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Sound City 120 Mk IV - can it run KT88 or 6550?

Discussion in 'Amps and Cabs [BG]' started by christw, Jan 11, 2012.


  1. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    Google and it's mastery of the internet and message boards is leading me to believe I might be able to run KT88 or 6550's in my SC120 after a rebias on the stock Partridge trannies. I want to ask:

    1) Would that work?
    2) Would it make a noticeable improvement in power? Those six EL34 walk all over 100w tube amps already.

    Ummm... that's it. I'm trying to decide whether to keep or sell this head right now and I'm leaning towards keep. If I can pull an extra 80w out of it way down the line it's definitely staying. :p
     
  2. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    The six EL34s the amp came with draw 1.5A heater current each. Originally both the KT88 and 6550 had the same. New production tubes seem to have up to 2A of heater current which will eventually toast the power transformer.

    Why would you want to use other than the EL34? You'll not get any usable power increase from the change.
     
  3. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    Tubes will not give you more power - you need to go to a larger PT with a higher VA rating. That said, if you can get sufficient bias voltage and there is enough space between the tubes (especially for KT88s) it should work. Primary impedance of the OT may not be ideal.
     
  4. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    I won't bother if the PT isn't up to snuff. Thanks guys.
     
  5. You won't see much more power coming through those transformers. The 200w SC's have significantly larger transformers and run four KT88's. I wanted to mod my 120 for this as well, at one point, but decided to just get an amp that runs off 6550's which I much prefer now. The SC will not put out more power or volume (maybe a slight bit but nothing huge like the numbers you're throwing). My 120 with six EL34's was easily running around 130w. If it's only power you want, I would not look into modding it for that. If its a different sound you're after than I wouldn't mod it for that either. If you decide to do so, I can't vouch for it working properly with or smoothly with the active EQ. Maybe using four KT88/6550 is a little more reasonable for the transformers in there, though I still think it's a lot of change for not much outcome difference in that circuit/amp. Maybe I'm wrong and someone else will pitch in.

    A stock 120 with a 12AT7 in the PI and a nice set of power tubes is one of the best bass heads I've heard/had. Only one I've liked more is the Univox I'm playing now.

    Either way, good luck.



    edit: late post. people pitched in already. haha.
     
  6. It might be okay, but after changing screen resistors and maybe bias circuit for the 6550/KT88 it would be a shame to have to revert back if the PT overheats.
     
  7. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    I love the tone from it. I was just wondering if I could make it a heavier hitter than it already is. On top of my '72 810 this thing is a monster. It has an incredibly forgiving high voicing that brings out the best, even in dead strings. On the other hand, roll it off and man does that thing do fat.

    I don't really need to get more from it. I don't want any big difference in tone. It's just about right. I trying to decide between doing one of these:

    1) Keep it and do a full rebuild on it later to make it road-ready and bring it with my Marshall 410 to wherever I go as a do-all guitar/bass rig. Those old CTS speakers sound great in there!
    [del]2) Keep it and do a retube later if I can draw more power[/del]
    3) Sell it as part of my sell off, probably at a decent loss

    I'm pretty happy with it's voicing and I don't think I could replace it with anything I know of. I'm probably going with option 1...
     
  8. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    A proper service will do far more for the sound than bodging in the wrong valves ever will. Uprating the filter caps might help, mine have 470uf ones instead of 200. It you have to have KT88s, look for a Sound City 150.
     
  9. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    It's been serviced twice to bring it back from a rather sketchy state of affairs. I'd trust it to go a long time as it is. I think Randy had suggested a tone stack rebuild at some point when I was complaining about something but I don't know what it was anymore or why he suggested it. My only issue right now is having more noise than I'd like when I roll in the treble to most any degree. It's not an unreasonable level but I know it can be improved upon.
     
  10. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada
    The Partridge power transformer IS "up to snuff" for what it was designed for.
     
  11. christw

    christw Get low!

    May 11, 2008
    Dayton OH
    I want to be Tesla (tinkerer at Dayton Amp Co)
    Okay, let me rephrase that: I don't want to take any risks that might damage the PT and if changing output tubes could damage it, I'll pass.

    I've been just a little mod-crazy because I'm wasn't working much thanks to the arm. :smug:
     
  12. I had mine for a while, toured with it, and it took all the abuse I threw at it. Just get a road case for it. Sure the wiring is messy and you'll get noise from the active EQ, but unless you want to re-do the circuit or it's in a ridiculously poor condition, it shouldn't give you any problems on the road in a decent case.

    If you like it, I say keep it. Sometimes I miss mine for nostalgic reasons, but then I plug into my univox and it doesn't bother me after that. Regardless, I LOVE bass pushed hard through a stock SC 120 through a big cab.
     

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