is the kind of sound i get, (taken with my phone at 10 cm from the speaker, input 1 vol pot at 5 and master volume at 5, trebble & bass pots at 5 also)
          in fact it's not really parasitic oscillation (maybe i should change the title of the thread)
          you 're right it sounds more like "a FM tuner off the station or a tv on an unused channel"

          Cheers N

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          • #6
            Whats the value of the resistor hanging across the signal input?
            It looks like a 1.2k (1K2) ?.. if so take it off .. turn everything down .. and try again...

            Comment


            • #7
              i removed the 1k resistors across the signal input, the noise is the same,
              i found this weird interaction between the trebble pot (between +1/+2 and master volume at 6-7), if you could hear the attached audio file at 6sec&10sec,
              weird isn'it? Sounds like the Exorcist movie!

              the problem has to be before the master vol cause i can control the sound right?, i already replaced the caps around trebble&bass pots exept the 150pf (c14)
              maybe a dead pot..

              m40tonestack.m4a

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              • #8
                Hi Notea,

                First Bit.

                "I removed the 1k resistors across the signal input, the noise is the same."

                I didn't expect the noise level to change but the proportion of guitar signal/level to noise to increase i.e. the guitar signal
                to become louder compared to the noise.

                Say you have a Fender Guitar with a 250k pot. If you turned that down so there was just 1K across the guitar output jack
                there would be considerably less guitar signal coming out.

                In the original diagram it shows the 1k going to a terminal and not connected to the input.

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                This was to make it easy to connect Line or Mic to that input by simply bridging within the plug without having to open the chassis.

                These days most pa's have mic and line sockets sitting beside each other.

                So I'm surprised you didn't hear any dramatic gain change.

                Perhaps you can elaborate on that before we move on.

                "the problem has to be before the master vol cause i can control the sound right?"

                I'm going to say no!.... more later as have to go out.

                Later:

                Re your questions in post #5 ..can't quite grasp/understand re o/p sockets
                With those black plastic ones cliff or rean etc just run one big thick wire through the front
                connections of all 4,both the switch contact and the sleeve connection and run it to chassis
                ground where the output transformer secondary is grounded.
                The 15 ohm tap goes straight to the tip connection in the 15 ohm socket.
                And all the back contacts for the two 6 ohm sockets can be joined , and connected to the 6 ohm tap.

                The grounding seems ok .. those insulating washers are more to break a magnetic loop as I understand it


                Just noticed this :-

                "I soldered the 1Meg resistor between the ground & the tip, on the input jack, it has reduced the noise a bit
                but i have lost the crunch."

                Well that's possibly reducing the extra noise picked up by the guitar as well as lowering the signal input
                slightly eliminating slight overload or "crunch".

                The existing one meg to ground that's there at the input grid should be sufficient.

                The white noise generator seems to be later in the circuit.

                Your audio clip "m40tonestack.m4a" has a sound commonly known as "motorboating" usually associated
                with instability and sometimes , reversed wiring at the output transformer.

                I think there's a fault in the negative feedback circuit or no neg f/b at all.

                Definition of Negative Feedback = the return of part of an output signal to the input, which is out of phase with it,
                so that amplifier gain is reduced and the output is improved.

                Firstly as you have another unit compare the output transformer wiring of that one to the suspect one.

                It should be as in the schematic diagram with one end grounded and two six ohm (why six?) and one 15 ohm output.
                Running from that six ohm tap is the negative feedback connection leading to a 200pF capacitor and a
                16K resistor in parallel (C18 and R44).
                This leads to Ro4 cathode via a 2k resistor with a low value 47 ohms also to ground.

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                All this needs to be checked , the 100v line output on the transformer you wont need and at the opposite end the transformer should be grounded.
                Measure the 2k and 16k and 47ohm .. It would be easiest to disconnect the wire maybe at c18&R44 or the 6 ohm tap to make the measurements
                otherwise the grounded transformer will affect the 16k reading in circuit.

                On the primary side check all the taps go to the correct places on the corresponding tubes.
                Last edited by oc disorder; 05-15-2016, 09:08 AM.

                Comment


                • #9
                  Hi Oc,

                  step1: my fault,last time when i removed the 1k resistors i didn't have the chance to plug the guitar in, i just listened to the amount of noise that came out of the speaker,
                  now i tried to play a bit and yes indeed the fact of removing those 1 k improved the sound a lot lot more, now i got a louder and crispy sound, it has removed "the veil" from the guitar/amp sound.
                  thank you for sharing the schem I feel stupid i didn't think about looking at the other M40 diagram before, the one with the input jack socket, it just didn't come into my mind..
                  - Now the proportion of guitar sound vs noise have changed, with a guitar sounding louder than the noise


                  step 2: i followed the OT secondary wires of the spare unit and compared to mine, i didn't notice any differences,
                  in the attached pic this is how the secondary is wired to the output jack sockets in my working unit


                  step 3: i rechecked the ground, found one loose bolt, resolder the 6 ohms tap on the OT secondary and applied fresh solder on it, switched on
                  -it seems that the "motorboating" effect disappeared but it didn't affect the noise level which is still loud.

                  note1: in both units (the working and the spare ones) the 16k (R44) is in fact a 33k
                  the 200pf caps is an old fashion cap with a resistance in it? cause it's labelled 200k (in the attached pic)
                  note2: the OT is not grounded directly to the chassis but to the grounding bridge where are connected the center tap of the power tube sockets+ the PT ground via the negative leads of the filtering caps and from here a ground wire is going to the chassis.
                  note3: in the jack plug unit diagram, it seems that it is the 15 ohms tap that connects to Rö4 whereas in the older Din plug unit like mine it's the 6 ohms tap


                  thank you Oc for the explanation with diagram, now i understand better the negative feedback, the OT seems to be wired correctly,could it be that the ground bridge which concentrate a lot of different ground leads could carry also parasitic noises, especially when one of the heater leads of a power tube is also connected to this?

                  Cheers

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                  • #10
                    "Now the proportion of guitar sound vs noise have changed, with a guitar sounding louder than the noise"

                    Well that's a good start so I presume you can turn the master down and the pre-amp up and have a usable sound although still noisy? We all make obvious mistakes from time to time and kick ourselves later !

                    I should point out here that with high gain amplifiers you are always going to have "tube" noise.
                    These amplifiers Boogie, Marshall etc have fancy muting circuits hanging off the switching contacts
                    so the amplifier is very quiet with nothing plugged in!
                    In the days of Boston (the band) when soaring distorted guitar sounds formed layers like some crazy orchestra
                    the amount of talk about noise reduction was huge.
                    Fancy power conditioners and all manner of noise gates were the hot topics. It's unfortunate that the low output and high impedance of a guitar and lack of a low noise balanced circuit make it rather a dinosaur of the modern age.

                    But then if guitars changed impedance and connections which wouldn't fit existing pedals and amps but had incredibly low noise coupled with a new system who would be the first to buy them ?

                    All the existing would become like vintage steam driven cars !

                    Les Paul did produce a few low impedance guitars but I gather they are not big sellers!

                    If you went into a music store armed with a loose unterminated jack plug and plugged it into a high gain amps
                    and put all controls on 5 I think you would find a familiar sound "wooooooooosh" but of course you have to pay thousands to get an amp like that ! :-)

                    So bearing in mind that we are always going to have some hiss we can see by experimenting how we can lower it.

                    I'm sure other owners of a M40 have encountered this and perhaps may chime in with their views.

                    You are going down the right path replacing possibly suspect old anode resistors in the pre-amp ,the first ones in the chain most likely to be the culprits!

                    Hopefully by now you have changed the anode resistors for Tube 3.

                    I hear the "white noise whooshing cloud" (supposedly it's electron noise!) and usually when an anode resistor
                    is faulty there's a what I call "spitting noise" in amongst the cloud.

                    Verbal descriptions of sound are often difficult. Doesn't stop the advertisers though!

                    To answer some of your questions..
                    "could it be that the ground bridge which concentrate a lot of different ground leads could carry also parasitic noises, especially when one of the heater leads of a power tube is also connected to this?"
                    As long as all connections are good can't see any problem there in relation to hiss.

                    step 3: i rechecked the ground, found one loose bolt, resolder the 6 ohms tap on the OT secondary and applied fresh solder on it, switched on
                    -it seems that the "motorboating" effect disappeared but it didn't affect the noise level which is still loud.
                    Well , an example of a poor connection creating instability.The noise is coming from the tubes.
                    Some tubes are noiser than others so later it may pay to experiment with different brands , the pre-tested TAD ones I see down here seem to be consistently quiet but I may have come across only good ones!

                    You could try a 12AT7 or 12AU7 in the tube 3 slot as you mentioned the noise disappears when V3 is removed.
                    With all 4 volume controls turned down at the input the noise must be coming from tube 3.
                    Have a look here at tube gain
                    Preamp Tube Gain Factors And Substitution Chart - 300guitars.com

                    If you look at my redrawn schematic it makes sense.

                    One thing you could try here just to satisfy my curiosity .. well if its easy and won't bump into anything else
                    is to remove the wire to the grid of valve 3 and instead wire a 250k resistor to ground and that ground has to be the same point as the cathode ground via resistor R53 2k i.e. where that resistor is grounded also ground the 250k and see if there is any difference in noise.

                    Can you see why I chose 250K?.... 4 x 1m resistors going to 4 500k pots with the wipers turned down to touch ground.

                    It's an equivalent circuit but with a closer?? ground to what is normally there.

                    This is just a test and obviously the inputs are disabled.

                    One other test here while that input wire coming from the 4 x 1megs is loose.. with tube 3 removed tack (lightly temporarily connect)
                    that wire to the input of the next tube for the EQ .. where r23 (200K) is.

                    Now we have removed that 3rd tube from the circuit as if we have erased it from the diagram.

                    The gain should be drastically reduced and the hiss banished !

                    Here I'm trying to get an overview of the amp.... reducing r23 would also reduce the gain and hiss.

                    Return it to how it was if it makes the amp lifeless but may have to alter the gain later.

                    I'll state here that I have never seen one in the flesh and never worked on one.

                    There must be many that have and encountered this before and hopefully they can chime in with some tips.

                    "note2: the OT is not grounded directly to the chassis but to the grounding bridge where are connected the center tap of the power tube sockets+ the PT ground via the negative leads of the filtering caps and from here a ground wire is going to the chassis."

                    That must be their "Star Ground" where the grounds all converge , sometimes resembling a star pattern.

                    That should all be ok.

                    One oddball component that could be replaced as cheap is the small value capacitor from one phase invertor anode to Ground.
                    It's C22 500pF and could be a small round brown disc a ceramic type or like that grey tubelar one in the f/b circuit which is also a ceramic type.
                    These are not known to have problems.i.e. normally no problems but as we are starting to "scratch around" and for the cost - worth changing for a new one, preferably a 500 volt type.

                    "note1: in both units (the working and the spare ones) the 16k (R44) is in fact a 33k
                    the 200pf cap is an old fashion cap with a resistance in it? cause it's labelled 200k (in the attached pic)
                    note3: in the jack plug unit diagram, it seems that it is the 15 ohms tap that connects to Rö4 whereas in the
                    older Din plug unit like mine it's the 6 ohms tap."

                    That 200pF cap is actually a tubular ceramic.. (have seen some in Japanese amps) usually reliable.. some of the numbering systems
                    deviate from our present standards but in this case K is just the symbol for 1000. Ten to the power of 3.(10³) Kilo
                    0.0002uF ,milli 10(-6) ~ 0.2nF nano 10(-9) ~ 200pF pico 10(-12)
                    Usually expressed as uF or microfarads. Maths is a whole new topic...!

                    These are small caps and easily pass high frequencies. Note the value of the bright caps across the volume controls on popular amps.
                    It's often 250pF or 500pF.

                    When working on negative f/b putting that cap there allows more "high frequency" to get through and as its reversed , cancels high frequencies !

                    That 500pF mentioned above on the phase inverter/splitter (it inverts one side and splits the signal into 2 halves for the output stage shunts ,um directs, er um - forces the high frequencies to ground eliminating them.
                    Obviously a single ended output stage (one o/p tube) doesn't need a P.I. .. by the way!

                    With that R44 the smaller the value the more negative feedback can get through thus more cancelling and conversely with a larger value less of the negative f/b can get through thus more gain and noise.

                    Looks like they changed it, increased the R value and changed the tapping !

                    I suggest you experiment with this.. just unsolder one end of the 16k resistor and temp. put the 33k in there ...
                    should make it worse.... :-)
                    Now try moving it to the 15 ohm tap ... better?
                    Now try putting back the 16k there ... too clean ? not enough gain?

                    Try and find the best compromise between noise and guitar sound ... and in the history of guitar amplification you won't be the
                    first or last to do this!

                    There's 3 main types of capacitors with those small values.
                    Polystyrene,ceramic and silver mica.

                    I like the silver mica nice strong leads on them .
                    The polystyrene ones melt with the soldering iron have flimsy leads and have totally confusing different standards for numbering!

                    -------------------------------------------------------------------------------------------------------------------
                    just put this here for ref re Polystyrene (copied from net)

                    "For Styroflex brand caps the color rings give the rated voltage, but the values are different, depending on the

                    capacitor type. See reference books, e.g. Siemens Pocket Book.

                    In general the blue ring is the lowest rating, about 25V =;
                    next is yellow for up to 63V=
                    red ring is for ca 60V~ /100 to160V=;
                    green for 160 to 250V=, and
                    black may stand for200V~/500 to 630V=.

                    Polystyrene Capacitors
                    Most that I have seen do not have coloured bands and usually have the legends located so as to need a mirror to see!
                    There seems to be no consistent marking scheme.

                    LCR Components who made many of the capacitors in the UK, identify with the capacitance in pF and two letter
                    code:-
                    Tolerance
                    F 1pF
                    H 2.5%
                    J 5%
                    K 10%
                    M 20%

                    Voltage
                    Z 30V
                    X 160V
                    V 400V
                    U 630V

                    Hence 470 JX would be 470pF +5% 160Volt dc working.
                    Attached Files
                    Last edited by oc disorder; 05-18-2016, 03:42 AM.

                    Comment


                    • #11
                      Hir Oc,

                      thanks for all, really interesting, i learn a lot!

                      "You could try a 12AT7 or 12AU7 in the tube 3 slot as you mentioned the noise disappears when V3 is removed.
                      With all 4 volume controls turned down at the input the noise must be coming from tube 3."

                      this works, it has reduced the noise!, i had a nos tube that i put in there and actually it improved the sound,
                      i wish i could test the unit in real situation to put it louder.. as soon as i can i will do a better test/description

                      i changed also the 500p with a silver mica cap may be it has contributed to change the noise.

                      I have to study all of this, didn't know about that, swaping caps with different value in this amp stage to improve the sound, brillant!

                      i wanna try also your suggestion with the 250k to bypass the valve it's a great idea, but i need to order new parts,

                      i will try this as soon as i got them!

                      all the best

                      Comment

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