Two amps with diodes ,one loves pedals ,..the other hates them !...Why ?

Discussion in 'Marshall Amps' started by Kelia, Feb 17, 2020.

  1. Kelia

    Kelia Well-Known Member

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    Hi guys,.........I was wondering why my 2205 get along real good with any kind of OD or Fuzz pedals while my 2555SL Jubilee can only take my Klon .
    Both having diodes ,the 2205 is really surprising , when adding pedals, I can sculpt warm organic tones really easily like I used to do when I had my JTM45 with KT66 GEC's ,....but it's a completely different ball game with my Jubilee ,...when I hit it with and OD or a Fuzz ,...it sounds like Fart Fest and there's a big clash of diodes hitting diodes in the overall tone .

    So my question is Why ?..........why do some amps with diodes take pedals better than others ?

    Tx
     
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  2. DragonCrestPC

    DragonCrestPC Member

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    Love/own both 2210 and 2555, man do i love my 2210. I don't use boosts but that thing is so warm to my ears :shred2:
     
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  3. ampmadscientist

    ampmadscientist Well-Known Member

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    I think the 2 amps are 2 different circuits.
    In most current amplifiers, the power supply is made with diodes. That's a given.

    The audio may or may not have diode clipping circuits.
    Some amps do have the diode clipping, some do not, sometimes people take the clipping diodes out.

    And so if there is a problem with (the sound of) diode clipping, it "can" be removed and often is.
     
  4. Crikey

    Crikey Well-Known Member

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    I love my 2210 and for me can’t see adding an additional distortion pedal to it. But that’s just me.
     
  5. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    I think you need to tell us what diodes you are talking about, you just cant say "diodes" without explaining which ones they are.
     
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  6. Kelia

    Kelia Well-Known Member

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    Those two amps both sound awesome on their own and they don't really need any pedals,
    I don't know which diodes I'm talking about but wish I could , I'm not a tech but I am very interested in trying to understand how things works and why they sometimes don't if that makes any sense .
     
  7. d95err

    d95err Member

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    I'm sure it's the clipping diodes used in both the 2555 and 2205 the OP is referring to.

    We also need to note that there are two versions (earlier and later) of the 2205 which are completely different circuits with different topology. Both use clipping diodes though.

    I don't think the difference has anything to do with the clipping diodes. There are other factors that affect how amps take pedals.

    One common reason why an amp does not like pedals is if there's a bright capacitor across the gain pot. The bright cap can sometimes enhance harsh frequencies caused by dirt pedals, which would have been filtered out otherwise.

    The 2555 and early 2205 circuits have a large bright cap. The later 2205 does not. Don't know if the bright cap is the culprit here though.
     
  8. Nik Henville

    Nik Henville Well-Known Member Premium Member

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    ampmadscientist and d95err nail it...

    2205 JCM800 Split Channel Reverb, 50W head with 5x ECC83 & 2x EL34, circa 1983...)
    Boost has 1N4007 x2 back to back, to ground, after V2b just before the Volume pot. Simple clipping topology.

    2555 Silver Jubilee, 100W/50W head with 3x ECC83 & 4x EL34 (circa 1988...)
    seems to have a whole mess of diodes between V1b and V2a, but a resistor to a back-to-back pair of 1N4007 and a capacitor to ground just before V2a seems to be the clipping circuit. More complex and much fussier topology.

    JTM45 - god-like tone when loud. Valve clipping. The best :cool:

    :hippie::pirate::uk:
     
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  9. Kelia

    Kelia Well-Known Member

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    Thanks for your input ,...my 2205 is from 1988
     
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  10. Matthews Guitars

    Matthews Guitars Well-Known Member

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    It might be a matter of level. Some amps take high level boosted input signals very well and get a good overdrive when you do that.

    Some others sound like garbage when you give them a really hot input signal. Too much level will of course cause clipping, and depending on the circuit, it may be a really bad sounding type of clipping. Some people call it blocking distortion.
     
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  11. Kelia

    Kelia Well-Known Member

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    Yeah,.......really bad sounding type of clipping ,....really sounds awful !
     
  12. JTM1963

    JTM1963 Active Member

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    Because of gain structure and topology in an amp.
    You are dealing with two very different amps with very different gain structures, let alone diode clipping circuits.
    The 2550 lead channel takes cleanish boost in a good way (f.ex. a TB type OD with minimal gain but a slight volume boost) but if you already have a good amount of gain to begin with and then add more gain with an OD or fuzz you get the result as described. Doubly so if you scoop the mids.
    The 2555 has a diode clipping circuit as an integral part of the Lead channel gain.

    The 2205/10 lead channel is actually two gain stages. Gain is a dual pot placed before and after the V1a valve and feeds into the lead channel volume which feeds into V3 and then into the tone stack. The balance between the two has a lot of impact on the distortion and tone of the amp. The diode clipping circuit is different to that of the 2555.
    Basically there's a bit more headroom in the 2205/10 preamp section and therefore it takes certain pedals better.

    Both amps are completely different animals compared to a JTM45 as a pedal platform.
     
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  13. Kelia

    Kelia Well-Known Member

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    Thank you ,.....I appreciate your input !
     

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