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Discussion in 'Amps and Cabs [BG]' started by Lonesomedave, Jan 22, 2012.

  1. howdy.

    just wanted to share news about a piece of equipment i recently purchased for my son.

    it is a THD Univalve pre amp/power amp.

    here is a picture:


    it's real function is as a 15 watt guitar amp, TUBE AMP, NOT SOLID STATE....admittedly not too exciting for a site devoted to high power bass amps, but in it's secret identity it is a monster piece of gear.

    it has one 6550 power tube and two 12AX7 pre amp/driver tubes, and herein lies to secret to it's amazing potential.

    the THD has what is called a dummy load. this means that, unlike most tube amps, you can use it as a pre-amp WITHOUT ANY SPEAKERS ATTACHED!

    further (and i admit i don't really understand how it works, but it does), it has the ability to either be used as a pre amp for a true power amp or as a pre amp stage before a guitar or bass amp. in this case, my son uses an acoustic B450 combo bass amp.

    the thing functions flawlessly, without any hiss or noise at all! it is amazing.

    Now, for the truly jaw dropping aspect.... this amp/pre amp has the ability to self bias with almost any power tubes or pre amp/driver tubes! just plug and play. want to try a combination of a kt88 power tube with one 12 AX 7 and one 12AU7 pre amp tubes? just pop the tubes in and away you go.

    admittedly we won't be using the power tube swap feature because we are using it just as a pre amp, but the thing can take about 20 different power tubes and the same number of pre amp/driver tubes: 2 12AX7's no problem....2 12AU7's.... 1 12AX7 and 1 12AU7, any combination at all, ... still no problem.

    just plug and play.

    I popped it open and found the pre amp tubes were an old RCA tube and one no-name chinese tube, both 12AX7's. who knows how old the RCA tube is! 30 years? no idea but they have not been made in decades. (we bought the unit used from GC) I swapped the no-name chinese tube for a groove tube 12AX7, and, obviously, left the RCA tube right where it was. the sound, again, is just fantastic.

    we love it and it sounds great.

    let me know your thoughts.

  2. someparts


    Nov 22, 2010
    Seems like the perfect platform to make a two-tube bass version.
  3. Cathode bias SE (Single Ended) amps are self bias. No secret magic there. Nice unit though!
  4. wcriley


    Apr 5, 2010
    Western PA
    My thoughts: $900 ("Open box" / "Warehouse resealed") is way more than I ever see myself spendind on a preamp.
  5. don't blame you there, but we got ours used at GC for about 500.00....still a lot i grant you, but for what it can do, i could not pass it up.


    ps... and on ebay they run from 6 to 800.00...still pretty expensive.
  6. Well, it's not a dedicated bass amp, but don't let that stop you. Plenty of guys have gotten great tone out of amps that weren't labelled "bass."

    THD Electronics
  7. looking at it more closely, i can see the jet city amp does not have all the features of the univalve.

    the univalve has the famous THD "HOT PLATE" which has the effect of turning up the gain (or anyway, raising distortion) without raising the volume. THD sells these separately.

    the jet city amp also does not have the "attitude" which, while i don't know exactly what it does, according to THD it determines how the driver tube responds to the signal and alters how distorted or clean the sound is.

    also, the univalve has two inputs, one of which has an extra gain stage. if you use the other, you bypass one gain stage and the sound is cleaner. the jet city amp pictured just has one input.

    all of these features are engaged before the line out signal, so are available to you when you use the univalve as a pre amp, as we are doing.

  8. WretchedExcess


    Jul 29, 2013
    > looking at it more closely, i can see the jet city amp does not have all the features of the univalve.

    looking really closely, the jet city amp is also missing the grid leak resistor on the third gain stage. lack of a ground reference makes the third stage unstable. although it's supposed to be there (it's on the schematic), there was a last minute change in the amp's specs to add a bright switch (not on the schematic). the result is that there was some miscommunication so that when the bright switch was implemented the grid leak resistor got left off. this made the amp unstable. it's one of the reasons that 333 decided to halt production on the amp and remove it from the product line. yesterday musicians friend just finished blowing out all the lemons at $99.

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