5E5 Pro Input circuit question

Discussion in 'Shock Brother's DIY Amps' started by tweedy_woodpecker, Apr 6, 2020.

  1. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    I am planing my 5E5 build and decided to use the unusual input stage (1 double triode for each channel, no grid stoppers, shared cathode resistor for all inputs).

    Heres the schematic:


    Bildschirmfoto 2020-04-06 um 16.43.09.png
    I am trying to get my head around this, because it seems quite different from the usual Fender input stage and I found no real coverage of this input circuit...

    My questions:

    1) Am i right that the input impedance of input 2 of each channel is the two 1M resistors in series?
    This would mean that wehave 2M Ohm impedance for input 2 and 1 M for input 1 of each channel.

    2) Since there are no grid stoppers present, am I right that the impedance is the only difference between input 1 and 2 of each channel?

    3) if i plug into either input one channel, does some signal also pass through the second 1M grid leak/impedance resistor into the grid of the other triode?

    Heres the more common 5E5-A input stage for comparison:

    Bildschirmfoto 2020-04-06 um 16.51.39.png
    Thanks!
     
    Last edited: Apr 6, 2020
  2. Snfoilhat

    Snfoilhat Tele-Afflicted

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    When Input 2 has a plug inserted, the path to ground is 1M and then through the closed contact of jack 1, so 1M total.
     
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  3. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    You are of course right, @Snfoilhat. Been staring at my computer screen all day, so I totally missed the switch to ground connection... :confused::lol::eek:
     
  4. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    But if I plug into input 1, do you think there is a little bit of signal going throught the 1M resistor into the grid of input 2 or is this neglibile?
    Ohne Titel.png
     
  5. Snfoilhat

    Snfoilhat Tele-Afflicted

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    I think that between the interconnections of the grids and especially the cathodes, any signal at any of the 4 jacks will result in all 4 triodes doing something, though I couldn't tell you what.

    Could you share more with everyone about why it might matter, or what you're aiming for?
     
  6. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    I am just trying to understand whats going on here opposed to the regular input stage of a 5E3 and if it does do something the other one does not or vice versa....especially in terms of interaction, since it takes two preamptubes where usually on suffices.
     
  7. D'tar

    D'tar Friend of Leo's

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    I would say the signal would travel straight to the grid of the other triode..... Around the 1M.

    There's that darn shunt again!

    Looks like input 1 runs the triodes parallel w 1M grid leak, and input 2 runs a single triode w 1M grid leak.
     
  8. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    You are right.....
     
  9. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    As D'tar said input 1 would parallel the two sides of the tube. Input 2 would just be one side of the tube.

    As Snfoilhat said it is hard to say what each tube is contributing from the tied together cathodes.

    (If you decide to use this circuit, RFI could be an issue. A 15k - 33k grid stopper directly on each tube grid pin would be the most efficient way to combat RFI.)
     
  10. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    Thanks, I already plan to use gridstoppers directly at the grids of V1 and V2...

    Its an interesting circuit, one part of me says try it and part of me says just use the proven 5E5-A/5E3 input stage and save a whole 12AY7.
     
    Last edited: Apr 6, 2020
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  11. David Barnett

    David Barnett Doctor of Teleocity

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    Not just some, all - if you use input 1, both triodes run in parallel. The switch on 2 shorts the 1M resistor and connects your guitar directly to the grid.
     
  12. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    Correct.
    So this would be the signal path/impedance for the two inputs:
    Input 1 is red and dark green, Input 2 orange and light green


    Bildschirmfoto 2020-04-06 um 18.10.22.png

    So is using input 1 similar to jumpering channels Bright and Normal in a 5E3? Except that the mixing of signals happens at a later stage...
     
    Last edited: Apr 6, 2020
  13. Nickfl

    Nickfl Tele-Afflicted

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    Not exactly, if you jumper the channels on a 5e3 you are running signal through two separate gain stages in parallel, input one on the 5e5 is running your signal through one gain stage that is made up of two triodes in parallel.

    The difference is that the two halves of the 12ay7 in the 5e3 have separate anode load resistors and generate voltage across those resistors seperately (though they will be creating very similar output due to receiving the same input signal and sharing a cathode resistor). The two halves of the 12ay7 in the 5e5 share all of their components, including the plate load resistor, so they are working together to generate signal across the same resistor.

    There is also the fact that the parallel gain stages in the 5e3 are mixed after their independent volume controls, which can be adjusted, whereas the parallel triodes in the 5e5 are mixed at the load and there is therefore no adjustment possible.

    In my experience, parallel triodes like the 5e5 has make a pretty subtle difference. Parallel triodes add a little bit of volume and "fatness" (or muddiness). Its been a long time since I tried jumping the channels on something with 5e3 type inputs but I also remember that being subtle and kind of uninspiring... so, in the end they do probably have a similar effect, though what is happening electronically is different.
     
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  14. The Ballzz

    The Ballzz Tele-Afflicted

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    @tweedy_woodpecker
    I too am exceedinly intrigued and curious about the actual effects of the use of multiple triodes processing the same signal in parallel. I'd very much like to know the actual "hands-on" results. On top of that, it would be interesting to know what happens when "Y-Jacking" into the #1 input of both channels, as jumpering won't actually work, given the switching configuration? My limited knowledge/experience suggests that while there may be some increase in overall "gain" to the next driver stage of V3-A, the real benefit may be a more full range complexity to the overall tonal response. Of course then again, there may be diminishing returns by having such a strong signal become a bit too linear and sterile.

    The 5E5 is one of a few examples of this input arrangement, along with the 5D4 Super and 5E8-A Low Power Twin, among others. It should be noted that all four triodes in the preamp section share the same cathode cap/resistor. Some other models, like the 5D8, 5C4 and others look similar (at first glance), but notice that the jack switching functions differently.

    And in case you're not aware of this, here is a fantastic resource for many if not most of the earlier schematics and layouts:

    https://vintagefenderamprepair.com/pages/library-schematics-layouts

    If there is anyone here with a generally bone stock version of this preamp (original or clone), I would be indebted to them for a willingness to try a couple of specific configuration experiments to answer some of my questions.

    Thanks To All,
    Gene
     
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  15. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    @The Ballzz :

    I started the build today.
    Once I am finished I can do some audiosamples with various input configurations.
     
  16. The Ballzz

    The Ballzz Tele-Afflicted

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    Are you building it "generally" stock, according to the Fender schematic?

    Thanks,
    Gene

    Pro-5E5-Schematic.jpg
     
    Last edited: Apr 7, 2020
  17. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    Yes. I will probably just add a choke like the 5e5-A has plus an adjustable NFB Loop.
     
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  18. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    159A8050-67DB-43D0-9214-B5B902664EEC.jpeg

    Here‘s where I am at right now....
     
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  19. tweedy_woodpecker

    tweedy_woodpecker TDPRI Member

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    DB91EAA0-CAB2-4EE6-9340-8C0ED4EF07A0.jpeg
    Slowly getting there...I must say I appreciate the longer chassis of the 5E5 since I am mostly used to the smaller 5E3 size...more space :)
     
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  20. The Ballzz

    The Ballzz Tele-Afflicted

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    @tweedy_woodpecker

    Nice work so far. :cool: I like turrets so much more than tag/eyelet boards! ;)

    Thanks For Sharin'
    Gene
     
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