Twin Reverb Mods

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sandman, Jan 17, 2006.


  1. sandman

    sandman Member

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    Took my amp to a tech for retubing-rebiasing and he showed me some mods for my Twin on the fly.

    I have zero knowledge on this but he tweaked it to sound Vox-like,Tweed-like and Marshall-like with (to my ears) cool results.He used Normal channel for this and he can rewire the bright switch to be the selector for two sounds I choose(Vox,Tweed).

    He just did some quick soldering job and added a little wire...Do you have a clue what did he do?

    I wouldn't argue about how close it'll sound to Vox or Tweed but I don't use the Normal channel at all and adding some different sounds for little cash seems cool to me.

    Mods are reversible.

    I'll post the schematics as soon as I get 'em.

    Gimme some opinions please...
     
  2. darkbluemurder

    darkbluemurder Member

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    Sandman,

    Did your tech already perform the mod or did he show you the mod on a different amp?

    Does the amp still have the tone controls? What happens sonically when you engage the bright switch of the modded channel?

    For a tweed mod (a tweed bassman mod) I could assume that he rearranged the gain structure. The normal Twin structure is 1st gain stage, tone stack, volume, 2nd gain stage, Reverb, phase inverter etc. The Tweed structure would be 1st gain stage, volume, 2nd gain stage, tone stack, phase inverter.
    The Vox mod could use the same architecture as the Tweed but the value of the treble cap should be changed from 270pf/250pf to 50pf and of the treble pot from 250kA to 1MA. The bass and mid pots can probably stay as they are. If the tech used the 50pf cap and the bright switch to add a 220pf cap in parallel he may approximate both Vox and Tweed sounds. Of course it will never sound the same since the plate and cathode resistor values are way different, and there is no cathode follower before the tone stack.

    I suppose the modded channel either way has more gain than the Vibrato channel and is pretty loud ...

    Apart from that it is really difficult to tell unless one can look inside the amp.

    Hope that helps.
    Cheers
     
  3. hasserl

    hasserl Member

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    Well, you don't use the Normal channel anyway; and he mods are reversible, right? So why not do it. If it adds versatility to the amp you come out a winner. If it doesn't turn out so well and you don't use it you're only out a couple of bucks, no big deal. Just as long as he doesn't do anything irreversible!

    For the tweed mod he may just be bypassing the tone stack for a nice boost in gain and mids.
     
  4. Matt H

    Matt H Guest

    i'm guessing for vox/tweed it might not even be a tone stack BYPASS, but perhaps adding a second resistor in parallel to the slope resistor- that'd give the amp a very different feel.

    of course- it's also possible it was just a tone stack bypass- that could be done on a bright switch with one wire...
     
  5. sandman

    sandman Member

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    this is the schematic for similar mods he does to Orange amps using the 6-way fac switch(somewhere down in the scheme, upper part emulates Ampeg tones).Modes 2 and 3 are the ones he would do to my Twin.
    In fact there is no Tweed mod but something he calls Fender Custom(Vibroking kind of sound).

    does any of this make any sense?

    [​IMG]
     
  6. sandman

    sandman Member

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    that's exactly what I'm thinking! ;)
     
  7. Dave C

    Dave C Gold Supporting Member

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    Looks like your tech is providing switchable values for the treble cap in the tone stack. If you like the results and it's not very expensive I'd agree with some of the other posts and say try it especially since you stated he's using the existing bright switch and will be completely reversible.
    Dave C
     
  8. donnyjaguar

    donnyjaguar Member

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    I have to question the point of putting a master volume within the NFB circuit. I think you'd be better off replacing the NFB resistor with a POT control as I did on my amp'.

    DJ
     
  9. sandman

    sandman Member

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    After two days of playing my now modded Twin I can say I'm really happy.Mods gave me cool flavors with stock JBLs and even bigger difference with other cabinet i tried.I really like the EQ of modded channel.
    The best thing is that my tech sold me some NOS Mullard,Telefunken,Siemens and Philips tubes for a great price.I was surprised that he had some 'cause here in Serbia all we can get at local stores are current production Fender,Groovetubes and Sovtek tubes.

    Here's another question:

    My Twin is 1981 135w version.Actually, it was until someone replaced the ultralinear transformer and put a regular one.Is "blackfaceing" easier to do now and would it make any sense? My tech claims it would make minor difference in sound.What do you think?

    thanks
     
  10. AL30

    AL30 Member

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    I play a '72 Quad Reverb - same circuit as a twin. I played it stock silverface circuitry for close to a year before I did anything to it. I finally decided I hated the clean sound - way too thin and ice picky and was completely unuseable for me. Every time I played straight clean I had to add an EQ in front to tame the harshness. So, I blackfaced it and the sound difference was any but minor to my ears. The cleans are much warmer. No more ice-pick and no more EQ. I was really taken back by the improvement - I wasn't expecting that much of a difference.

    But, some people like the silverface sound. To each his own. The silverface did seem to take pedals better. If you never use the clean sound and get your sound from pedals it's probably not worth the effort.

    If you've already got a different transformer it should be an easy/quick mod that's reversable.

    AL
     

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