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Supro "Best Of 24" Build Thread

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Badside, Feb 16, 2017.

  1. Badside

    Badside Member

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  2. EFK

    EFK Member

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    My only comments would be (1) the later trem you are using is stronger at high volumes when compared to the earlier trem, although you can take the earlier trem (using only one triode) and run it's voltage off the screens node to get a much stronger trem without needing to go to a 2nd triode. then you have an extra half to use at-will. (2) The 500 pf caps at the inputs are absolutely, positively unnecessary in my opinion and only serve to dull the sound, especially when pretty much any guitar is rolling back the volume. Likewise the 2.2K at the input grids also don't seem to do anything, at least not in any real world situation I've ever encountered with any guitar. Keep the input as open as possible, there's no need to choke it back and the tone control at the input to the paraphase will do what treble nullifying needs to be done. (3) The later split cathode paraphase is even more unbalanced under drive conditions than the earlier shared cathode version. It's also brighter. That's a personal preference thing, just thought I'd mention it.
     
  3. Badside

    Badside Member

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    Thanks a ton for chiming in! This build wouldn't have been possible without all the knowledge you contributed on those amps.
    I actually asked questions about the trem and PI in the past but didn't get any replies, so I went ahead and decided to build the amp in a way that I can experiment with it.

    To be sure I understand, when you say "(1) the later trem you are using is stronger at high volumes when compared to the earlier trem", is that a good thing, a bad thing, or just something to know about? A stronger tremolo can be tamed down with the intensity knob right? Fact: I've never owned an amp with tremolo, I have no reference.

    2 - I put them as optional, don't see why I would need them indeed, especially with the 100k series resistor.

    3- The use of a 0.05uF cathode bypass cap on the first stage of the paraphase puts the boost at above 1.4Khz, that probably has a lot to do with it. What little audio samples I was able to find does show the later Dual-Tone version to be brighter and not as warm. The Dual Tone does seem like it would lose a lot of bite in the preamp, so perhaps they were trying to compensate for that. It will be interesting to both try a bigger bypass cap, and no bypass cap at all.
    The shared cathode resistor on the earlier version makes more sense to me, it seems that would help balance things out, but reading Merlin's article on Paraphase Inverter, he mentions it can cause some instability. Still have to think about it some more to wrap my head around this one!

    Again, thanks for chiming in, I appreciate it!
     
  4. cugel

    cugel Member

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    I love the grid-leak bias on the first stage. I did it to one channel of my national Val-Trem, which is pretty similar to your build. Maybe my imagination, but fuzz pedals and that grid-leak bias preamp get along very well.
     
  5. EFK

    EFK Member

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    Thanks for the compliment - very kind of you!

    The grid leak input is super-sensitive to the signal strength. Pedals seem to work wonderfully with the 1624T as it has a small (relatively) .01 input cap, and some of the smaller Valcos also used a .01 or even a .005. On the other hand, I have run into pedal issues with some Comets which used a .05 as well as most early grid leak Gibsons (.05). So just a seat of the pants thing, I think a pedal's ability to get along with a grid leak input is dependent upon the strength of the signal being put out and the size of the input cap.

    I think the later tremolo maintaining its strength at higher volumes is a good thing; it's very consistent regardless of volume. The earlier trem can 'fade' or get weaker when you really start wailing because the whole amp is sagging, including the power supply to the trem, and the earlier version relies very heavily on the power supply to control the intensity. It has a much narrower range of acceptable supply voltage.

    I've never come across any audible instability with a shared cathode paraphase and it does definitely help with the balance until the first stage begins to break up. At that point all bets are off; it maintains an excellent balance when clean but as soon as you begin increasing the volume and hitting it with a larger signal, like most paraphase inverters it goes bats**t crazy. Honestly, though, I think that's part of the charm of the 'valco tone'. One of the power tubes is being hit with a much larger signal than the other, so both are being driven at different points.
     
  6. Badside

    Badside Member

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    Here is the finished amp:



    And the schematic:

    [​IMG]
     
    EL34 likes this.
  7. Muzzy

    Muzzy Supporting Member

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    Great sounding amp! I love the 1624's. I have a Valclone and an Airline and really enjoy both of them.
     

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