Lil Bumble-B (2-channel micro)

Discussion in 'Shock Brother's DIY Amps' started by Lynxtrap, Jun 2, 2021.

  1. Lynxtrap

    Lynxtrap Tele-Afflicted

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    With all these great micro builds going on here, I couldn't resist it anymore :)

    Like the rest of us I have of course been looking at the designs by @robrob
    I began to plan a "high power" Twin, essentially the Bassman but with two pairs of output tubes, with half the 12AY7 as a switchable extra gain stage.

    Then I started thinking a 4 triode output stage kind of defeats the purpose of a "micro", and started rethinking it.

    I ended up with 5F6A/RobRob's circuit as the basic clean foundation, instead adding an overdrive channel that is sort of inspired by a Dumble ODS and also a design by Kevin O'Connor.

    The overdrive channel will most likely be tweaked when I get sound out of it, so this is more like a starting point. I put a switchable Framus mid control in there as well, I liked it in a previous build.

    I had already ordered a pre-drilled Marshall 18W chassis with five tubes. Haven't got it yet, I guess it might be a bit of a challenge to find room for the circuit, knobs and switches.

    The pre PI master and the presence control are left out for this reason.

    It's kind of Dumble, Bassman and it is little, so...a Lil' Bumble-B :rolleyes:

    Anyway, here is the first draft for it. Grateful for any input, opinions, suggestions etc!

    Lil Bumble-B rev1.png
     
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  2. sds1

    sds1 Tele-Afflicted

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    Looks fun! Seems like it will work but arm yourself with the parts and be prepared to balance out the various stages post-build...

    D3 and D4 are misoriented.

    The bleeder could be as small as 220K if you wanted a 4.5X faster bleed.
     
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  3. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Thanks, yes I think there might be a bit much gain in the drive channel, but had to start somewhere and tweak when I hear how it sounds and works.

    I see the error with the diodes now, thanks for pointing it out. I was kind of in a hurry to get it done, and drawing schematics on a computer is not my favourite part of amp building :oops: But they sure turn out easier to reed than hand drawn, and as Rob is kind enough to provide the DYIC file on his website I thought I'd go for it. I had to use a battery symbol for the electrolytics as I couldn't find the right one in the software :lol:
     
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  4. Lynxtrap

    Lynxtrap Tele-Afflicted

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    By the way, has anyone tried a bypass cap on the cathode resistor for the output tubes in a micro?
     
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  5. Asmith

    Asmith Friend of Leo's

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    I have but I didn't try without so I have no idea what differences there would be.

    Also the high gain mode will sound like a load of bees in a can. 4 gain stages even when most of them at 12ay7s is a lot.
     
  6. Lynxtrap

    Lynxtrap Tele-Afflicted

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    What value did you use for the cap?

    What makes you think that? IME you can have as many gainstages as you want as long as they are well balanced and attenuated at the right places.
     
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  7. robrob

    robrob Poster Extraordinaire Ad Free Member

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    The DIYLC capacitor has a check box for polarity which turns it into an electrolytic.
     
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  8. JohnnyCrash

    JohnnyCrash Doctor of Teleocity

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    I’m un-paralleling the 12AY7 and instead adding a JCM800 cascade switch on mine (Gain DPDT pull switch).

    I’ve had success cascading a 12AY7 in my full-size tweed Pro/Super head.

    Watching this thread!
     
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  9. King Fan

    King Fan Poster Extraordinaire Ad Free Member

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    Aha. Gotta look deeper in the 'schematic symbols' goody bag.... "polarized". Hadn't noticed that. Thank you, Rob.

    upload_2021-6-4_8-11-2.png
     
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  10. Lynxtrap

    Lynxtrap Tele-Afflicted

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    I got around to doing a layout.
    I have never built an amp with this kind of board before, with the components in a row. I ordered a board with pre-installed turrets, 30 on each side. And I managed to fit almost all components on it. On paper at least ;)

    The chassis is not this deep in reality. The tube sockets sit closer to the board, and the PT wires will come up underneath the distribution board.


    Lil Bumble B_layout_rev1.png
     
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  11. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Some small corrections and updates. The Clean/OD switch was not wired correctly, for one. Switches always mess with my brain...

    Lil Bumble B_layout_rev2.png
     
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  12. mountainhick

    mountainhick TDPRI Member

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    I'm a little confused about the schematic, I understand the circuit as I see it, but I don't understand calling both the input triode and the gain section, two tubes, for a total of three triodes V1.

    Input V1 "AB Parallel", and V1 gain circuit triodes are two different tubes, no?

    When engaged, the gain stage is in series with the input tube. What is the V1 "AB parallel" about?
     
    Last edited: Jul 4, 2021
  13. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Two V1 is a typo, and I hadn't even noticed that I forgot to draw the second parallel half of the actual V1! Thanks for pointing it out!

    I decided to have both triodes of the first tube in parallel like in Robrob's schematic. Because we can't have an unused triode, right? :D

    On the serious side, parallel triodes has advantages while not really adding gain.
     
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  14. mountainhick

    mountainhick TDPRI Member

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    OK, thanks, I'm not :confused: after all.

    What are these benefits of parallel triodes at unity gain? I could make a somewhat educated guess regarding current and tube longevity, but I'd just be guessing.
     
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  15. Lynxtrap

    Lynxtrap Tele-Afflicted

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    Actually gain migh be a little higher, but mu is the same as one triode. Since it is connected in parallel, there will be twice the current capability, capacitance, power rating and transconductance. Plate resistance is halved.

    The performance can of course be altered with component choises, and almost always is, like half the cathode resistor value (820R) in this case and in Robrob's circuit half the plate resistor value (47K).

    It can smooth out a 12AX7 a bit, since the performance of the gain stage will be more similar to a lower mu tube, but still with the gain of a 12AX7 triode.

    The 12AY7 has lower mu, so it remains to see how that tube works in this configuration. I guess it works fine since Robrob uses it in the micros, and many have built that circuit.

    Matchless is one maker who is fond of parallel triodes, they can be found in many of their amps and often run quite hot.

    I basically thought that the spare triode might as well be put to use.

    I will correct the schematic when I have the time.
     
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