preamp distortion VS Poweramp compression

Discussion in 'Amps and Cabs' started by prof2915, May 7, 2005.

  1. prof2915

    prof2915 Guest

    Hi

    I’ve always loved the sound of hard driven EL 84s.

    I’ve recently bought a Laney VC30 and I’m thinking of changing some tubes.
    This amp goes into ”preamp distortion” to early for my taste, and I would like to be able to ”clean drive” the power tubes instead.

    - Do you think that changing the clean channels ECC 83 into an ECC 81 would result in this?
    - Has anyone changed the stock EL 84s to something that sounded better?
    - What about the ”driver tube” (V3) what can be done there?

    I replaced the original speaker with a Celestion Vintage 30, which was an improvement, but I really would like more of an ”AC 30 compression” to the sound.
    This might be quite difficult though... Maybe it’s a matter of the output transformer?

    Any help will be appreciated!

    /Peter
     
  2. stratopastor

    stratopastor Member

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    hi Peter

    preamp tubes... yes, ECC81 has lower gain. Get one and try it in each ECC83 position - there should be at least one position in which it reduces the preamp distortion.

    speakers - alnico speakers aren't cheap but they 'compress' more than other types.

    Maybe you can get the sound you're looking for with an attenuator.

    That's all I know... good luck and best wishes

    SP
     
  3. gizmo

    gizmo Guest

    I'm in the process of doing the same thing to a Marshall. All those that I spoke with indicated that changing the V1 preamp tube would be most noticeable. My RCA 12ay7 replacement shoud be here Monday. :cool:
     
  4. cameron

    cameron Member

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    It's likely the compression you're thinking of is due to the speakers. Get an AlNiCo speaker, such as the Weber AlNiCo Blue Dog. These are pricy (almost twice the price of the Ceramic magnet versions) but will give you that compression when you drive them hard.
     
  5. prof2915

    prof2915 Guest

    Hi ya´all!

    Thanks for sharing your ideas.

    -"Stratopastor"

    The tube layout on my amp is:
    V1=Clean channel
    V2=Overdrive channel
    V3=Drive tube (and I have to admit that I really don´t know what that tube is doing... Some help maybe!?)
    So in my case it´s V1 that should be changed as it is the basic clean sound I´m referring to. (regarding the overdrive channel I´m thinking of getting the highest gain preamp tube that can be found as I would like more preamp distortion from this channel. Any ideas of such a tube?)

    Regardig speakers I sometimes play the VC30 through a 2x12 cab loaded with Vintage Jensens, and the compression in these are better!

    -"Gizmo"

    In order to get more tone and more dynamics out of the overdrive channel/channels (instead of "to much" distortion/gain) I´ve changed ECC83s into ECC81s in several amps with great results. I´m sure that the distortion in your Marshall will improve as well.
    When an amp uses several gain stages/tubes to create distortion the common opinion is that the "input stage" is where the effect of a tube change will be most noticable, but great Swedish Amp Guru: Tommy Folkesson once said: "Try the last stage, this is where most things happen!"
    You´ll have to try for yourself!

    -"Cameron"

    Don´t know if the Weber AlNiCo Blue Dog can be found here in Sweden, but if so I´ll give it a try!


    Once again: Thanks a lot!

    /Peter
     
  6. gizmo

    gizmo Guest

    Thanks prof...I'll have to do some swapping all around and see what works best.
     
  7. drbob1

    drbob1 Silver Supporting Member

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    The "driver" tube is also known as the phase invertor. The signal from the two first preamp tubes is a single, AC signal imposed on a DC voltage at the preamp tube's plate. If you just feed that signal into the power tubes, you'll get the same signal going thru both power tubes which will then cancel out when it goes thru the output transformer (which is center tapped to flip the output from one tube upside down compared to the other-you can then turn one tube off part of the time -class AB-and still have both positive and negative deflections of the signal amplified). The driver tube flips one copy of the signal upside down to feed 1/2 the power tubes. It can do this a number of different ways, most amps use a "long-tailed pair" approach, which allows the driver tube to amplify the signal a bit too. Changing the driver to a 12AT7 or 12AU7 won't likely give the effect you want-it'll just make the amp a bit quieter.
     
  8. enharmonic

    enharmonic Old Growth Gold Supporting Member

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    Pushing a preamp is fun, but nothing sounds quite like a power section opened up nice and wide :)
     
  9. prof2915

    prof2915 Guest

    - "drbob1"

    You seem to have done your homework on tubes!
    Do you think that also changing the clean channels ECC83 into a ECC81 will result in just a quieter amp, or will volume remain the same but with less preamp distortion?

    Maybe - as suggsted elsewhere - putting a cleanboost in the FX loop (to cleandrive the powertubes) and set the clean channel to a "non distorting level" is the best way to obtain some poweramp compression.

    -"Enharmonic"

    Yes you´re right, poweramp distortion is "the real thing"!
    When running my VC30´s clean channel on max, I get a very nice "raw" distortion.

    Thanks to both of you!

    /Peter
     
  10. drbob1

    drbob1 Silver Supporting Member

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    In most amps a lower gain tube in V1 will clean it up a bit. It's not as simple as the gain factor of the tube (John Phillips has spoken to this a few times and he's WAY beyond me) because the coupling of the tube with the rest of the amp limits the effect on that stage, but to simplify a lower gain tube should clean up the amp. It's also MUCH easier to find NOS Telefunken or Amperex 12AU7 or 12AT7, so it's worth trying those. The amp will compress less but if there's distortion from the EQ and later stages it might not completely clean it up.
     
  11. prof2915

    prof2915 Guest

    So, now I´ve done some testing.

    -I settled for a 12AT7/ECC81Y from RubyTubes in V1.

    The result is less preamp distortion (as well as a little less volume) and better clean headroom.

    - I ended up with a 12AU7A from National GoldLine in V2. :eek:

    This might be considered as a strange chocie, but the reason for this was that the "drive sound" was too fat :confused: - in a muddy way - when compared to the clean sound.
    The volume and distortion is now MUCH lower, but as I use pedals for my distorted sounds it was nice to have the drive channel deliver a nice and clear breakup sound, rather similiar to the clean sound. (as this is a 30 watt EL84 amp, the clean sound is close to the point of breakup as well)

    - I tried some tubes in V3, but changed back to the original one.

    - I´ve tried a cleanboost in the effectloop, and by doing so I can coax a little more "clean" volume from the amp. My RC booster is still in front of the amp, but I might get another one for the loop.

    All together I´m right now much happier with my amp! :D

    /Peter
     
  12. gizmo

    gizmo Guest

    Sounds good prof! I like the change to a 12ay7 in V1 of my marshall too. A little cleaner, rounder tone.

    The nice thing about these changes are are how easy they can be done with just replacement tubes.
     

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