HELP! New 5F1 build testing voltages...now no sound

Discussion in 'Amp Tech Center' started by Battle Axe, Jul 19, 2020.

  1. Battle Axe

    Battle Axe TDPRI Member

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    I have assembled a 5F1 from scratch and I'm at the stage of testing voltages and plugging in the tubes. I'm following Robrob's startup procedure https://robrobinette.com/Tube_Amp_Startup.htm

    I'm almost there!

    I got to testing the DC bias voltage on pin 5 of the power tube socket and I got a reading of 18.6v. Sometimes fluctuating to 15v and 22v. His instructions say I should get a voltage between -30v and -60v. My readings are way off in the positive. Does anyone know whats gone wrong? I don't want to plug in the power tube and have it blow up!

    Here are my testing notes...

    I'm using a light bulb limiter.

    With no tubes plugged in...
    Voltage at fuse 234VAC (I'm in Australia using Classictone power transformer wired for 240v)
    Rectifier tube heater wires 5.45VAC
    Preamp and power tube heaters 6.5VAC
    High voltage wire 333VAC x2

    Installed rectifier tube...
    Filter caps all test at 388VDC
    Preamp tube pins 1&6 388VDC
    Output transformer 390VDC

    Installed preamp tube...
    Preamp tube pins 4&9/5&9 5VAC ***is this low? robrob says I should have between 5.9v and 7v
    Preamp pin 1 259VDC
    Preamp pin 3 1.75VDC
    Preamp pin 6 261VDC
    Preamp pin 8 1.83VDC
    Power tube pin 5 18.6VDC <------01000110 01010101 01000011 01001011 (me swearing in binary)

    HELP!!!!
     
  2. jhundt

    jhundt Doctor of Teleocity

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    I wouldn't expect a negative voltage there. This is NOT a fixed-bias amp. Rob's instructions, point # 7:


    "7. If your amp has fixed bias then carefully measure the DC bias voltage at all the power tube grid pins

    Amps with fixed bias will show a negative voltage on the power tube grid pins. If the bias voltage is not present at any of the power tube grids inspect the bias circuit."

    the 5F1 is cathode biased, not fixed bias. Your readings sound correct. Fire it up and play!
     
  3. Battle Axe

    Battle Axe TDPRI Member

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    HAHAHA! And all this stress for nothing! I'm going to plug the last tube in and continue...

    Thanks jhundt!
     
  4. jhundt

    jhundt Doctor of Teleocity

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    Go get 'em, Battle Axe!
     
  5. Battle Axe

    Battle Axe TDPRI Member

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    OK. Hasn't blown up on me yet, but I get no sound!

    No hiss
    No hum
    but the speaker makes a crackle when I unplug the guitar, even with the volume turned down.

    I've checked the input and output wiring, and the wiring between components along the signal path for continuity.

    Guitar cable gives me 1.4Mohm when plugged into Hi and 140Kohm plugged into Low.

    The only things I have done different to the standard 5F1 is a power switch and filter caps are 30uf, 22uf and 10uf.

    Any ideas?
     
  6. D'tar

    D'tar Friend of Leo's

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    Lol! No swearing!!!!

    18.6 sounds good if it were on pin 8 not pin 5. Can you give all pin voltages for both the 12ax and 6v6.

    Pictures are always welcome!!!
     
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  7. dan40

    dan40 Tele-Afflicted

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    A quick troubleshooting test is to fire up the amp (be sure speaker is plugged in) and start probing the tube sockets with your meter, starting at the power tube. Pin 3 is the plate and you should hear a "pop" through the speaker when you touch it with the meter probe. If you don't get a pop sound, you have a wiring issue around your OT and output jack. If you do hear a pop, perform the test on pin 6 of the 12ax7. This is the second gain stage of the circuit. No pop hear means that you have a wiring issue between this stage and the power amp. The final spot to check would be pin 1 of the 12ax7. Let us know what you find with the test.

    If you happen to have more than one output jack in this build, be sure that you didn't accidentally plug into the shorting jack by mistake. A stock 5f1 will only have a single jack so this wouldn't be an issue but if you deviated from the standard layout and used two jacks, this could be causing the "no sound" problem also.
     
  8. Battle Axe

    Battle Axe TDPRI Member

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    Been poking around the amp all morning and realized that my description of the problem might not be correct. Full volume and tapping the tip of the guitar jack - I get a slight tapping sound through the speaker.

    OK. Here are some voltages...

    JJ 12AX7
    Pin 1 336VDC
    Pin 2 349VDC
    Pin 3 2.83VDC
    Pin 4&5 6.5VAC
    Pin 6 336.5VDC
    Pin 7 0VDC but increases with volume
    Pin 8 2.57VDC
    Pin 9 6.5VAC

    JJ 6V6
    Pin 2 6.5VAC
    Pin 3 454VDC
    Pin 4 417VDC
    Pin 5 40VDC jumps around a lot
    Pin 7 6.5VAC
    Pin 8 49.4VDC

    6V6
    Pin 3 No pop, but slight scratching sound when I drag the probe across it at full volume.

    12AX7
    Pin 6 Had a couple of pops with a flash of light around the V1 Load resistors but very hard to replicate. Occasional crackle for about 5 seconds after I stopped poking. Slight scratching sound when I drag the probe across it at full volume.
    Pin 1 No pop, but slight scratching sound when I drag the probe across it at full volume.

    ....and only one output jack.

    Photos coming...
     
  9. tubegeek

    tubegeek Friend of Leo's

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    How confident are you that your solder joints are good? "Jumps around," "crackle," and "no sound" are not quite dead giveaways but certainly indications you may have cold solder joints, possibly in multiple places.
     
  10. Battle Axe

    Battle Axe TDPRI Member

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    Board.JPG 6V6_output.JPG AX7_output.JPG 5Y3.JPG Inputs_vol.JPG
     
  11. Battle Axe

    Battle Axe TDPRI Member

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    Pretty sure, but can never be 100% confident in such tight spaces. I have gone back and re-flowed a few joints out of desperation. Check the pics.
     
    Last edited: Jul 20, 2020
  12. Battle Axe

    Battle Axe TDPRI Member

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    While checking that I have used the correct parts I have discovered that the power tube cathode resistor is 470K ohm instead of 470 ohm! :confused: Would this cause super quiet volume?
     
  13. jhundt

    jhundt Doctor of Teleocity

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    you are right, I jumped the gun on that one.
     
  14. D'tar

    D'tar Friend of Leo's

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    470k would surely be an issue. Makes for lower than bedroom volume playing and your tubes will last forever!:)
     
    Last edited: Jul 20, 2020
  15. D'tar

    D'tar Friend of Leo's

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    Just checking......

    You must have some under board ground wiring?

    I recommend moving the pt center tap to the 30uf filter eyelet. Fill those eyelets right full of solder, some have gaps and possible cold joints.
     
  16. Battle Axe

    Battle Axe TDPRI Member

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    Yes, under ground! I have already filled the eyelets when checking, rechecking and triple checking the solder and wiring. That's when I thought I better check the components.

    I was going to put the CT there, but the eyelet wasn't big enough for it, the cap, under board ground and ground wire to chassis. Does it make a noticeable difference? I can have another attempt and see if I can make it fit. Its not like I have a busy week!
     
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  17. D'tar

    D'tar Friend of Leo's

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    Give it a whirl. If you have hum..... I would start there.
     
  18. aabbs20

    aabbs20 Tele-Meister

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    349 volts on the grid (pin 2) of V1A is a problem for sure. It should be about zero. How is that voltage getting there?

    The voltage on the cathodes (pin3/8) are also a little high.
     
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  19. Paul G.

    Paul G. Friend of Leo's

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    [QUOTE="Battle Axe, post: 9981343, member: 150665"
    OK. Here are some voltages...

    JJ 12AX7
    Pin 1 336VDC
    Pin 2 349VDC
    Pin 3 2.83VDC
    Pin 4&5 6.5VAC
    Pin 6 336.5VDC
    Pin 7 0VDC but increases with volume
    Pin 8 2.57VDC
    Pin 9 6.5VAC

    JJ 6V6
    Pin 2 6.5VAC
    Pin 3 454VDC
    Pin 4 417VDC
    Pin 5 40VDC jumps around a lot
    Pin 7 6.5VAC
    Pin 8 49.4VDC


    [/QUOTE]

    Serious wiring issues here. V1, pin 2 is the grid should be 0VDC. Zero. So, something is miswired here for sure. This looks dangerous because that voltage appears on the tip of your input jack.

    On the 6v6, pin 8, the voltage should be about 19VDC.

    Are you grounded? There should be a jumper between the cathode resistor and grounding at input jack. Also a jumper between the last filter cap and the cathode resistor.
     
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  20. andrewRneumann

    andrewRneumann Tele-Meister

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    @Paul G. speaks truth.

    If the last filter cap had no return to ground, you would get a lot of voltage on that volume pot and any other circuits attached to it. Do I see insulated input jacks? It’s hard to tell from the photo.
     
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