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amp hum after mod - help!

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  • #61
    Originally posted by guitarzan


    The biggest difference between Vox & Marshall tone IMO is brought about with Marshall using global feedback but Vox using no NFB, aka open loop. There are way more than 5 things I can think of that are also different; Tone stack, cathode Rs & bypass Cs, Blocking caps, phase inverter, master volume, topcut/presence...


    It's true - there are a lot more variables at stake than just one switch... the Vox/marshall switch would just change the filtering in the tonestack really.. I will also have switching on the feedback circuit, and a cathode/fixed bias switch.

    I'm hoping that it will be possible to get close enough to almost any amps tone by tweaking the switches in the preamp and power amp to closley match the circuitry of the amp I want to "dial in." I could also math it precisely if I wanted to by just swapping cap values and such. (This is why I wanted to go with screw terminal strips - EASY modification)

    Yes, its crazy, yes its complicatied. But I want to be able to hear the difference that different circuit comtribute to "tone", and in many cases A/B them, without having to buy 7 amps!! (Although I have been thinking alot about the Vox AC30CC head...)

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    • #62
      Originally posted by gusfinley


      It's true - there are a lot more variables at stake than just one switch...the Vox/marshall switch would just change the filtering in the tonestack really.. I will also have switching on the feedback circuit, and a cathode/fixed bias switch.

      All those switch wires could bring about some scary noise issues and others will reap few rewards. Why not build a 2 channel amp or a power amp and play around with different preamps.

      I'm hoping that it will be possible to get close enough to almost any amps tone by tweaking the switches in the preamp and power amp to closley match the circuitry of the amp I want to "dial in." I could also math it precisely if I wanted to by just swapping cap values and such. (This is why I wanted to go with screw terminal strips - EASY modification)

      Without digital switching and some serious knowledge you may end up with just a noisy bunch of dangerious wires, if you do try this better stick to a Hiwatt layout and spacing. Just line up the screws where the turrets would be
      Yes, its crazy, yes its complicatied. But I want to be able to hear the difference that different circuit comtribute to "tone", and in many cases A/B them, without having to buy 7 amps!! (Although I have been thinking alot about the Vox AC30CC head...)


      That is why Orange used a FAC switch. The DC30 second channel has one as well. Easy designs for effective preamp voicing adjustment. This changes the low FQ roll off by just switching the value of the blocking cap.

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      • #63
        Originally posted by guitarzan

        All those switch wires could bring about some scary noise issues and others will reap few rewards. Why not build a 2 channel amp or a power amp and play around with different preamps.

        Without digital switching and some serious knowledge you may end up with just a noisy bunch of dangerious wires, if you do try this better stick to a Hiwatt layout and spacing. Just line up the screws where the turrets would be


        That is why Orange used a FAC switch. The DC30 second channel has one as well. Easy designs for effective preamp voicing adjustment. This changes the low FQ roll off by just switching the value of the blocking cap.


        I've been thinking about the two channel amp due to the complexity of the USA/UK switch... The premp is still in the design stage. I am really wanting to get the poweramp section done soon so I can used the DSL401's preamp and run it through some EL34's.

        This is really a two step process and this is why I don't want to commit it to a chassis yet....

        its may be one noisy amp when I get done, by I am really doing this for experimentation/ experience more than I am searching for "the tone", but hopefully I will find it along the way!!

        by the way whats, a FAC switch?

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        • #64
          Originally posted by gusfinley

          by the way whats, a FAC switch?


          Check out vintage Orange amps or the DC30 B channel.
          Besides a FAC switch the best way to alter tone IMO is to add stages. Easier said than done. In my amp I have the options of an extra stage before the tone stack, after the tone stack, or both.
          Also you could use an adjustable sweep control feeding the mid and bass like on the newer Marshalls and Krank advertises like they thought of it first. A CF fed tone stack keeping a middle value treble cap(250pf) works best IMO. Too small a value like Vox just is too thin, too big like some newer highgain amps has too much mids.
          edit,
          The most important thing after safety and getting the right parts is layout. Without a good layout it will be too full of noise to hear any good tones, or you will need to use tone sucking anode caps to make up for it

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          • #65
            reguarding the "USA/UK" switch, If I remove the cathode follower that is typical in a UK preamp setup, then I could use a 4PDT switch, which is produced and would be able to handle the current/voltage present in the preamp...

            I am just wondering how much you think the cathode follower effects the tone of a UK type preamp. How similar would it be if the tonestack in a UK type preamp where fed from the anode of the second gain stage rather than from the cathode follower?

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            • #66
              installed the new master volume dual pot and it sounds great!! No more hum!!

              took out the anode caps and its feeding back like a real amp!!

              put in the larger value coupling cap in the cathode follower and it rumbles!!

              Thanks for all the advice....

              now as soon as I get more parts I can get back to those bright switches........

              Comment


              • #67

                Originally posted by gusfinley
                reguarding the "USA/UK" switch, If I remove the cathode follower that is typical in a UK preamp setup, then I could use a 4PDT switch, which is produced and would be able to handle the current/voltage present in the preamp.... I am just wondering how much you think the cathode follower effects the tone of a UK type preamp.

                The CF is not a British thing, Fender used it in a Bassman to feed the stack before Marshall, Laney or Vox did. IMO a CF improves the impedance match greatly when feeding a Fender style tone stack. This allows the stack to simply work better . Why not have individual adjust on the perimeters rather than switching all 4 or 5 at once?
                Originally posted by gusfinley
                How similar would it be if the tonestack in a UK type preamp where fed from the anode of the second gain stage rather than from the cathode follower?


                IMO the best amps use more CFs to match up stages, not less. If you use a CF you do not have a phase shift. The CF can usually be connected without to many extra components. It is well worth having as the output is greatly improved over that which is possible by an anode output. The advantage IMO of doing DIY is that you don't have to deal with cost factors like you would if you were selling them. My amp even has one CF driving each output tube after the PI. This would not be cost effective in a production amp or even most boutiques.
                Just my .02
                other's milage may vary...

                Comment


                • #68
                  Originally posted by gusfinley
                  i

                  took out the anode caps and its feeding back like a real amp!!

                  put in the larger value coupling cap in the cathode follower and it rumbles!!

                  Thanks for all the advice....

                  You are most welcome. Dang those tone sucking anode caps So glad taking them out was good for you. Better bass now with the larger blocking cap too! Haha, gald I could help but lets maybe talk before you attack the bright switch. Looks like you're winning the battle so way to be, Mang!

                  Comment


                  • #69
                    Originally posted by guitarzan

                    Haha, gald I could help but lets maybe talk before you attack the bright switch. Looks like you're winning the battle so way to be, Mang!


                    Here are my plans for the bright swtiches - first I'll install some headers and swap in different cap values - I think I have 5 or 6 ranging from 100pF-470pF - then when I find one I like in that particualt channel I'll wire up a switch and plug it in the headers....

                    Since I've already drilled the wholes for the bright switches - I might as well put them in, or I'll have to find something to plug the wholes!!

                    Comment


                    • #70
                      When I talk about headers I am talming about these:


                      You can just break them to whatever size you need. The components in my amp need three pin sockets, but I have to break the lead off of the middle one as there are only two holes in the pc board.

                      After this is installed you can EASILY add or remove components without desoldering and resoldering.... which I have found to be quite detrimental to PC boards!!

                      Comment


                      • #71
                        Originally posted by gusfinley


                        Here are my plans...

                        Ok so which caps(#s from schematic) on your amp are you fiddling with? Marshall typically hard wired some treble caps in the circuit that Fender uses in their Bright switch. I would start with those.

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                        • #72
                          The bright caps that I am messing with are C97 on the clean channel and C 100 for the distotion channel....

                          actually looking back at the schematic... again which is a bit confusing... I'm not sure what c100 is doing!! The schematic that I posted does not really follow the circuitry of my amp. My amp is somewhere between the one I posted (issue 9) and issue 4....

                          Mine would appear to be like issue 4, but with the transistors for A/B ducking in place... except for the fact that I took them out...

                          So this schematic better follows my amp.... http://cc.usu.edu/~davbradshaw/imghost/dsl401.pdf

                          in this issue C87 looks to be the bright cap for the gain channel.... but I can't find c87 in my amp.... I see a C86 and a C88, but there is no C87 in between them..... so that is good...

                          SO in this case what would be the "bright cap for the gain channel? would there one I could simply replace rather than just putting one across R107 in the tone stack?

                          hmm, glad you asked.. looks like I need to track down the REAL schematic that goes with my amp...

                          Comment


                          • #73
                            C97, definately a bright cap bypassing high FQs around VR1. Note that when VR1 is maxed this cap has no effect. This would be a good cap to use a switch on, like a Fender somewhat. C100 is simular but uses R122 to limit the level of high FQs bypassing R121. If you use this as a bright switch maybe try changing the value of R122 so you can adjust the level of highs allowed to bypass.

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                            • #74
                              C87 seems to be the same idea but across VR5. according to your latest link. If it is not in your amp then I would not concern yourself with it. Hope that helps.

                              Comment


                              • #75
                                Originally posted by guitarzan
                                C97, definately a bright cap bypassing high FQs around VR1. Note that when VR1 is maxed this cap has no effect. This would be a good cap to use a switch on, like a Fender somewhat. C100 is simular but uses R122 to limit the level of high FQs bypassing R121. If you use this as a bright switch maybe try changing the value of R122 so you can adjust the level of highs allowed to bypass.


                                Finally, I see how C100 is affecting the frequency response!! I didn't notice that it was being fed into the other section of the tube from the other anode.

                                Rather than adjust R122, I think I will play with a few different values of capacitors in this location... I already know that 470pF sounds too shrill... Then when I find the one I want I'll give it a perminant install.. I believe the orignial owner of my amp modded it too a 100pF which sounded pretty good... if I don't find something I like than varying R122 will be the next step...

                                Oh, and another question - what exactly is C68 doing? It is between the anode and the cathode on the first stage of V1. I don't think I've ever seen a cap connected like that in any other preamp schematics....

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