Supro Thunderbolt Build

Discussion in 'Shock Brother's DIY Amps' started by Nickfl, Jun 13, 2017.

  1. Nickfl

    Nickfl Tele-Afflicted

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    I had a thread going a while back where I was asking advice working out the schematic for this build, but I thought I should go ahead and post a new thread for the actual build, so here we go.

    Here is my schematic:

    TB insp schematic.png

    and layout:

    TB insp layout.png
    As you can see there are a few mods from the original thunderbolt, including: single and parallel triode first gain stage inputs, switchable tone slope, a bright switch, a cut control (switchable to a master volume), a raw switch, and switchable tube/SS rectification. I am also probably going to put a bypass cap on the power tube cathode resistor, and I've upped the first filter cap size a bit since I'm using a rectifier that can handle the extra capacitance. Special thanks to tubeswell for helping me out a lot in my schematic thread and to robrob, as most of these switchable mods come directly from his website.

    I've already started building the cab, pictures of that to come. And the actual amp build should begin any day now...
     
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  2. Nickfl

    Nickfl Tele-Afflicted

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    Here's the cab, ready for tolex.

    image.jpeg

    It's roughly the same dimensions as the original, but with a straight back instead of being sloped on both sides and with a top cutout for the 5e3 chassis I'm using. I'm going to use a Weber 15F150B speaker.
     
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  3. arielyitus

    arielyitus Tele-Meister

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    Nice, subscribed!
     
  4. Nickfl

    Nickfl Tele-Afflicted

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    Here's the cab with the Tolex on and hardware installed. The color doesn't come through well in the pictures, but I used sky blue Tolex, since neither the vintage or reissue Supro style covering is available anywhere as far as I know. I am going to do the baffle in silver sparkle, but I'm waiting on the speaker so I can measure for the t-nuts before I do the grille cloth.

    Front, with a beam blocker like the original:

    image.jpeg

    And the back with my assistant:

    image.jpeg

    I also installed turrets and mostly populated the circuit board, but of course I ended up missing a couple of caps that I somehow forgot to order, so I couldn't quite finish it up and move on to wiring yet...
     
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  5. Dan Miller

    Dan Miller Tele-Meister

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    Good thing AES has a 15% off all caps sale until the end of June (https://www.tubesandmore.com)

    ;)
     
  6. Nickfl

    Nickfl Tele-Afflicted

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    I made a lot of progress Saturday afternoon, got the controls wired up and installed:

    image.jpeg

    And the board installed, aside from the couple of caps mentioned in my previous post, which I am still waiting on:

    image.jpeg

    My last two builds were in blackface style chassis, so this was a rude reminder of how tight it is inside a tweed. I had also forgotten how much those tiny solder lugs on the switch of a DPDT push pull pot suck, having not used those in a couple of years. I also wired the PT primary and tested all the secondaries, which are all producing proper unloaded voltages. Now I just need to wire up the sockets and get my OT and missing caps!
     
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  7. Nickfl

    Nickfl Tele-Afflicted

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    Speaking of the OT, I am about ready to order it and I can't quite decide what to get. Originally I was planning to use the 40W Supro reproduction that classic tone sells with a 5K primary. This afternoon I plotted some load lines for my estimated voltages using the interactive data sheets at BM amps, and it looks like a 4K primary might actually get the tubes to preform more like the original thunderbolt. With my estimated B+ of 380v and a 5k primary I get a line right at the knee of the graph, but when I plot the original thunderbolt at 425v and 5K I get a line a good bit above the knee. If I substitute a 4K primary in with my 380v the lines moves up above the knee and looks more like the Supro. The best I am able to interpret this is that with my voltage and the 5K I will get less headroom, more distortion, and slightly less output wattage as compared to the 4k or the original thunderbolt with the higher voltage and 5K. Part of me wants to use a 4k transformer to get closer to the supro, but I also kind of like the idea of letting it be a dirtier amp and I certainly won't miss 5 or so watts of output in a 40 watt amp. The price is not meaningfully different and they both will require me to make an adapter plate to fit the chassis, so that doesn't help me decide.

    Suggestions please, I'm really torn here and I'm not really sure how much difference it will make.
     
  8. Nickfl

    Nickfl Tele-Afflicted

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    Nobody has any thoughts on the transformer? I am planning to go ahead and order a a classic tone 40-18006 tonight, I think the 4K primary impedance will help it sound more like the original at these voltages. So, if anyone has any opinion to the contrary, speak now or forever hold your peace. :p
     
  9. tubeswell

    tubeswell Friend of Leo's

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    It is interesting to note that for push-pull cathode bias where you are running the output tubes at 100% dissipation at idle (i.e. Class A Push-pull operation, which the t-bolt certainly does), you get more output power with a reflected load that is twice what you would normally calculate for a fixed bias push-pull stage, than if you ran the Class A PP amp into the same load.

    So, if you get a PT with (say) a 4k primary and 2R, 4R and 8R secondaries, you can always switch the speaker to a different secondary tap to make the Primary load 8k. (or at least, you have options). These Heyboers are perfect for this (and are what I have put in several of my T-bolt style builds).

    https://www.tedweber.com/hy018343

    YMMV
     
    Last edited: Jun 29, 2017
  10. Nickfl

    Nickfl Tele-Afflicted

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    Thats a good point about having options with the multiple secondaries. It looks like that heyboer is pretty comparable to the classic tone I'm looking at, except I was looking at the 4/8/16 version.
     
  11. Nickfl

    Nickfl Tele-Afflicted

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    Over the weekend I wired the sockets, heaters, and all the PT secondaries.

    image.jpeg

    Now I just need my missing filter cap and the out put transformer and I'm done!

    My speaker arrived today (Weber 15F150b), so I can finish up the cab while I'm waiting on the last few parts to arrive.
     
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  12. Nickfl

    Nickfl Tele-Afflicted

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    I finished the cab yesterday, installed the baffle:

    image.jpeg

    And test fit the chassis:

    image.jpeg

    I wish I could have gotten something closer to the vintage Supro vinyl covering, but I'm still pretty happy with how it came out aesthetically.
     
  13. danlad

    danlad Tele-Meister

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    For this Supro/5E5 inspired head I use I really, really wanted it as dead on by the textbook as possible so ran a 4k OT with my 380v supply voltage. Then tried it with 8k and it sounded better.

    I know, I know. You can't quantify 'better', but I really wanted to play at being a real old timey engineer & use the datasheet but instead ended up finding the received wisdom about going higher than you think for cathode bias turned out to be.

    Not that 4k sounded bad mind.
     
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  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The amp and cab are looking great.
     
  15. tubeswell

    tubeswell Friend of Leo's

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    That looks cool
     
  16. Nickfl

    Nickfl Tele-Afflicted

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    Thanks guys!
     
  17. Nickfl

    Nickfl Tele-Afflicted

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    Well, I got the last of my parts Friday and finished putting everything together on Saturday. Tonight I fired it up and everything went well until one of the power tubes started red plating :(. Switched the tubes around and checked various connections and long story short it looks like one of the new JJ 6l6s I used to test the amp is bad tube with an intermittent internal short of some sort. I'll pull some others from a different amp and try again tomorrow as I don't have the motivation to keep messing with it tonight.
     
  18. tubeswell

    tubeswell Friend of Leo's

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    With the tube unplugged, check that you have negative bias voltage on that socket's signal grid pin (Pin #5). Maybe you need to re-tension the pin clamp (with the amp off, discharged and unplugged)
     
  19. Nickfl

    Nickfl Tele-Afflicted

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    Its cathode biased and I checked all the socket connections to make sure nothing was loose or touching an adjacent pin. I had 29.5 volts across the cathode resistor and I had stable voltages for a couple of minutes and based on that (389v plate to ground, I believe) I was at about 85% diss. A couple minutes after warming up the amp started humming a bit louder, my plate voltage on the V4 power tube started dropping and it began to red plate. Switched the power tubes so the tube that redplated was in the other power tube socket and the issue followed the tube, all of which leads me to think it is the tube rather than the socket. I will go ahead and tighten the socket before I try a different set of tubes, that is a good point.
     
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  20. tubeswell

    tubeswell Friend of Leo's

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    Oops - forgot this thread was about a t-bolt LoL. Of course its cathode biased!

    Yes it seems like a bad tube at this point.

    FWIW 389V on the Plate and 29.5V on the cathode = 359.5 plate-to-cathode, which is what you use to assess plate dissipation. You also need to know exactly what the cathode resistor's resistance is, before you can get a reliable calculation of cathode current (which will be shared by 2 tubes in this case)

    A 5881 screen runs at about 5mA when its set up at about 400V on the plate/screen (so this should be subtracted from the cathode current to get the plate current for each tube).

    What sort of 6L6/5881s are you running? (Some of them are only rated for 19 or 20 Watts)

    Check that you have the correct load resistance for the tubes. For this type of Class A operation, with plate voltage of 389VDC, and assuming you have (say) 20W 5881s*, you want to be running a load resistance of 7k5 plate-to-plate (or slightly higher).

    *For 30W 6L6GC, you'd use 389/(30/389) = 5k plate to plate


    And if the shared Rk measures 200R exactly, the shared current is 29.5/200 = 0.1475A, which is 74mA per tube - less 5mA g2 current = 69mA. 359.5 x 69mA = 24.8W. Not quite full Class A for a 6L6GC, but overheated for a 5881.
     
    Last edited: Jul 10, 2017
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