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My 5F11 Build (aka The amp who wouldn't be)

Discussion in 'Shock Brother's DIY Amps' started by brucerbc, Mar 5, 2021.

  1. Scooby Snax

    Scooby Snax Friend of Leo's Silver Supporter

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    Insulated washers are just nylon washers and can be had at Home Hardware, Canadian Tire etc...
    Not the cheapest, but may be faster than ordering online
     
  2. 2L man

    2L man Tele-Meister

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    Two easiest and best methods to improve amp signal/noise ratio is to do grounding like it is instructed in electronics (preventing return current flow thru chassis) AND use shielded cable between input jack(s) and V1 G1(s) and solder a low noise Metal Film grid leak resistor(s) to tube socket without routing cable(s) thru chassis.

    For example I am not satisfied of few of my Choke installations and I think generally Choke cost factor is poor but those two I always do.

    Building solid ground bus is simpler and amp comes more reliable than drilling and mounting more solder lugs to aluminium chassis which can corrode.

    Using shielded input cable cost only little more and workload increase some but I think it is worth it. If anyone has two input amp and grid leak resistors are mounted to turrett board for test other is easy to replace and and hear the effect. Naturally first both inputs needs to be compared to each others before modifying other because they might be different. I looks modern house has so much switching power supply appliances that there comes lots of electromagnetic noise to air and to mains network. Even battery operated flash lights often use switching power supply and all LED light pulbs have one.

    To test grounding between where power amp stage is "grounded" to separate chassis lug and pre amp to chassis at input jack VS. Single Ground Bus require one SPDT switch temporare installation. When switching it is good to turn amp Standby just to be sure nothing breaks. I have test this only on 5E3 and although difference was not huge it was clear that avoiding current flow thru chassis did reduce hum.

    I wrote "grounded" because technically power amp is not grounded to chassis when its negative, power supply negative and chassis are connected together because no power amp current flows to chassis. It is the pre amp current which flows thru chassis to that lug.
     
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  3. brucerbc

    brucerbc TDPRI Member

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    Struggling to get this 5F11 head up and running. Extra eyes and advice would be greatly appreciated.

    All.jpg


    Pots 1.jpg


    Pots 2.jpg


    Top 1.jpg


    Top 2.jpg


    Tubes 1.jpg


    Tubes 2.jpg


    Amp is fully assembled but producing no sound … except if I short the heater wires to ground (i.e, jumper V3 pins 2+8 or 7+8), in which case I can hear my guitar signal with a LOT of background electrical noise.

    I can also effect the guitar signal with Speed and Depth knobs if the heater wires shorted to round.

    Not sure what this means? Is it diagnostic?

    Also, some voltages are more that 20% off from Fender layout. And the 6V6s look like they may be red plating (though it's the first time I think I've seen that, so perhaps I'm an unreliable observer)

    I know I must have errors, but am having a hard time tracking them down … and am not sure what some of my observations mean (e.g., getting signal when shorting heater wires).

    Build is on Mojotone eyelet board in Mojotone chassis and was assembled following Mojotone 5F11 Layout with bias pot with following exceptions:
    • Hammond 290CAX power transformer (used 330VAC primaries)
    • Hammond 1760E output transformer
    • AA964 bias circuit:
      • bias voltage from pin 6 of the rectifier
      • first resistor before the diode changed to 100k
      • 10k bias pot with 20.8k bias resistor (33k+56k in parallel)
    • AC input wired per RobRob 5E3:
      • hot to fuse tip, fuse side on to switch, then to primary A
      • neutral spliced to primary B (A and B being arbitrary)
    • Negative feedback on switch
    • 4R and 8R speaker outputs on switch
    5F11 - Layout w Bias Pot - Mojotone1024_1.jpg



    Startup followed Tube Amplifier Startup Procedure and used a light bulb limiter. All observations as expected until all tubes in and speaker and guitar connected … then no sound.

    All voltages relative to ground except heater wires (wire-to-wire):
    • V1-V4: ~6.53 VAC
    • V5: 5.06 AC
    Voltages.JPG


    Again, any assistance or advice would be appreciated.


    Thanks!
     
  4. Linkjr

    Linkjr Tele-Meister

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    Have you under board wiring between the negative ends of the filter caps? Cant see in the pics in the mojo layout they located under the speed and depth pots
     
  5. jmp81sc

    jmp81sc Tele-Meister

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    You should have about -30 volts on Pin 6 of your 6V6 power tubes.

    I think it's strange that the heater wire grounding effects the sound. Make sure they are wired to the correct pins of the tube sockets.

    Do you get a pop when you measure your voltages at the preamp tubes?

    Your build looks clean and the solder joints look good also so I am thinking it is miswired somewhere.

    Hang in there, smarter guys than me will chime in.
    John
     
  6. brucerbc

    brucerbc TDPRI Member

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    [Edited for "-" voltage]

    Thanks Linkjr and jmp81sc for replies! Responses below:

    Yes, negative ends of the filter caps are wired per Mojotone layout and continuity checked.

    I do have minus 30-32 VDC on pin 6 of both 6V6 tubes.

    Definitely strange behavior re: sound only when grounding 6V6 heaters to pin 8 (and lots of background electrical noise when I do that).

    No pop when measuring voltage on preamp tubes.

    Any tips or troubleshooting steps to fix would be greatly appreciated!
     
    Last edited: May 4, 2021
  7. Linkjr

    Linkjr Tele-Meister

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    I had a problem once with no sound at all, no change to the amp wondering what caused it then remembered i took out the tubes for storage , putting them back the tube socket contacts were not making good contact with the tube pins believe it was my 6v6, also a mojo tone socket too.
     
  8. jmp81sc

    jmp81sc Tele-Meister

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    Just to confirm that is Negative 30 to 32 volts. I have had some very strange things happen with fixed bias amps when I had mistakes in the bias circuit. I would make sure that the bias circuit is correct first without any tubes installed.

    Have you confirmed that the tube sockets are wired correctly? Make sure your not off by one pin or something. Also check for solder blobs or stray wires that may be touching socket pins they shouldn't be. You should get a loud pop when taking your voltages on the preamp tubes.
     
  9. brucerbc

    brucerbc TDPRI Member

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    Thanks John.

    Sorry, my mistake (now edited in earlier post): definitely negative 30 to 32 volts on pin 6 of both 6V6 tubes.

    Re: tube socket wiring, I've reviewed it repeatedly and can't find a mistake. Have also checked each pin for continuity to destination on board. I mean, a mistake must be there ... but I can't see it.

    And no pops with preamp tube probes unless heater wiring grounded as described earlier.

    Sorry if this seems like a ridiculous question ... but, could this problem be related to me using shorting jacks for the guitar input and speaker output jacks? I've tested by inserting unwired 1/4" plugs, but thought to ask anyway.
     
  10. 2L man

    2L man Tele-Meister

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    G1 voltage is too high negative and shuts down anode current. There is no voltage drop on OPT primarys. Depending OPT there is 5...15V over primary on idle.

    Try if bias pot effect g1 voltages. Perhaps its range is not enough?

    Anyhow fully dimes amp should still produde sound but operate around B-class and distort bad so you might have other issue as well?

    Voltage from cathode to g1 should be around -20 for 6V6 but Screen voltage has effect to Bias point too and it is just an estimate but -30V is too high Vgc
     
    Last edited: May 5, 2021
  11. 2L man

    2L man Tele-Meister

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    If you did not install 1 ohm resistors to both power tube cathode circuits you can still measure anode currents. When amp is disconnect measure resistances from OPT CT to both power tube pin-3. Then power anp and measure voltages from these same points. Anode current = voltage / resistance. Screen current is very small without signal so it does not effect amp biasing too much.

    Naturally be careful because there can be voltage on electrolyts when measuring OPT resistances but their current do not flow thru OPT and resistance measuring can be done.
     
    Last edited: May 5, 2021
  12. 2L man

    2L man Tele-Meister

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    Before powering amp measure resistance from both power tube pin 8 to ground. On Fixed Bias amp there should be zero ohms or one ohm when "bias measuring resistors" are installed. Multimeter cables have small resistance increasing effect when very low resistances are measured.

    On your voltage table you have measured fluctuating +20V which points the amp being Cathode biased???
     
  13. 2L man

    2L man Tele-Meister

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    Amplifiers can oscillate on very high frequency which loudspeaker can't produce or OPT pass to speaker. Or amp tech can't hear which might be the most obvious reason ;)

    Anyway wrong phase on Feedback circuit can cause it because transformer wire colors can nit be trust fully. But there are more possibilities. For me it was bad soldered loudspeaker jack which did not connect reliably and together with wrong FB phase it did destroy power tube in few seconds. Then I did run amp using a variac increasing voltage slowly and using an oscilloscope I saw it was wildly oscillating 250kHz already about half mains voltage.
     
  14. jmp81sc

    jmp81sc Tele-Meister

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    Shorting jacks are fine, you just need to check that the shorting tab doesn't touch the tip when a plug is inserted.

    You may have two issues going on. Input jack/output jack wiring, thus no signal going in to the amp or getting out. Probably output jack since no pop when measuring voltages. Also check your output transformer wiring.

    The other issue has to do with your high voltages. Check your resistor values in your power amp area and your phase inverter tube. Make sure you don't have a much higher or lower value resister in place, for instance I used a 470K resister once instead of 470.
     
  15. 2L man

    2L man Tele-Meister

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    First measure that between power tube pins 8 and ground is zero ohms. If it is then adjust bias so that power tubes begin to conduct! Tube conducts more when voltage between control grid (pin 5) and cathode (pin 8) comes smaller. Grid voltage is more negative than cathode voltage.

    5F11 is fixed bias so g1 is negative against ground.

    On cathode biased tube both grid and and cathode usually are positive against ground but cathode is smore positive.
     
  16. mrfitz98

    mrfitz98 Tele-Meister Silver Supporter

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    No sound at all. Let's disregard that by shorting things that shouldn't be shorted you can make sound, let's look at the things we can check. The voltages, while not great, should be enough to get sound. If you have the amp off and plug something into the output jack (like a guitar cable) do you have ground from the sleeve to the mains ground? Have you double checked all of your OT connections? It really sounds to me like there is a ground issue in the output section.
     
  17. brucerbc

    brucerbc TDPRI Member

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    Thanks all for suggestions and advice!

    I worked through them and this post solved it:

    I hadn't connected 6V6 pins 1 and 8 to ground per the layout diagram. I thought I'd read here somewhere that this wiring was only needed for a metal jacket 6V6.

    Clearly, I was mistaken. I tried this with test leads and immediately had clean signal.

    Apologies and appreciation for all assistance.

    I now have signal and it sounds good. On to biasing!
     
    Last edited: May 7, 2021
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