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Let's call it a "μ-GA-9T" or "μ-Vibro5E1" - parallel SE design questions

Discussion in 'Shock Brother's DIY Amps' started by gigante, Feb 13, 2021.

  1. gigante

    gigante Tele-Meister

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    OH NO! I mean "micro vibro 5c1"!

    After building a little 6SJ7 into 6AK6 SE amp last year, I want to do it again. Last one I just put together some circuits I found online, but I am trying to be a little more deliberate this time. I'm mostly happy with the previous amp, but want to make some changes.

    This time I want to do parallel SE output. Using the datasheet suggestions for the 6AK6, each wants to see 10kohms, which means the two together should want 5kohm, correct? Probably the hammond 125ASE.

    I may put individual cathode resistors to allow me to pull (or switch off) a tube for half power (and double impedance).

    I also want to add tremolo. I was looking at the Kalamazoo Model 2 for a one-triode trem. Could I use the other half of the 12ax7as a gain stage? Would there be noise issues sharing a gain stage with the trem in a single tube? The triode gain would probably be before the pentode stage, because I want to get most of the preamp character out of the pentode. The Gibson GA-8T uses parallel output and has trem, but I don't know if I like how it's done. Or I'll just do the two triode

    PT will probably be the antek toroidal into at bridge rectifier.

    Am I making any bad choices here?
     
  2. NTC

    NTC Tele-Holic

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    A 5E11 and 5F11 Vibrolux shares the oscillator with the preamp. No issues.
     
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  3. gigante

    gigante Tele-Meister

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    Thanks, good to know.
     
  4. RollingBender

    RollingBender Tele-Afflicted

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    I recently built a single channel version of a blackface Deluxe but I subbed in a Princeton style 1/2 tube tremolo and a one tube reverb to keep the tube count down. The other half of the tremolo tube is being used as the last gain stage before the PI. I have no noise issues with this setup.
     
  5. gigante

    gigante Tele-Meister

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    Thanks for the additional confirmation.

    I read this somewhere, but the schematic I see looks to be PP. So it looks more like the princeton. GA-8 is parallel SE but GA-8T is PP. At least in the schematics I can find. I know Gibson liked to change things within a particular model?
     
  6. gigante

    gigante Tele-Meister

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    Per my other thread, I'll be going with the datasheet class A operating conditions.

    Here's my planned loadline: Load 10k per tube, 180V on the plates, 14mA per tube. Grid Bias will be -10.33V.

    As I understand Keuhnel, the grid can swing from -22.66 to 0V, and therefore my headroom will be 10.33V peak (17.3dBV). This would be clean(-ish), correct? So I want master volume control at 50% to give 10.33V peak, or close to it. I think.

    It should be obvious I am trying to learn all this, so don't be afraid to tell me I'm doing it wrong.
    6AK6 180V 10k.png
     
  7. 2L man

    2L man Tele-Meister

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    Looks good but perhaps you should use other Giuseppes calculator (Universal) where you can play with "headroom" and then you see distortions

    Grid can become positive and on datasheet they are on AB2 tables. Grid drive to zero volts and it is AB1. On Universal Loadline Calculator you get both powers. I have noticed that Screen voltage effects how "high" g1 as 0 rise.
     
  8. gigante

    gigante Tele-Meister

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    I'm using the universal calculator. When I put in 'headroom' values, I get the green area under the line, but I don't really understand how to use that info to make decisions.

    Headroom set to 100V

    at max g1:1.21
    at g1=0:1.21
    at class A/A2:0.98
    at headroom:0.50

    2nd:4.64 3rd:2.01 4th:0.30 THD:5.07

    Headroom set to 155V

    at headroom:1.20

    2nd:4.30 3rd:11.05 4th:2.59 THD:12.13

    Headroom set too high like 225V

    at headroom:2.53

    2nd:9.38 3rd:19.43 4th:1.96 THD:21.67

    Obviously as the output stage gets pushed harder, distortion increases, but I don't know what those distortion levels sound like.
     
  9. gigante

    gigante Tele-Meister

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    I have my circuit planned out mostly. A few things missing from the schematic, but it's mostly here. Tremolo stolen from the Vibro Champ. I'll try it as-is unless someone has a reason not to. The interstage attenuation will be figured out once I'm making noise. 6SJ7 bypass will be on a 3-way switch, as will the OT primary impedance to give 4k or 5k (with a change to speaker impedance).

    Will be my first try with the LND150.

    PT is the Antek AS-05T240 and OT is Hammond 1760C.
    uGA-9T - 041521.png
     
  10. NSB_Chris

    NSB_Chris Tele-Meister

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    Very Cool! I have never seen the power tubes driven off of the furthest B+ node (beyond the preamp). The data sheet that I found shows the 6AK6 max plate is 300V. Any reason you did not select a more traditional configuration and drive the power section with the 318V or 300V node and cascade down from there?
    Is there a reason you decided not to combine the cathode into a combined resistor&capacitor?
    Not judging, just interested in the basis of decisions and of course interested in the outcome!
     
  11. gigante

    gigante Tele-Meister

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    Thanks. I built another amp with the 6AK6 at 180V and I don't really want to push the little fellas too hard. It also had the 6SJ7 in the preamp, but only the one stage. I want another gain stage. I also want this amp to be about 3W.

    I want to see what happens if I bias them a little bit different from each other. My thought is that it could make the sound a little more complex, but it's an experiment. They may eventually be combined. Depending on what I learn with this build, I want to build another that may just be a single output tube/no trem/different OT. Who knows? Not I, that's for sure.
     
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  12. printer2

    printer2 Poster Extraordinaire

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    The 300V spec is for non-inductive loads. Seem to recall the tube was used to drive some radio transmitter tubes in portable equipment.
     
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