2203xman's Plexi amp rebuild

Discussion in 'The Workbench' started by Guitar-Rocker, Aug 6, 2012.

  1. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    I see at times a lot of interest in amp building here on the site, and people who kind of want to follow along with a build, so with 2203xmans permission, I thought we could log a few sections of his up coming rebuild. To date, I had built a 18W 6V6 power tubed Parallel/ Cascade for myself to keep before last years Christmas, then decided to let it go. 2203xman (Mike) purchased the amp from me, and we became friends after the sale. Mike's amp is a very nice amp in it's own right now, as is, especially with the Mullard mustard caps and Piher resistors. But with the discussions of some of the various combinations that I had tried, Mike inquired if we could mod his amp a bit, to more closely dial into the types of tones that he enjoys the most. At first we were going to just alter the amp's Volume, Tone, Gain tonestack, but after looking into the ins and outs, decided in the parallel mode that the tone stack would rob too much of the signal. After a lot of discussions, we came up with a plan. We are going with a twist on the JTM 45. It will have just one input jack, mixed Volumes I and II, Treble, Middle, Bass, Presence tone controls. We are putting in a 1 wire mod, leaving the parallel triode values as they were, one biased hot, one cold. When Mike now pulls out on the Volume knob, the amp goes from a stock JTM45 to a 1 wire modded RR version...almost a 2203.. The existing amp has a ClassicTone 18W PT, and an oversized MPS 45va OT that has a set of 8K primary taps to give you that "Fat Tone". Those will stay as is, and we're adding another preamp tube to accomidate the changeover, along with a new layout turretboard, so a few pics are in order:

    [​IMG]


    [​IMG]




    [​IMG]
     
  2. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Our progress so far is a customised nameplate with the 2203xman logo etched on it, and a turretboard staked out and jumpered, along with a schematic to follow:

    [​IMG]

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    Looks like this, except with just a single input jack.
     
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  3. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Oh yeah, the amp currently sounds like this:

    Hope Mike doesn't mind me posting a sound clip.


    [ame="http://www.youtube.com/watch?v=1aq77NWciUs"]movie 001.MOV 2203xman 18 watt marshall - YouTube[/ame]


    [ame="http://www.youtube.com/watch?v=VYq1IVhtQMs"]2203xman charvel & g&r 18watt 003.MOV - YouTube[/ame]
     
  4. 2203xman

    2203xman Well-Known Member

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    I know a lot of thought went into that turret board stake out,wish I was there to hold a light,or get you a brew :thumb:
     
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  5. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Tonight I added the B+ dropping resistors and capacitor's to make a B+ buss on the turrertboard. Since the turretboard we are using is standard size, space is at a premium. Normally you will see a row, top to bottom of components making up your bleed down resistor, 1st dropping resistor, your power tube screen resistors, and the second dropping resistor, followed by the bias cap & resistor (this is a 18W cathode biased amp). This is shown below in the photo of another amp framed in a green frame:

    [​IMG]

    That took up seven rows,too many. Add in the bias cathode/resistor shown in the orange frame below, and there goes another 2 rows:

    [​IMG]

    To save space I took the dropping resistor string, and the bias cap and resistor and turned them sideways, like below, in the photo of the dropping resistor B+ string




    [​IMG]




    Then the bias cap & resistor went below that string, like this:





    [​IMG]



    The end result is 3 rows saved. Note the Dale precision resistors in black, and that BC 1000uf cap looks like a depth charge hanging over the edge!
     
  6. Dimitar

    Dimitar Active Member

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    Nice sounding amp! I love it... and big like for the southern flag on the window.
     
  7. Thiez

    Thiez Well-Known Member

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    That's a nice looking rebuild! Gonna follow this one.
     
  8. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Tonight, I got the components soldered up through, and including the PI. Bear in mind, that right now, I'm only spending plus or minus about an hour a night, so the progress isn't real fast. And the current photo:

    [​IMG]
     
  9. Les Moore

    Les Moore Well-Known Member

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    I just love to look at these pics. Wish I had the knowledge to do one myself...
    Sometimes I´m thinking of taking a schematic of some amp, by the parts and try to put it together just by looking at the diagram and pics. Crazy, but I´m so fascinated by these things:)
     
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  10. 2203xman

    2203xman Well-Known Member

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    Fine progress,your time is at a premium as well my friend.What role does the 16uf cap play?Just trying to make notes of the board so far. :)
     
    Last edited: Aug 9, 2012
  11. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    The role of the 16uf cap .....We have went through the green 30K dropping resistor to take the B+ voltage down for the preamp tubes, and the 16uf cap helps keep the B+ where we want it at. More in a bit. OK,...We have the PT secondary HT of 290v-0v-290v, that goes to the tube rectifier, that takes the AC and rectifies it to DC, and then goes on to the standby switch (Not shown in the photo of Mike's new turretboard). From the standby switch, we go to the 32uf-32uf can cap for "smoothing", and also to the OT primary (also not shown in the photo), and will then go to the right hand side of the brown bleed down 220K 2W resistor in the photo of Mikes new turretboard. Now we can refer to the ongoing string as B+. On the new turretboard the brown bleed down resistor (the other end of the bleed resistor will go to ground), is jumpered to the black 2.2K dropping resistor on the right hand side. That point is where we route the B+ from the standby switch / cap can / OT connection to, allowing us to either drain the voltage off the system when power is turned off through the bleed down resistor, or during run operation to drop the voltage down going to the next state. To make the first voltage drop we go through the 2.2K (follow from the right to the left), the dropped B+ voltage now commonly feeds, (by the jumpers), both power tube screen resistors, and the next dropping resistor of 8.2K. Next B+ goes from left to right through the 8.2K dropping voltage. At the right hand side of the 8.2K the red wire routes B+ to the 33uf axial cap for "smoothing". The red wire leaves the 33uf cap going to the right, terminating at the junction for the plate resistors of the PI, and then goes on to the green 30K resistor (as mentioned above). Here we drop the B+ for the final time going to the Preamp tubes, and again have a "smoothing " cap....the 16uf cap . Please note that in the schematic below, the bleed resistor is shown to the left of the can cap, but on the turretboard, it's actually to the right hand side.


    [​IMG]
     
    Last edited: Aug 9, 2012
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  12. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    That is exactly how most folks start. The people selling kits to first timers, also usually coach a person through it.
     
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  13. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    And tonight's progress. I populated the turretboard with components, just lack 1 resistor and 1 capacitor. I'll start lashing the wires to the components tomorrow. A couple of pics below:














    [​IMG]




    [​IMG]
     
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  14. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Tonight, I received the amp back, took it out of the head cabinet, and ripped out eveything that's coming out on this rebuild. It was sort of sad at first, but I'm really excited about hearing the new tones once it's been finished, more pics below:

    [​IMG]

    The old turretboard, pots and a pile of wire removed from the amp.


    [​IMG]


    Where the new board will end up going, slided over to the left from the old board position, in order to make room to add another preamp tube,as noted by where the screwdriver is pointing.


    [​IMG]
     
  15. Brett Blackmore

    Brett Blackmore New Member

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    Hi all,

    this looks absolutly great! I wish I could do this and then, even more important, i wish I could understand how they work. Sadly I only play them...

    Many thanks for posting this. Brett
     
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  16. 2203xman

    2203xman Well-Known Member

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    Well,I can barely play them,and my understanding of tube electronics is taking giant steps forward,but I only know enough to really buger one up now.Terry(guitar-rocker),is a master at his craft,and is about as down to earth and friendly as they come.People like him who are willing to share their knowledge so generously,and unconditional are rare indeed. :)
     
  17. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Just a quick update. Unfortunately I had a small mishap on the build. I had the knockout punch freeze up and it left the chassis marred. I could not abide an ugly mark on the chassis, so I've pulled everything and dropped it all into a brand new chassis. As soon as I get the transformers and filaments rewired, I'll take off on this again, which will be very soon. No oops allowed here. (by the way the punch has been round filed and a new one purchased).
     
  18. Les Moore

    Les Moore Well-Known Member

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    I am so trying to understand how the signal flows, where and what things are that I´m getting a headache. :) Wouldn´t have it any other way.
     
  19. 2203xman

    2203xman Well-Known Member

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    I'm with you Les,it's downright genius when you see the workings of tube circuits.How'd Jim Marshall figure so much out?I've read that after copying the Fender tweed bassman,he couldn't get like components,and the substitue parts and tubes created the Marshall tone....:wave:
     
  20. Les Moore

    Les Moore Well-Known Member

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    I have the same info. Many believe that for instance that is the reason for the very high value on the V!a cathode cap, 250 or 330uF on the earlier amps. Sounds much better in the bass with 1 or .68uF. (!)
     

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