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  • It always helps to have information.

    The resistor divider network that consists of R7 (470 ohm) & R8 (6800 ohm) is not a mod, it is the factory circuit.

    The schematic indicates -51 Vdc coming out of the divider.
    So if you want it adjustable you have to change one of the R's.
    I chose R8 so if the adjustment pot happens to fail open, the bias voltage will still be valid to keep the tubes from redplating.

    As mains voltages are higher now, you will have to tweak the circuit elements.
    Measure the actual raw bias voltage, as the voltage that I used is what it would be with the known variables.
    (-51Vdc, 470 & 6.8K) Reenter the measured voltage, with the 470 & 6.8K R's in the provided link & see if it agrees with the actual bias voltage that the amp currently has.

    That said, if you lower the value of R8 to 4.7K & add a 5K potentiometer in series with it (it does not matter which component goes to ground) you will get a small adjustment range.
    A little hotter (less negative Vdc) or a little colder (more negative Vdc).


    Click image for larger version

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    Click image for larger version

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    Here is the link that I used: http://www.raltron.com/cust/tools/voltage_divider.asp
    Last edited by Jazz P Bass; 01-04-2015, 08:42 PM.

    Comment


    • thanks a lot guys for your precious inputs, my son played it and sound is incredible compared to his usual cube60 !

      there is a little blue light "band" on one tube when it's ON, don't know if it's normal.

      seems the message update alert is not working for me...

      best wishes,
      Phil
      Last edited by Bolt60; 01-06-2015, 10:42 PM.

      Comment


      • I'm checking a Bolt 60 and would like to alert (if it's needed for other people as me)
        that your standby switch board mod schematics is ONLY for Bolt 30 !
        as on Bolt 60 there is R1 resistance so the wiring has to be changed here is mine
        The standby switch modification which allows it to switch all the HT (high voltage supply)
        off instead of just the anode supply via the output transformer center tap is suitable for
        the Bolt 30 and the Bolt 60.

        The resistor you mentioned is part of a pair found in the Bolt 60 which simply go across
        the two main filter capacitors serving two functions.
        One is to help "balance out" any differences between the two caps to help share equally
        the smoothing function... and the other ..they bleed the high voltage away when the amp is turned off.

        They are 2 x 100uF 315volts in series which is equivalent to 50uF 630volts.

        The Bolt 30 which incidentally has smaller power and output transformers only has one filter capacitor, a 47uF 500volt one and doesn't need those resistors although having a bleed resistor wouldn't hurt as the HT would be discharged after turning off the amp and wouldn't have any nasty shocks waiting for the unsuspecting tech !

        The Bolt 60 has a HT of 490 volts and the Bolt 30 has an HT of 400 volts according to the figures
        written on the schematics provided.

        The other windings are the same that supply the bias and the pre-amp.

        I'm looking for adjustable bias mod and did not find something usable to me (Roland Bolt 60 pdf is not detailed about it)
        may someone know where to find proven Bolt 60 adjustable bias mod to start with ?
        J P Bass has covered this , I think there may not be quite enough negative voltage
        to begin with . Ideally it would be nice to have a range of say about 45 volts to about 58 volts.
        Attaching the diagram re-drawn.

        Click image for larger version

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        I actually doubt there will be - 51 volts bias there anyway , the cap C6 is only rated for 50 volts
        and the other half of the winding only has to supply 47 volts for the preamp.
        That figure was hand written on the schematic provided.

        I think a new bias circuit would have to be built to achieve a higher voltage but first we need to know
        the voltage between the junction of (D1, D2 , R7) and ground.

        If the amp is sounding good I would leave the original bias circuit intact unless there is a serious problem.

        Yes, maybe a standby switch should be called "The SuperQuick Power Switch II "

        The "Blue Glow" as I understand it is a result of impurities in the vacuum phosphorescing and is of no concern.
        Last edited by oc disorder; 01-09-2015, 07:04 PM.

        Comment


        • Originally posted by oc disorder View Post
          The standby switch modification which allows it to switch all the HT (high voltage supply)
          off instead of just the anode supply via the output transformer center tap is suitable for
          the Bolt 30 and the Bolt 60.

          The resistor you mentioned is part of a pair found in the Bolt 60 which simply go across
          the two main filter capacitors serving two functions.
          One is to help "balance out" any differences between the two caps to help share equally
          the smoothing function... and the other ..they bleed the high voltage away when the amp is turned off.
          Hi,

          thanks, my comments was because seems to me on the BOLT60 the switch, as it was mod placed here, will not be on the same place as on the BOLT 30.
          As it was here, on the BOLT60 the switch would have been between C1 and R1, so i made a cut change to keep it as on the design (between R1 and pin 7)
          i suppose there is no problem if switch is between C1 and R1, in fact seems to me it should be more energy efficient (no bleeding in standby)
          Originally posted by oc disorder View Post
          They are 2 x 100uF 315volts in series which is equivalent to 50uF 630volts.
          Yes i don't know if they have 35 years as the amp so i'm actually looking for long life (+5000h) capacitor replacement;
          wich is surprisingly not so easy task for axial ones;
          my friend is a pro (he plays near every day) and so this amp need new long life components.

          Originally posted by oc disorder View Post
          ....
          ....
          J P Bass has covered this , I think there may not be quite enough negative voltage
          to begin with . Ideally it would be nice to have a range of say about 45 volts to about 58 volts.
          Attaching the diagram re-drawn.

          [ATTACH=CONFIG]32282[/ATTACH]

          I actually doubt there will be - 51 volts bias there anyway , the cap C6 is only rated for 50 volts
          and the other half of the winding only has to supply 47 volts for the preamp.
          That figure was hand written on the schematic provided.

          I think a new bias circuit would have to be built to achieve a higher voltage but first we need to know
          the voltage between the junction of (D1, D2 , R7) and ground.

          If the amp is sounding good I would leave the original bias circuit intact unless there is a serious problem.
          copy that ! thanks for such precious information.

          Originally posted by oc disorder View Post
          Yes, maybe a standby switch should be called "The SuperQuick Power Switch II "
          I fully agree, off should be "wait here 20s if i'm cold" and ON should be "then you can play"

          Originally posted by oc disorder View Post
          The "Blue Glow" as I understand it is a result of impurities in the vacuum phosphorescing and is of no concern.
          thanks I don't need to worry any more, this light is beautiful to me !


          As i told upper my friend is a pro (he plays near every day and love this amp so... don't use others)
          so now the overdrive part is already down, turning volume doesn't even make the led going red anymore...
          though he still plays it without the overdrive hi lo gain part
          but >sometime< even with no guitar connected, there is a big hummm, near lound as guitar sound if connected (these explanation so...)
          seems to me there is maybe another broken wire moving somewhere.
          as with caps probably time to change all these tiny old wires too
          ...
          he will give it to me again for repair once he is really bored with this as actually he love it so much he still plays it despite problems...

          thanks again
          best wishes
          Phil

          Comment


          • Hello czech-one-2,
            sorry to revive this old thread.

            The old Bolt60 website was mine, I made it around 2003 to help Bolt users get the schematics for these amps, nowadays the schematics are available everywhere but at that time no Bolt schematic was available.
            I lost interest in maintaining the website after I got so many crazy and ridiculous emails over the years and also started to work and have other interests in my life.
            A lot of people were looking for mods that "improved", but I really regretted to have ever published other peoples mods on that website because some they didn't improve anything and were just based on personal/subjective opinions of the authors, some people took it for facts and destroyed their amplifiers trying to change it into something they're were not.

            Personally I think the Bolt series are great sounding amps they way they are, great clean sound, good EQ and good reverb. And if someone doesn't like the overdrive channel, that's understandable just don't use it, I never used it, there's millions of cheap pedals for overdrive or distortion that will sound great through this amp.
            But changing the circuit to try to improve the sound thats already good will not improve anything will just change. Change is different than improvement.
            Besides changing the standby switch position and installing a Bias Pot , I don't advise on any other mod.

            After so many years I don't know if you still have the amp or if you were able to solve the Hum problem, but I will write here for future reference.
            I know these amplifiers quite well
            The noise you were complaining about is probably from Electromagnetic Interference from the Power Transformer into the Preamp PCB.

            Click image for larger version

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            I also spend a lot of time with a Bolt 60 amp trying to discover the source of hum, I wired all the heaters with new flying twisted heater wires, I moved and changed the wiring arrangement and nothing improved the hum.
            I discovered the problem while using a sheet of transformer shield and tried it in different areas of the amp.
            this is the sheet I used:

            https://www.don-audio.com/trafo-shield_3

            When I put this sheet between the Power transformer and the Preamp PCB end the hum went away.
            So my advice is to put a metal plate with trafo shielding riveted to it where the red line is in the photo:
            if you put want to try it before installing anything put a piece of metal in the diagonal area of the line and you will hear the hum being reduced a lot

            Click image for larger version

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            Let me tell you that I have the Bolt amp for almost 20 years, the hum was never a problem in a Practise room with a band or in a live situation, even not a problem in recording, so it' I notice it more when I'm playing at home. I just wanted to take care of it because I know it's there, most of my friends didn't notice it.

            Comment


            • A detail about these amplifiers is that transistors 1 to 8 are not 2SK233 as indicated in the schematic, but 2SK223, already discontinued. Yesterday looking for a substitute for a defective one (firecracker noises) I found an equivalent: a 2N7000 (same pinout) that worked perfectly in a Bolt 30.

              Comment


              • Originally posted by Pedro Vecino View Post
                A detail about these amplifiers is that transistors 1 to 8 are not 2SK233 as indicated in the schematic, but 2SK223, already discontinued. Yesterday looking for a substitute for a defective one (firecracker noises) I found an equivalent: a 2N7000 (same pinout) that worked perfectly in a Bolt 30.
                you can also use PN4393

                Comment


                • hey there, all!

                  thank you all so much for this great topic ... what a ressource! thank you

                  I've got a used bolt-60 as well, and there was big static buzz. 50Hz (cycle in austria)

                  I found the culprit to be one of the orange cables, from the to terminal 3 on the power board. as soon as I pushed it down to the bottom of the chassis, below the surface of the pre-amp board, the buzz got reduced by 70% ... it's still there a little, little, but maybe there is also someplace else. and it's exactly the same, when plugging into the main out, no buzz at all. just plain nice tube power amping.

                  well, the thing is, the effect loop might be another problem. i won't need it, with its crappy switch, and switching it changes the volume, without anything connected to the effect loop. in the "post vol" setting the hum almost vanishes, but the volume is also a little lower ... less amplification.

                  I would like to get out the whole effect loop ! circuit. is that possible?? to just disconnect all the effect loop cables from the PCB terminals, unsolder the switch and off you go, without the bad stuff originating from this crappy design?

                  or will this block and sabotage the whole preamp signal chain??


                  thank you so much! you've given me back the bolt-60. was quite dissapointed when turning it on at home. had to change the power tubes, but that wouldn't matter. but the descriptions here also drastically helped with the buzzzzzz. the 12at7 is still the original RCA ... and working just fine. ; )

                  Comment


                  • well
                    I've just ditched the whole switch ... simply not robust enough for a guitar amp / stage material .... (?! ROLAND)

                    does anyone here have any clue on what contacts on the PCB I need to jump with a wire, to get the settings (I would simply leave it fixed in one setting, any recommendations?)?



                    thank you so much for any help ... this would be a great step in keeping this guy running ... what a great amp !

                    thanks

                    Comment


                    • schematic attached. Unfortunately, I don't think it's good enough quality for what is needed.
                      Attached Files
                      "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

                      Comment


                      • hey g1

                        thank you very much indeed

                        great ... more and enough hi-res than what I've found so far ...

                        I will try to understand the standby switch mod ... that bugs me a little.

                        but it seems my bolt-60 is from 82. maybe the standby flaw was solved by then? do you know details on that...

                        thank!!! you


                        Last edited by meiko; 12-02-2020, 04:20 PM.

                        Comment


                        • mh. reading about the standby mod, it's simply very confusing for an electronics amateur like me ...

                          what exactly was the problem with rolands bad design?

                          what is the harm if I keep it like that?

                          how could I recognize if my bolt-60 from 82 or later maybe has this problem already solved?


                          thank you !!

                          Comment

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