6G2 Trem/Pots Question

Discussion in 'Shock Brother's DIY Amps' started by jejj21, Apr 23, 2019.

  1. jejj21

    jejj21 TDPRI Member

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    Hey Gang,

    After having so much fun last year with the 5E3 build, I decided to do a 6G2 this year. I had Blues Jr cabinet with no chassis in the garage, so I decided to build a one-off chassis and put the 6G2 circuit inside.

    Having studied King Fan and the Professors builds, I reworked the layout 6G2 to be similar to a 5E3 style I did last year. I grabbed the raw materials and have been hard at it for several days.

    I finally got my last piece installed and ran through Rob's startup procedures and never had an issue. I check voltages along the way and made sure things were good.

    I play my first chord, once it all checks out and the amp sounds great. I don't hear much tremolo even with the speed and intensity dimed. I turned down the Volume all the way and I hear a hum from about 2 down to 1. I start messing with the intensity pot and if I go 0 or 12 the hum stops, but it is there anywhere in between. I also notice a hum from the Tone knob if the Volume and intensity are both down.

    If I set the Volume on 3 or higher, the hum is completely gone. I'm assuming a grounding issue, but I'm not sure where to look.

    So, any suggestions would be appreciated. I have exhausted, poked and prodded on this thing until I'm brain dead.

    Thanks, Joey

    6G2_Tube_Guitar_Amp.png
     
  2. aerhed

    aerhed Friend of Leo's

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    If your bias is too hot the trem won't oscillate.
     
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  3. jejj21

    jejj21 TDPRI Member

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    I kind of figured the bias setting would effect the tremolo. I have sweeped across the 10k bias pot from -24 to -36 and I don’t really hear the tremolo at either range. I know the original schematic shows -35, but I’m not sure the best way to measure the bias.
     
  4. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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  5. jejj21

    jejj21 TDPRI Member

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    Thanks for the link. I have read through all of the bias calc information multiple times. My Klein MM400 doesn’t get me granular enough readings that I have found to be able to leverage the calculator usefully.

    I have the JJ6V6S power tubes and no red platting happening. Since the reading in the bias circuit is in the -35 range, I’m assuming I’m close.

    I’ve seen people suggest taking the power tube value (14W) and the plate value at Pin 3 and dividing the two then multiplying by .7 and .6 to get the bias ranges. Is that method good enough to get me in the ballpark?

    Could the bias voltage be causing the interaction with the pots?

    Thanks for the help guys.
     
  6. Wally

    Wally Telefied Ad Free Member

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    That bias voltage is not going to tell the story since different tubes will yield different current draw at any one certain amount of negative voltage. IMHO, if one is going to build or work on these amps then one needs to learn how to take voltage and bias measurements. In fact, the first thing one should do after getting the amp fired up is to make a voltage chart....and that will include bias numbers....current draw.
    There are a number of ways of going about taking bias measurements. I am old school and use the transformer shunt method...it is the quickest method. It is accurate enough for my purposes. It is also the most dangerous, perhaps; but it is no more dangerous than taking voltage measurements on a live circuit, ime and imho.
    Regardless of how you take bias numbers, it has to be done.
     
  7. jejj21

    jejj21 TDPRI Member

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    Thanks Wally. I have been checking voltages as I walked through the startup process, but I didn’t record the information in a formal manner. I will make that happen when I get home tonight.

    I will try the transformer shunt method tonight as well with the meter I have. If I can’t get what I need with the equipment I have, I’ll start looking for a Fluke.

    Thanks!!
     
  8. jejj21

    jejj21 TDPRI Member

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    As promised, I went through and gathered the voltages into a table and checked the bias using the information on Robs site. Please give things a once over and let me know if there are any apparent outliers that may be causing my issues with tremolo and pot interactions. (This is with the 10k bias pot full CW from inside the chassis, so 59% is as high as I can go.)

    Any and all help will be greatly appreciated. I'm so close to having this working, it's wearing on me not being able to figure it out.

    6G2 voltages.png
     
  9. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    You will have to change the resistor between your diode and pot to bring yourself into an area where you have range to move. I like this description:
    https://el34world.com/charts/Biascircuits.htm
    Rob probably has a good explanation as well.
     
  10. jejj21

    jejj21 TDPRI Member

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    I’m assuming you are referencing the 22K resistor and not the 100k 3W resistor.

    I have a spare 27k resistor, would that be enough of a change, given where I’m currently at or should I be looking to do something with two resistors in series?

    Thanks!!
     
  11. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Honestly I have to look it up every time I run into a range issue. Something tells me maybe to try the 27k parallel first. If it’s the wrong direction- like you can only get to 53% put them in series.
     
  12. jejj21

    jejj21 TDPRI Member

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    Ok, I added a 56k resistor in parallel with the 22k and was able to get my bias adjustment from 53% - 75% which is a much better sweep. Resistor is now about 15.8k vs the 22k I started at.

    The bias now has room to be changed, but has had no effect on the tremolo or the weird interaction with the pots, bias pot included.

    The layout I posted is a duplicate of what I have setup currently. Is my grounding on the bias circuit to the lug of the PT and not the Power Bus an issue? Should it ground to the PreAmp bus, since the intensity pot is involved.

    Again, any and all help is appreciated.
     
  13. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Having the range now is good and takes that part of the equation. Usually a ground location is chosen for hum. I wouldn’t think that that will fix things. I can’t really read the values on your layout photo from my telephone, so I’m not able to double verify components from the original. I bet you have though. Also how did your voltages line up with the published circuit from Fender? I see you have a pot on the rear as well. Does that use parts of the trem circuit?
     
  14. jejj21

    jejj21 TDPRI Member

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    So, progress!!! I found where one of my ground points was not soldered. Made the connection and a very beautiful tremolo came rolling out of my amp. I played for a solid 5mins just basking in the wonderful sound.

    I turned off the volume to go get my wife and there is still something going on with the Volume/Tone pots. If I have the Tone pot any where other than full and I reduce the volume below two, it cuts the guitar sound out and there is either a hum or a ghost tremolo coming through.

    Volume 2.5 and on, no issues and dead quiet, but I drop back the volume below that with any tone and it goes crazy. Bad Pot or Pots? Brand new CTS 1MA pots.
     
    Last edited: Apr 25, 2019
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  15. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    What value cap did you use for “intensity” cap? Original shows a .05 and you are showing a 47uf. Not a .047. Not sure if that would do it. I’ll keep checking as I saved your photo and can now read it.
     
  16. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Oh wow. Awesome job! I didn’t notice you got it!!!
     
  17. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Same with cap from tone to ground. Original shows .0047
     
  18. jejj21

    jejj21 TDPRI Member

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    Thanks for all the help Mr Ridesglide. The image I uploaded didn't come across very clear. The image should show the Intensity is 47nf and the tone 4.7nf in the annotation. The caps are 0.047uf and 0.0047uf respectively, I just didn't have room of the drawing to put all the zeros, so I converted the unit.

    All that is left now is the Vol/Tone Pot issue.
     
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  19. Mr Ridesglide

    Mr Ridesglide Tele-Afflicted Gold Supporter

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    Ahhh. Your math better than mine. :)
     
  20. fastedtex

    fastedtex TDPRI Member

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    Which input are you plugging the guitar into?
    If you have not, try the other one.

    The way your layout is the ground on the right jack is going to the switch then the to sleeve.
    When you plug into the right jack the switch is removing the ground.
    Not sure the 1M resistor is correct either.
    HiLow.jpg
     
    Last edited: Apr 25, 2019
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