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Mojotone Studio One inspired build

Discussion in 'Shock Brother's DIY Amps' started by reggiepe, Oct 2, 2020.

  1. andrewRneumann

    andrewRneumann Tele-Holic

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    Careful doing this. Without a path to ground, the grids can slowly accumulate positive charge leading to abnormally high current. Shouldn’t be too big a deal with cathode bias, but I thought I’d say something. Normally, to remove a signal from the grid, just ground it directly.
     
  2. reggiepe

    reggiepe TDPRI Member

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    Thank you all for the input on this!

    What the? are you serious? That is the exact hum I am hearing "filament hum"
    Learned something new today. After work, this is the first thing I tackle today.

    Ok, that makes sense. Along with changing the heater wiring I will be connecting the cathodes together.

    I always have to sit down with google to figure out reflected imepdance...do I go up and set it at 16 ohms, or do I go down and set it to 4 ohms.

    The Mojo has the Grid leak at 330k for a single tube. Should the 330k be left alone when they share it, or does that need to be changed?

    It was for a quick couple of seconds. When the tube started conducting and I heard the hum, it got shut down.
     
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  3. andrewRneumann

    andrewRneumann Tele-Holic

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    What do you think @Lowerleftcoast ?My read on the matter is that heater phase could make a small difference, but I have seen it argued it makes no discernible difference, depends on the individual tube, etc. Depending on how painstaking this will be, I would maybe save this idea for later. Seems like something bigger is going on—grounding issue etc. ???
     
  4. andrewRneumann

    andrewRneumann Tele-Holic

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    330K should be just fine, I wouldn’t change it.
     
  5. reggiepe

    reggiepe TDPRI Member

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    @andrewRneumann Ok, I will hold off on the phase reversal then. If the hum would not be loud like an open circuit at full volume, then that probably is not it. Kinda glad for this as the heater wires are buried under everything else on the socket. I have already removed everything once and rewired (actually multiple times) and it's not so "pretty" anymore.
     
  6. andrewRneumann

    andrewRneumann Tele-Holic

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    I think you right @dan40 . The datasheet says 3.5W per section, but Rob’s calculator shows 7W. A quick check of the datasheet with the voltages given by OP gives an estimated 6W dissipation per section. That’s 170%. Now I know why you were asking about redplating. :)

    When breaking out the cathode resistors like @reggiepe did, the value should have been doubled (from 470R to 1K) since there was half as much current going through each one.

    With about 305V on the plates, the bias should be around -13V to -14V for 100% dissipation.
     
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  7. reggiepe

    reggiepe TDPRI Member

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    I am working from home today and work is slow. so I tried some things and noticed something else:

    I coupled the cathodes on each socket with a 470R. I am at 321V on the plates and 11.81vdc across the cathode resistor with a single 12BH7. This gives me zero hum. I moved the tube to the other socket and get 321V on the plates and 11.82vdc across the cathode resistor. With Rob's calculator, I am a little cold at about 57% dissipation. But that is fine for now.

    New discovery....with both 12BH7's socketed, I get the hum. Nothing plugged into the inputs the "clean channel" (just volume and tone) starts squealing when you start turning the volume up. It does not exhibit this behavior with a single power tube installed. I pulled the first preamp tube (disables that channel) and it still hums.
     
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  8. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    From regiepe's description:
    It has low frequency hum with both tubes in place after they warm up.
    It does not hum with just one.
    The bias calculations seem to be cool enough with no visual red plating.

    It hums when the second tube is installed. So... what has changed? Well, there is an extra three dB because of the added tube and the heaters may be out of phase. Possibly a noisy tube?

    Perhaps there is a grounding issue but would an extra three dB exacerbate the ground issue resulting in this low frequency hum? I would think a ground issue would be present even with one tube installed.

    Maybe the design is on the edge and the extra three dB is enough to start an oscillation. We don't know yet.

    There is a hypothesis where the heaters would induce hum.
    I hypothesized, with the added tube, an induced low frequency hum may lead to a low frequency oscillation.

    As you suggest, and reggiepe affirms, changing the heater phase may be a painstaking endeavor and there may be a totally different reason for the hum.

    I am just throwing ideas out there. For sure the fix will be the last thing reggiepe tries.;)
     
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  9. andrewRneumann

    andrewRneumann Tele-Holic

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    I think when Rob’s calculator asks how many tubes share a cathode resistor, it literally means a single tube with 2 triodes inside. So in this case you would select 1 tube for one resistor. Here’s what I get:

    F0BC9F50-D917-4007-AA8D-8E8D9A2BC6F0.png

    Datasheet says 3.5W per section, Rob is using 7 watts for a single tube with 2 triode sections.

    D19516D1-6819-4665-A289-4E7C8AB40C3B.jpeg
     
  10. reggiepe

    reggiepe TDPRI Member

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    @andrewRneumann

    With that....I am driving these tubes way too hard and probably need to change out that 470R that Mojo has in their schematic.
     
  11. andrewRneumann

    andrewRneumann Tele-Holic

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    Me too. :)

    What if @reggiepe kept the signal phase within each tube the same? Right now he’s got each half of each tube working opposite phases. Something like this might work better? Realizing this is probably as painstaking as rewiring the heaters. o_O

    What about the guitar signal? Are you getting any guitar signal with both tubes in? Any with one tube plugged in?

    image.jpg
     
  12. andrewRneumann

    andrewRneumann Tele-Holic

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    A little too hard maybe. Any idea what plate voltage Mojo was running at? I don’t think you should worry about being 12% over if you aren’t redplating. Things like this get fudged all time.

    After you combined each tube onto a single 470R cathode resistor, all you would need to do is run a jumper from one tube to the other tube to connect all the cathodes and test my theory out.... :rolleyes:
     
  13. reggiepe

    reggiepe TDPRI Member

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    With both tubes the guitar signal is very weak. With a single tube it sounds fantastic and everything works as should. It is just the addition of the second power tube in parallel that is wreaking havoc.

    I like your schem, I was almost there when I tied pin one and six on each socket and sent the ot to pin 1 on each. The only thing I didn't do was tie the cathodes. THe signal was weak, but there was no loud hum.
     
  14. reggiepe

    reggiepe TDPRI Member

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    318

    Layout with voltages attached
     

    Attached Files:

  15. reggiepe

    reggiepe TDPRI Member

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    @Lowerleftcoast , @andrewRneumann ,@dan40

    BOOM!

    The filament phasing on those two sockets matter. Totally disappeared with a pair of 12BH7's after I reversed one of the sockets. Just a hair louder, but I wasn't going for lots of volume, I was going for the "let's see if we can make this work"

    Thanks so much for all of the assistance.

    I tried a pair of ECC99's just to see and I get a different hum out of them (individually they are fine). Strangely enough THAT hum disappears when I touch the cathode on V4 to get a reading. I will need to dig into that a bit more.
     
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    So maybe a cathode cap might help?
     
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  17. andrewRneumann

    andrewRneumann Tele-Holic

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    Glad to hear it’s working! I’m confused though... you said with two tubes there was a loud hum and the guitar signal was weak. Now you reversed the heater polarity on one tube and the guitar signal came back and the hum went away?! I’m befuddled. :confused::confused: What is the final wiring that made it work?
     
  18. reggiepe

    reggiepe TDPRI Member

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    I have read that with self split you can't use a bypass cap on the cathode. If I'm not mistaken the out of phase signal is coming though there and fed into the grid connected to ground. I may be wrong though as this self split is very new to me.

    The wiring that had no hum but a weak signal was one that you suggested few post back with having each tube (instead of each triode) working the push pull. I had wired it that way to try before I posted here, but did not join all of the cathodes together....no hum, weak guitar signal.

    The final wiring was a duplicate of mojo's in the schem and layout and bridging pin 1 and pin 6 between the sockets. When i swapped pins 4&5 with pin 9. The hum disappeared.

    Hope I described that properly.
     
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  19. reggiepe

    reggiepe TDPRI Member

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    Thanks again to all that helped resolve this issue. I worked with the Mojo schematic and edited it in Paint to show how this amp was built. I used a common 18 watt chassis to house the amp. This gave me 2 separate channels without the relay switching that Mojo had originally designed. With that I mixed the channels using a 100k and 220k resistor with a cap over the 220k as it was sounding a little dull. The cap brightened it up a bit. The 2nd 12BH7 doesn't add much volume, but running though a 2x12 and 4x12, this amp is huge.
     

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  20. andrewRneumann

    andrewRneumann Tele-Holic

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    I will say it one last time and then shut up ... to ensure equal load sharing amongst the valves, I would still keep all the cathodes connected together. I’m curious if this would change the sound in any detectable way. As you have it now, the anodes are shared, but the cathodes are split so *i think* you could be getting some negative feedback if the triodes aren’t all amplifying the same amount (or if the cathode resistors aren’t exactly the same). You don’t have to try it, but a simple jumper would be an easy enough experiment!

    Props to @Lowerleftcoast on the heater wiring suggestion!
     
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