5f2a (another beginner with the same questions sorry)

Discussion in 'Shock Brother's DIY Amps' started by totterer, May 24, 2018.

  1. totterer

    totterer Tele-Meister

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    No pops from any grids. On the 12AY7 pins 2 and 7 grids are silent. I get a lot of noise from pin 6 plate, voltage now 125v DC. The other plate pin 1 on the 12AY7 is silent. Voltage there is 138v DC.

    On the 6V6 Pin 5 grid is silent also. I get a little noise from pin 4 screen. Sorry to be confusing, but I misread the pin #s in my earlier post. Its the grid that is slowly ticking up in voltage which makes sense I guess. I turned it off at about 158mV. So electrons are building up on the grid, correct? They should be passing through it seems maybe?? The cathode is near perfect at 20.6v.
     
  2. bigguy12321

    bigguy12321 Tele-Meister

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    Hey @totterer , Sorry to sound like a broken record but are you sure that lead from the left post isn't touching the back of the pot?

    IMG_20190516_093614586.jpg

    That may cause the first triode to be silent. Other than that keep chop sticking for noise.

    Keep pushin' on!... Carefully...

    a
     
  3. totterer

    totterer Tele-Meister

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    Nope its good. I just triple checked. So now I suppose Im gonna start reading up on some serious troubleshooting. It seems the obvious has been eliminated. Not sure where to go from here. I know I could check for continuity in places where it should be (not that I know exactly where it should be). I should go back through the schematic and double check everything. I guess I was just kind of assuming it would work but now I'm stepping into a bigger world I don't really understand very well. Funny I remember before deciding to do this, I read a bunch of amp building disaster threads, now mine feels like its turning into one.
     
  4. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Since you don't get a pop from the power tube grid but you do get speaker noise there has to be a problem with the power tube circuit. Closely examine every solder joint, component value, leads--everything.
     
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  5. totterer

    totterer Tele-Meister

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    Okay I'll focus my attention there.

    Just thinking about the amount of confusion I feel for this small simple circuit, really puts into perspective the difficulty of troubleshooting larger more complex amps. Especially for beginners. Im glad I started with the 5f2-A!
     
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  6. totterer

    totterer Tele-Meister

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    Found something Im not sure about. I remember when I was going through other 5f2A builds I noticed a ground that is not on the Mojotone layout. I added it and now Im wondering if this might be the problem?? It is Is this ground necessary? Its the one just below the 1 MEG resistor on the 1st input.
     
    Last edited: May 19, 2019
  7. totterer

    totterer Tele-Meister

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    IMG_20190518_120244161.jpg
     
  8. totterer

    totterer Tele-Meister

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    I went through and checked every resistor value. Oddly the two 68k input resistors are reading 33.5, I swear I checked them before I put them in. I can't get a good reading on the 1M resistor on the input. It checked out before. Other than that they all check out. I can't find any obvious cold solder joints. There was a possible short on the shielded cable ground on the back of the volume pot, but this didn't seem to effect anything. I soldered the speaker connection cables to the speaker instead of the spade connectors (recommended on Robrobs troubleshooting site). Still nothing. Maybe this ground is a problem??? Running out of suspects to interrogate.
     
  9. totterer

    totterer Tele-Meister

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    image.png
     
  10. totterer

    totterer Tele-Meister

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    image.png
     
  11. totterer

    totterer Tele-Meister

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    Last edited: May 19, 2019
  12. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    With nothing in the inputs, the pair of 68k Ohm resistors are in parallel giving 34k Ohm resistance: just like your measurement. Put a plug/cable in one or two inputs (nothing on the other end) and you should get 68k Ohm for both resistors.
     
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  13. totterer

    totterer Tele-Meister

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    Thanks, I thought it might be something like that. I still working on the basics.
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Your 22k NFB resistor is solid black. Is that a 22k resistor?
     
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  15. D'tar

    D'tar Tele-Afflicted

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    A pop on v1b p8 is as good as a pop on the grid of 6v6, this would indicate the signal path is good from that point onward through to the speaker. sometimes it presents only as a scratching sound on the output tubes grids and plates. I'm not sure what you have going on with your tone switch mod. What are the specs on that switch? Be sure your volume is turned up. Also, what values are you using for NFB mod. I would crank up the pot for the least amount of NFB or even disconnect the nfb tap while troubleshooting. Your signal is likely grounding out through the volume tone controls. Can you ohm from v1p7 to ground and rotate the volume pot... You should see 0-1Mohm range.
     
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  16. totterer

    totterer Tele-Meister

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    its a 10k 1% metal film resistor (RN60D whatever that means) I got from the local supply place. https://www.surplussales.com They have a lot of cool old tubes and military equipment. I thought it was weird cause I was expecting it to be blue with stripes?? But it tests out at around 10k resistance. Its a 10K because I was lowering the threshhold of the the NFB to increase the range with the 50k pot. If that makes sense.

    The tone switch mod is from this build http://www.tdpri.com/threads/building-a-5f2a-here-goes.822616/page-4 The switch is a JBT DPDT mini 5amp Rnd toggle. Also from surplussales.com. Values for NFB mod are 10k resistor and 50k audio taper pot.

    Does the amp need to be on? Im having a little hope return:) Answered my own question. 0 -.97 M amp off
     
    Last edited: May 19, 2019
  17. D'tar

    D'tar Tele-Afflicted

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    Thats a good reading. Yes amp off caps drained.
     
  18. totterer

    totterer Tele-Meister

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    OH BABY!!! I have no idea what just happened, but all the sudden it works!!!! I tried everything and nothing worked now all of the sudden everything works!!?? Not sure if is something I changed or maybe there is a short somewhere that I jostled loose, I have no idea, but I guess for now Im just going to play with my new toy!:cool: Sorry for leading everyone down my bad amp building acid trip. I just went to heaven for a few moments, so thanks to everyone for all the guidance. I may be back with more questions but for now a lot of new sounds to explore.:D Ill try to get some sound samples or something. Such an amazing group here! THANK YOU!!
     
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  19. totterer

    totterer Tele-Meister

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    IMG_20190519_115935232.jpg I know your jealous of my guitar. Its a 2017 Maestro 23 1/2" scale "hollowbody electric" I call it my Tyrdland. Sorry no Tele..
     
  20. totterer

    totterer Tele-Meister

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    IMG_20190519_120006094.jpg
     
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