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JCM800 Micro 1w heater elevation question.

Discussion in 'Shock Brother's DIY Amps' started by jtcnj, Dec 21, 2020.

  1. jtcnj

    jtcnj Tele-Holic

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    My Pt does not have a heater center tap (Hammond 290 CAX). I am using the 2 100R resistor artificial center tap.
    Do I need the 65v heater elevation components: 180k, 39k, 10uf?

    I understand the hum reduction of elevating the center tab, but @robrob also mentions it affects the cathode follower.
     
  2. Nickfl

    Nickfl Friend of Leo's

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    It's still a good idea. Without the elevated heater, a cathode follower can exceed the allowable heater to cathode voltage limit for the tube. Using an elevated heater supply reduces this difference and will make the tube in that position less prone to failure.
     
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  3. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Connect the artificial center tap resistor junction to the 83v heater elevation eyelet like this:
    (I don't know why but I seem to have lost the ability to post images from my website)
    [​IMG]
     
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  4. jtcnj

    jtcnj Tele-Holic

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    Got it, thanks rob and nick!

    EDIT: Just notice there are some changes to the circuit from the layout I printed last year.
    At a glance I notice an additional treble peaker and the MV is wired differently.
    I caught the NFB resistor change earlier.
     
    Last edited: Dec 22, 2020
  5. robrob

    robrob Poster Extraordinaire Ad Free Member

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    I didn't add a treble peaker on purpose. Are you sure?

    I did revert to the Marshall pre-phase inverter master volume.
     
  6. jtcnj

    jtcnj Tele-Holic

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    My bad; I didnt recall some of the board components from memory.
     
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  7. jtcnj

    jtcnj Tele-Holic

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    Got my chassis running with no issues through start up.
    I did raise the heater artificial CT as per rob's design.

    My voltages are similar to what rob's earlier version had, but my B+1 was 385v at the power tube plates versus 370v.

    I used a 470Ω power tube cathode resistor as what I had on hand, instead of the listed 510Ω.
    (From memory) - at around 13v at the cathode I had about 10.6w total = >5w per triode.

    I didn't see redplating, but figured the tube was pushed really hard.
    I upped the cathode resistor to 1k, bringing the B+1 up to around 395.
    This brought the dissipation to about 3.1w per tube.

    Sounds great; really liking it, still learning balancing the 3 signal volumes - including guitar volume / boost pedal, etc..

    The sweep of the master volume pot is really smooth and usable over the range. For some reason the similar MV on my 5e3 and 6g2 builds doesnt do much until about 12:00 and tapers quickly from around 10:00 down.

    Still need to finish the faceplate and build a head cab.
    Not as pretty as others, but it brings the goods.
    Almost zero hum, fair amount of hiss full on.


    20201228_184053.jpg 20201228_195858.jpg 20201228_200421.jpg
     
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