2104/JCM800 Micro Amp

Discussion in 'Shock Brother's DIY Amps' started by robrob, Mar 12, 2019.

  1. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I'm not yet finished building the first Blackvibe Micro but I couldn't help myself and stuffed a JCM800 into a Princeton Reverb chassis with a 1 watt 12AU7 power tube.

    The Master Volume Micro is full-up JCM800 to the power tube circuit. It uses a Princeton Reverb chassis, power transformer and cab. I swapped the not-so-useful pre-phase inverter master volume for a Trainwreck Type-3 post phase inverter master volume (PPIMV) so the phase inverter's gain is added to the master volume control and we have more control of how hard the tiny power tube gets hit.

    Please let me know if you see any errors in the schematic or layout.

    [​IMG]
    [​IMG]
     
  2. bftfender

    bftfender Friend of Leo's

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    trainwreck 3 is the best MV on a 2204 circuit, mine has a switch to bypass and work very well from barley audible to dimed..only thing it does is it gets a lil nastier as MV comes down but for these amps thats a plus..especially with the new bedroom craze
     
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  3. Nickfl

    Nickfl Tele-Afflicted

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    I think this one will be popular, the JCM 800 seems like a great candidate for the "micro" treatment
     
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  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Here's the layout:
    [​IMG]
     
  5. jtcnj

    jtcnj Tele-Holic

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    I am seriously considering this build next, thanks once again for all your fine contributions.

    Is the cap can 40/20/20/20?
    If so should not the first filter in the schematic be 40µf?

    jcm800micro 1st filter cap.png
     
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  6. jtcnj

    jtcnj Tele-Holic

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    Isnt that the 6v6 22w layout?

    EDIT; I will probably use the Hammond 290CAX PT and 156L choke with the 125B OT.
     
    Last edited: Oct 17, 2019
  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    JTCNJ, the 40uF filter is right and below your red circle by 2 inches "B+ filter 40uf".

    You're right, I did post the 6V6 layout. Here's the 12AU7 Micro layout:

    [​IMG]
     
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  8. shortcircuit

    shortcircuit Tele-Meister

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    Have you considered using a pair of 6AN5 tubes? Unlike the more well known 6AK6, the little tube can take high voltages. I've been playing with it and it sounds great.
     
  9. dan40

    dan40 Tele-Afflicted

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    I built this circuit just last week and can attest to the fact that it does sound great. I got greedy though and wanted more gain so I ended up redoing the preamp in the style of the Splawn Quickrod. Great little amp for high gain tones at tv volumes but it can get pretty loud if crank it up. I did end up using a 12bh7 as the power tube for a little extra juice.
     
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  10. dan40

    dan40 Tele-Afflicted

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    Can you give an idea of the circuit and OT specs? Been looking to do another small push/pull circuit with a couple of pentodes.
     
  11. jtcnj

    jtcnj Tele-Holic

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    Sourcing 1W parts now.....
    I am likely going with these Hammond parts from Hawk Electronics, after I go over it all again:

    PT: 290CAX
    OT: 125B
    Choke: 156L
     
  12. dan40

    dan40 Tele-Afflicted

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    I got my choke and OT from this company. First time using them but the OT sounds great and they get good reviews from everyone that has used them.

    https://www.musicalpowersupplies.com/products/


    Went with the OT5PP+ in my build. He does wind these to order but I had both items in less than two weeks.
     
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  13. jtcnj

    jtcnj Tele-Holic

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    I got the OT for my 18w build there, no complaints here.
    Forgot about that supplier; will give it a look too as I have not ordered mine yet.
     
  14. shortcircuit

    shortcircuit Tele-Meister

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    Yep, that's the exact same OT that I used in my little PP amp that I plugged the 6AN5's into.
     
  15. gmdattilo

    gmdattilo TDPRI Member

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    Has anyone attempted the Blackvibe micro yet. I've recently finished a Bassman micro and after playing mine my coworker is now pretty excited about building his own. In his research he found the Blackvibe micro and approached me about helping him with a build. Looking at the schematic I was pretty surprised to see that the B+ is listed at almost 400V. Thinking this was a typo I looked at the power transformer and found it to be 275-0-275 so 400ish volts is reasonable. Its reasonable except that I know the plate voltage on my Bassman micro is under 300V and I'm running a 12BH7. I decided to build a spice simulation this evening to see what the dissipation on the 12AU7 in the Blackvibe would be. I found it to be over 10W which is not reasonable. I was pretty sensitive to this issue from my Bassman build because I used a power transformer that I had laying around. I had to add about 2.1k of dropping resistor before the first filter to get my B+ down to a usable value. I don't see a big dropper in the Blackvibe micro so I'm thinking that if built according to the schematic the output tube is gonna red-plate and fail pretty quickly.

    Hopefully Robrob will see this and and comment. I think if anyone plans to build a Blackvibe micro they will need a lower voltage power transformer or plan on adding some significant dropper. Capture.PNG
     
    Last edited: Nov 4, 2019
  16. dan40

    dan40 Tele-Afflicted

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    I have not built the Blackvibe but I did recently finish the micro JCM 800 from his website. I used a spare Classictone PT (40-18051) that I had laying around which was rated at 255-0-255 @ 55ma. All of the dropping resistors in the B+ string are the values that he recommended to hit the proper voltages in the preamp. I ended up with close to 340vdc on the plates of the 12bh7. I can't remember the exact dissipation numbers at the moment but I do remember that it was running a bit hot with the stock 500ohm cathode resistor. IIRC, I ended up raising the value up to 700ohm or so and the amp has been running great for several months now.
     
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  17. gmdattilo

    gmdattilo TDPRI Member

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    I agree your numbers. With 750 ohms on the cathode 340ish V on the plates puts you right at the 6.5W dissipation rating of the 12BH7. With the normal 510 ohm cathode resistor you'd have been in the same situation as me. I decided to drop the B+ instead of changing the bias. Now you've got me wondering what it would sound like if I raised the cathode resistor and removed the droppers. Hmmm...
     
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  18. dan40

    dan40 Tele-Afflicted

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    I think it's important to keep the preamp voltages up to the original manufacturers values if you are shooting for the same sounds. The triode output stage does change the sound and character of the amp quite a bit but little 1 watt JCM 800 sounds convincingly similar to it's larger brothers.
     
  19. decsnspace

    decsnspace Tele-Meister

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    I've got this on my up next to build and now beginning to source parts for it.
    To keep from having to use a bunch of different suppliers I was thinking of getting these transformers from Antique Electronic Supply. I believe they should work based on their specs.
    Choke: 194A https://www.tubesandmore.com/products/choke-hammond-fender-deluxe-deluxe-reverb-etc
    OT: P-T125B https://www.tubesandmore.com/products/transformer-hammond-universal-push-pull
    PT: P-T290AX https://www.tubesandmore.com/products/transformer-hammond-guitar-amplifier-replacement-fender

    Also, I'm unfamiliar with the secondary wiring of the OT Hammond 125B. Is it possible to wire up this OT for switchable or separate 8 and 16 ohm taps?
     
  20. dan40

    dan40 Tele-Afflicted

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    I believe that you can only use the 8 ohm tap when wiring the Hammond OT for a 22k plate load. The OT that I linked to above will allow 4/8/16 ohm options at 22k. They also sell an appropriate PT and choke at very reasonable prices.
     
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