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Roland JC 200 & Bugera G5 Infinium

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  • Roland JC 200 & Bugera G5 Infinium

    Hello again,
    Has anyone got any details or schematic on one of these Bugera G5 Infiniums ?
    I thought they were another single ended tube amp but no... they appear (without pulling it apart
    and pouring over it for hours ) to have a pair of 600v 10A Mosfets - NK60Z in push pull into a tiny
    o/p transformer. They originally had a 3 year warranty which is very generous so I guess they are one
    of the bedroom rock star throw away amps?? A lot of SMD stuff on the board and a 12AX7 & a 12BH7A.
    Also while you're there although I have fixed it now, anyone ever seen a Roland JC 200 service manual
    or schematic ? The guys here say to their knowledge there has never been a PDF made for that model !
    It was basically shorted output transistors & drivers- emitter resistor and some of those little white
    fusible resistors they use. Seems to be happily chorusing away now although at that volume it's more like a tidal wave !
    Last edited by oc disorder; 10-12-2020, 06:17 AM. Reason: added Tube nos

  • #2
    NO WAY you can pull 5W RMS out of a single 12BH7, even less Class A, even less if single ended.

    Just checked the JJ datasheet, pushing them to death and practically in Class B , in push pull, with +V=300V and idle current 2 mA (Class B) they can swing 180V and 28mA for 5W peak so 2.5W RMS

    Now if they are used as *drivers* for a pair of beefy MosFets then the sky is the limit.

    But then it´s not a real "tube amp".

    At least post a gut picture.

    You sure those Mosfets are not some kind of switching supply?

    Or a tube driven Class D amp?

    Is the "tiny transformer" iron or ferrite?

    You keep us in the dark.
    Juan Manuel Fahey


    • #3
      RE: the JC-200, I would figure out when it was made and look at the corresponding JC-120 schematic (note there are many variants of the JC-120). I would guess that there are a lot of similarities. Then again Roland is known for doing some wacky stuff.


      • #4
        Juan , no way was I suggesting that the 5 watts was extracted from a 12BH7 but you must admit at first glance at the outside of the chassis I could be forgiven for assuming it was a single ended amp possibly with a EL84.Seeing as you put it so nicely and I'm aware of the help you give to others on this and other forums and in the interest of science etc etc I have stayed up way past my bed time to take edit and send some photos ! I would think it is an iron transformer and after de-gluing the primary plug I did find a discrepancy between the outer windings and what I assume is the center tap. i.e. it has been run too hard without a load and the windings are shorted on one side. Really I was just putting my toe in the water to see if anyone happened to have a schematic as this is the place to ask and wasn't really expecting the Argentinian Inquisition ! I have inquired through the local dealers re a replacement transformer .. but I'm not holding my breath ! No of course its not a real tube amp but the marketing suggests otherwise and many a bedroom guitarist is probably quite happy with it until he can afford a Marshall Major !

        And Glebert the power amp design is very close to the attached schematic extract but the effects board seems to be a completely different layout with the MN3004 chip.
        It has beefier 2SD608/2SB628 drivers with T03 2SD555 (250v 10A) output transistors.Most of the diagrams seem to show pin 1 on the MN 3004 bb chip as the supply. On the data sheet it's pin 11.
        Click image for larger version  Name:	image_52098.jpg Views:	5 Size:	1.03 MB ID:	915630 Click image for larger version  Name:	image_52099.jpg Views:	5 Size:	540.5 KB ID:	915631 Click image for larger version  Name:	image_52100.jpg Views:	5 Size:	136.8 KB ID:	915632 Click image for larger version  Name:	JC 120-160 .jpg Views:	0 Size:	183.8 KB ID:	915634
        Last edited by oc disorder; 10-13-2020, 12:39 AM.


        • #5
          Just since it seems to worry you

          * not claiming you said anything, but commenting on what Bugera says and strongly hints all over the place, many times. Such as:
          5 beautiful, tube-soaked, Class-A Watts ... true Class-A amp head with an ECC83 front-end and a 12BH7 power stage. A masterfully crafted push-pull tube amp ....
          so please don´t get defensive when nobody´s attacking you. Quite the contrary.

          wasn't really expecting the Argentinian Inquisition ! I
          ALL are TECHNICAL questions trying to get some useful data to answer this riddle.
          Read them again.

          * back to the Tech area: still suspect 12BH7 directly drives those MosFets, there´s many ways to do that.

          FWIW they talk BS about
          Revolutionary INFINIUM Tube Life Multiplier ... our INFINIUM Tube Life Multiplier Technology can extend the usable lifespan of your amplifier’s expensive power tubes by up to 20 times
          WHICH power tubes? None there, expensive or otherwise. Pure crap.
          This revolutionary circuit automatically and continuously monitors the performance of each output tube, and dynamically drives it towards the target operating point, for an evenly distributed load.
          Bullshit, I wasted time downloading the datasheet and designing a PP power stage around them, optimized for maximum power and nothing else, posted physically possible voltage and current swing.
          Waste of time.

          INFINIUM also compensates for the effects of aging, automatically adjusting current levels to keep your tubes sounding as good as new – even when AC line voltages drop below nominal levels.
          Plain lie, period.
          Just some crap trying to "explain" presence of power Mosfets in the power amp and no power tubes, when inevitably some tyro opens it up and posts what he finds in some Forum.

          I would think it is an iron transformer
          NOW, SEEING it, I think so too.
          But based on this VERBAL description:
          a pair of 600v 10A Mosfets - NK60Z in push pull into a tiny o/p transformer.
          I´d first think some kind of SMPS rather than a "5W Class A Tube amp" any day of the week.

          I did find a discrepancy between the outer windings and what I assume is the center tap. i.e. it has been run too hard without a load and the windings are shorted on one side.
          What does "outer windings" mean? .... primary? .... secondary? .... (yes, some extra "inquisition")
          What does discrepancy mean?
          If resistive difference, please say so and post measured values.
          the windings are shorted on one side
          How would you know that?

          Juan Manuel Fahey


          • #6
            Thank's again for the response Juan. I'm trying not to get too worried. Excuse the Australian sense of humour infected with some Monty Python wit !
            I was just inquiring about a schematic.When I first got the amp to look at I fed a sine wave into it and noticed that half of it was chopped off when observed on a scope
            running it into a dummy load. I can't now remember which I sent a message back to the owner which took ages inquiring if he still had the receipt and was it still under the 3 year warranty..
            It looks to be in pristine condition as one would expect from mega bedroom concerts! I measured the primary of the output transformer from the centertap to the outer winding's (had to do it again just now.. ) 465 ohms 0 350 ohms .I concluded that the 350 ohm side was too low and caused (from distant memory) the negative half of the sine wave to be virtually non existent .
            I have contacted the agents and of course they don't have a replacement and dont know if they can get one etc. I know they bullshit in the adds but it seems we are surrounded by this... just look at some world leaders and the fact that Samsung can buy out JBL,AKG, Soundcraft, Studer , Infinity,Mark Levinson, Revel , Harman Kardon, DigiTech etc. I really don't want to spend more time on this than I have to. Cheers


            • #7
              Many OT's are wound one winding over the other, so primary resistance is different from each side to center tap because one side has more wire. Resistance is different than impedance. I doubt the OT is your problem. It's more likely half of your push pull outptut section is not conducting. I'd start by comparing/checking voltages on your output transistors.
              "Yeah, well, you know, that's just, like, your opinion, man."


              • #8
                Thanks for that, I'm aware of that phenomena but I thought 115 ohms dc resistance was proportionally a bit more than it should be.. I did at the time do a quick once over and came to the conclusion
                that without the transformer both "sides" seemed to be the same. I know that without a load sometimes I could get fooled .. but I have other things to do that I do have a working knowledge of and the job really isn't high on my priority list at the moment. I haven't even removed the board and am quite happy to hand it back at no charge. Except now more time has been spent on it than I envisaged.

                I know impedance and inductance are not the same thing well they are spelt differently for a start but on my incredibly cheap Chinese TC1 multi-tester (see link below)
                I got 10,3 henrys and 31.2 henrys ! Seems a henry too far.... !


                STOP PRESS ! Suggestions in other forums say it is a copy of a Blackstar HT5 .The HT5 schematic doesn't show the symbol for the 12BH7 or the wiring to the primary of the o/p trans.

                Last edited by oc disorder; 10-13-2020, 03:54 AM.


                • #9
                  I hope that Bugera did not copy that goofy mosfet phase inverter from Blackstar.

                  I will say that Blackstar did finally change it (though a schematic is not available)
                  Attached Files


                  • #10
                    Originally posted by oc disorder View Post
                    STOP PRESS ! Suggestions in other forums say it is a copy of a Blackstar HT5 .The HT5 schematic doesn't show the symbol for the 12BH7 or the wiring to the primary of the o/p trans.[/URL]
                    It does show the power supply on pg.2 though. There is a point (SP6) that it says connects to the OT primary. The other side of OT primary would go to power tube plate. The rest of the pins of the power tube are called out at CON1 on the same page.

                    "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


                    • #11
                      The OT is drawn a little clearer on the HT5C that I posted.


                      • #12
                        Thanks guys looks like I'm going to have to look a little closer but I still have that gut feeling its the o/p transformer. This is starting to become bigger than Ben Hur !
                        There are two .1uF 400v caps between the 12Bh7 and the mosfets. I'm not normally a betting man but I'm starting to believe the rumors on the other forums !


                        • #13
                          Are the MOSFETs you talk about T16 and T17? I have a hunch they handle very limited power because - and this is the important detail- without heatsinking the power they can dissipate reliably is not much. They sure look like drivers to me.


                          • #14
                            Bugera G5 Infinium

                            Attached Files


                            • #15
                              Thanks for the scheme.


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