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  • Quote Originally Posted by Edge11
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    not the same thread and/or repost. shouldn't have abandoned the old one or else you'd have been right.



    My Clips
    BLACKSTAR* BRIGADE
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    Way Huge Green Rhino Mk-II OD
    AMT P1
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    Home-made pedal board

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    -Ibanez RG321 w/SD Blackouts

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    • Quote Originally Posted by rugbyguitargod
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      not the same thread and/or repost. shouldn't have abandoned the old one or else you'd have been right.







      Let me bang bro




      http://www.youtube.com/watch?v=rkN8V2Xogsw
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      • Quote Originally Posted by NCarmona
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        Hi!



        I recieved my Jet City JCA2212 a couple of weeks ago. Yesterday I recieved a matched pair of EL844's. I already e-mailed Jet City asking if it was possible to swap EL84 with EL844, EL84M, and all EL84 variants. They told me there would be no problem and that I wouldn't need to adjust bias since it is a fixed bias amp.



        When I instaled the EL844 the amp sounded distorted and even sputty at all volume levels. Even at its maximum, the amp volume was very low (not higher than what I usually get with the master volume at 1,5 - 2). Sometimes the volume would fluctuate. After a couple of minutes of playing I decided to turn off the amp and install the EL84 back again: sounded perfect again.





        My question is: is this a problem with the valves? or do I need a tech to adjust the bias when installing the EL844's?




        I seem to recall the 844s can't really handle higher voltage. Don't really know stick to 84s man.
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        • Hi, all,

          I don't wish to hijack this thread with an off-topic post, but...



          Is anyone here modifying, or is thinking of modifying, a JCA5212RC ? There doesn't seem to be much interest on the internet about these beasts, which I can't understand why?

          The JCA5212RC, I would of thought, is a great platform to start from; it even has a spare 12AX7 triode sitting idle, just waiting to be unleashed.



          If anyone is interested, I would like to hear your thoughts ?



          Cheers

          --Boffo

          Comment








          • Quote Originally Posted by Boffo
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            Hi, all,

            I don't wish to hijack this thread with an off-topic post, but...



            Is anyone here modifying, or is thinking of modifying, a JCA5212RC ? There doesn't seem to be much interest on the internet about these beasts, which I can't understand why?

            The JCA5212RC, I would of thought, is a great platform to start from; it even has a spare 12AX7 triode sitting idle, just waiting to be unleashed.



            If anyone is interested, I would like to hear your thoughts ?



            Cheers

            --Boffo




            Hook it up with a schem.

            To my knowledge it has a clean and the s.l.os crunch channel right?
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            • Quote Originally Posted by Edge11
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              Hook it up with a schem.

              To my knowledge it has a clean and the s.l.os crunch channel right?




              It's supposed to have Soldano's Lucky 13 clean channel. Would love to have that clean channel on the head model!
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              • Quote Originally Posted by Edge11
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                Hook it up with a schem.




                Hi, Edge11

                Do you mean it's OK to post a Schematic on this thread or start a new thread ?

                I do have a schematic of the JCA5212RC, a big thanks to Doug from Jet City Amplification for sending it to me.

                However, I'm not sure if it is OK to publicly post it as I remember years ago Mike Soldano didn't like having his designs freely available on the internet. Have things changed in recent times as I see you have posted the schematics of other Jet City amplifiers (and thanks for that) ?



                Also, I am building up SPICE models of the JCA5212RC to work in LTSpice, so maybe it would OK to post the resultant schematic of that ?








                To my knowledge it has a clean and the s.l.os crunch channel right?



                Yes, it has a "clean" channel and a "crunch" channel, but it is not the Super Lead Overdrive type circuit. One big difference is the exclusion of the "Rk=39k? cold clipper" stage. That's what I am thinking of using the spare triode for to get a SLO channel, or at least close to it.

                The design of the JCA5212RC is very different to any other Soldano design, the closest that I can see is the Yamaha T50W/100W amplifiers.



                Cheers

                --Boffo

                Comment


                • Played around with the JCA5212RC over the weekend; managed to get a "cold clipper" stage working using the spare triode. Sounds great, but now their is a problem with the reverb getting some kind of echo effect. Could be a phasing issue, because the added stage is now 180

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                  • none of those images are legible.
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                    • Quote Originally Posted by Edge11
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                      none of those images are legible.




                      Sorry about that. I'm trying to work out why this forum's software keeps reformatting any images I upload.

                      So please bear with me, will I attach another image; and delete the previous two.

                      The original Jet City drawing is the same as I received it. I believe that it has been deliberately down graded so as it is only useful for people whom actually have the amplifier.



                      EDIT: I give up on trying to post a good quality image. There must be some kind of size and file type limit for new members ?

                      Clicking on the file attachment URL seems to produce a little better quality image.

                      LTspice produces good quality 4bit(16 colour) images; which are also very small in size.

                      I have labelled the component values which are missing from the original schematic.

                      I will see if I can find a third party server to upload the files to.



                      Cheers

                      --Boffo

                      Comment








                      • Quote Originally Posted by Boffo
                        View Post

                        Sorry about that. I'm trying to work out why this forum's software keeps reformatting any images I upload.

                        So please bear with me, will I attach another image; and delete the previous two.

                        The original Jet City drawing is the same as I received it. I believe that it has been deliberately down graded so as it is only useful for people whom actually have the amplifier.



                        EDIT: I give up on trying to post a good quality image. There must be some kind of size and file type limit for new members ?

                        Clicking on the file attachment URL seems to produce a little better quality image.

                        LTspice produces good quality 4bit(16 colour) images; which are also very small in size.

                        I have labelled the component values which are missing from the original schematic.

                        I will see if I can find a third party server to upload the files to.



                        Cheers

                        --Boffo




                        upload the images on to imgur.
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                        • OK will see if this works any better...



                          Caveat : This is a preliminary schematic image that was produced by LTspice; it may contain errors, in fact I know it contains errors. For those whom do not use LTspice, or any other type of simulation software, it may be a tad difficult to understand, because it is drawn for functionality; and, not how a draftsman would normally draft and electrical drawing. It has the benefit of using "node tags" rather than drawing lines all over the schematic that criss-cross each other.

                          Stay tuned; further updates will be posted along with the rest of the circuit showing the clean channel





                          Cheers

                          --Boffo (the not so great)

                          Comment


                          • ...And here is the "Clean channel" :







                            Also, I have attached a parts list so people can match up the component values with the original schematic. Not all the parts listed are actually in the amp, of course, they are imaginary components for modelling purposes so that LTspice can function properly.



                            We should now be able to see how the JCA5212RC differs from all other Jet City amplifiers.

                            First major difference is the input stage; it has a 100k anode load resistor (instead of the usual 220k). The reason for this is that is has to drive the high insertion loss BMT tone stack along with the input of the crunch channel.

                            Therefore: any modifications to this section has to be done carefully, as doing something to one channel will, also, affect the other channel.



                            The output transformer is the same as the 50watt head amplifier; it has a 16-Ohm tap, but is not connected; nor is shown on the schematic. So we have the option of adding another speaker output socket. And/or connecting the negative feedback to the 16-Ohm tap when doing the resonance(depth) mod it you want greater response from both presence and resonance controls.



                            So far, I have activated the unused triode(V3b) by using it as a "cold clipper". Tried it as a anode follower; and, as a cathode follower. As an anode follower the amp lot more of the SLO sound. As a cathode follower it has slightly less gain, but gives it a bit more bottom end oomph!. I haven't decided which I prefer, perhaps I will put in a switch arrangement to have both options.



                            Anyway... must run; will continue with the mods tomorrow.



                            Cheers

                            --Boffo

                            Comment


                            • This schematic is showing where I implemented the cold clipper. It is actually quite easy to do: simply lift jP7 from the circuit board; there you have where to insert the wires for the input and output of the the cold clipper stage.

                              I decided to increase the value of the cathode resistor to 56k Ohm to keep the gain(voltage amplification) under control.

                              There are, also, some variations of that stage which can easily be done by using a dual six pole rotary switch.

                              Variations include: back to standard; cathode follower; anode follower; anode follower with cathode by-pass capacitor ground; and/or, capacitors of different values to alter frequency response; a fixed bias version(no longer a cold-clipper).

                              The fixed bias version is a distortion monster, perhaps an out of control monster.

                              I will post a schematic of the variations at a later date.



                              Enjoy ;-)

                              --Boffo

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                              • I would like to reduce gain and compression on my JCA100 - which mod do you suggest ?

                                has anybody tried to play with the cathode bypass caps on V1-V2-V3 ?

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                                • kaoslord
                                  kaoslord commented
                                  Editing a comment

                                  Up ! Anyone knows a simple mod to reduce gain and to bring clarity to the OD channel ???

                                  I think that the gain is not usable past noon,

                                  there is too much low end "ompf" and compression.

                                   

                                   













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