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5G9 Trouble Shooting

Discussion in 'Shock Brother's DIY Amps' started by jmp81sc, Sep 11, 2020.

  1. jmp81sc

    jmp81sc Tele-Meister

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    Hello
    I am having some issues with my homebrew 5G9 Tremolux.

    Background - I built this amp and it worked fine but it was a little noisy and I didn't like the chassis I had it in, so I redid it in a Hammond aluminum chassis. This is built as a head unit with a Classic Tone multi tap OT and a Classictone PT used for a Deluxe Reverb, but they also state it is good for a Tremolux.

    Symptoms - No guitar

    When both volumes are at 0 the amp is silent, but there is a crackle/pop when going from 0 to 1 and from 11-12, tweed scale. NO trem thump at 0 on both channels

    With both volumes turned up the amp is has very low floor noise.

    With one channel up and the other at 0 there is mild trem thump, and the tone and trem contols affect the thump.

    With both channels turned up no trem thump at all.

    With a guitar plugged in -

    Both volumes turned past 1 it sounds normal, no trem thump, but the trem is weak.

    With one channel turned up and the other at zero it sounds normal and the trem acts normal except there is a slight modulation going on even with the trem controls at 0, and there is mild trem thump.

    It is a stock circuit except I have placed the diode across the depth pot, and I changed the coupling capacitors at V1 to .02. I do not have the trem foot pedal jack wired in. I have the bias set at about 60%.

    I've triple checked everything and I can't find the problem. I am sure it is a wiring mistake, soldering blob in the wrong place or poor soldering on my part but I can't figure out where.

    I can post photos and voltages.
    Thanks
    John
     
  2. Wally

    Wally Telefied Ad Free Member

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    Photos and a voltage chart would be helpful,,perhaps.
     
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  3. Wally

    Wally Telefied Ad Free Member

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    And I take it that the straight signal...no trem...is good as is with no problems??
     
  4. jmp81sc

    jmp81sc Tele-Meister

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    Straight signal is good as long as both volume pots are turned up some. If one of the volume pots is at zero the there is some trem thump.

    Voltages
    B+ 408

    v1
    Pin 1 227
    Pin 6 229
    Pin 8 1.81

    V2
    Pin 1 219
    Pin 6 218
    Pin 8 28.1

    V3
    Pin 6 390
    Pin 7 fluctuates from 223 to 275 unless Depth and Speed pot are maxed then steady at 260

    V4
    Pin 3 386
    Pin 5 -30.2
    pin 6 390

    V5
    Pin 3 385
    Pin 5 -30.2
    Pin 6 390

    All heater voltages at 6.3 and rectifier at 5. I am using a new 5Y3 rectifier to get the voltages down some but it was not much lower than the 5U4GB.
    I'm not sure why V1 voltage is higher than V2, it should be 35 volts lower. IMG_0932.jpg IMG_0933.jpg IMG_0934.jpg IMG_0935.jpg IMG_0936.jpg IMG_0937.jpg IMG_0938.jpg IMG_0939.jpg IMG_0940.jpg IMG_0941.jpg
     
  5. jmp81sc

    jmp81sc Tele-Meister

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    Schematic and layout tremolux_5g9_schem.gif tremolux_5g9_layout.gif
     
  6. dan40

    dan40 Friend of Leo's

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    Have you tried putting some space between the two wires under the tape? Two wires from different stages running parallel to each other may cause some unwanted coupling.
     
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  7. Wally

    Wally Telefied Ad Free Member

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    +1 on figuring out why your plate voltages on V1 are out of whack. Fwiw, your cathode voltage is low there, too. One would be curious as to the cathodes on V3...especially pin 8. Do you have a switch to take the trem circuit out of operation? All you need is a plug that is shorted.
     
  8. jmp81sc

    jmp81sc Tele-Meister

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    I'll keep looking, the tape is temporary and it helped tame down louder trem thump. The blue wire going to the grid of V2A pin 2 was the problem. Moving it down and next to the red wire from pin 6 made it much quieter.

    I took the wire off of the trem jack to eliminate that as an issue. I can hook it back up and make a shorting jack and see if that changes anything.
     
  9. jmp81sc

    jmp81sc Tele-Meister

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    I also forgot to mention that I am using a 12Ax7 in V1, new JJ.

    I hooked up a scope and am tracing the circuit still, but it seems to go wonky at the phase inverter, but I am still tracing it out, and am relatively new using a scope so take that for what it's worth.
     
  10. elpico

    elpico Tele-Afflicted

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    Any piece of bare wire, or tinfoil, or gum wrapper or whatever can be used to short the terminals of the foot switch jack. You have access to the inside of the amp so you don't need a plug. Not sure how much that helps you though. It won't thump when the oscillator is disabled.
     
  11. jmp81sc

    jmp81sc Tele-Meister

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    Ok I made a shorting jack and when inserted it makes a high pitch very loud squeal when both volume knobs are turned up even slightly and it gets louder when turned up. One knob at a time no squeal. Tone knob changes frequency of squeal.

    I switched the ot primary blue and brown wires at the power tubes and no change.
     
  12. jmp81sc

    jmp81sc Tele-Meister

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    Here are some scope shots, 500 hz input, volumes set half way up, tone half way up, shorting jack in Trem pedal input.

    Looks like strange clipping post phase inverter which is worse at the speaker out.

    Volts/Division scale was changed as the signal was amplified to fit on screen.

    I swapped the phase inverter tube, no change.

    Wave form coming out of lug 1 of microphone volume pot, normal channel

    IMG_0948.jpg

    Same place bright channel

    IMG_0947.jpg

    Coming out of the phase inverter at the 220 k resister and .02 capacitor normal channel

    IMG_0945.jpg

    Same place bright channel

    IMG_0946.jpg

    At speaker out normal channel

    IMG_0950.jpg

    Bright

    IMG_0949.jpg
     
    Last edited: Sep 12, 2020
  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Is this connection soldered?

    Inked5g9_LI.jpg
     
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  14. jmp81sc

    jmp81sc Tele-Meister

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    It was poorly soldered, I redid it, no difference.

    As I said I am a newbie especially with the scope, but it does not appear that the signal is inverting coming out of V2.

    The phase of the signal at pin 5 of the power tubes appears to be in phase, which I think explains the high pitch squeal.

    I traced out the wiring and checked all of the resistor and capacitor values and they all are good. Must be a poor soldering joint or something shorting out, but I can't find it.
     
  15. jmp81sc

    jmp81sc Tele-Meister

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    Ok baby steps. Moving the wire that goes to pin 2 of the trem tube V3 got rid of the squeal with a shorted plug inserted. The amp acts normally now with no trem input.

    With the shorted jack removed the trem thump is gone, but the trem does not work if both volume knobs are turned up. Trem only works when the volume of the unused channel is turned all the way down, then it is a normal strong trem.
     
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  16. jmp81sc

    jmp81sc Tele-Meister

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    spent some more time with the scope. The odd behavior seems to start in the phase inverter then progress to the speaker out. I'm thinking that the .02 coupling capacitors may be leaking negative voltage from the trem circuit in to the phase inverter. With the shorted jack in the trem, the signal is normal all the way through the signal path.

    Is there any way to test for this, can I just clip in new capacitors in parallel? That will double the capacitance correct, if I use the same value?
     
  17. jmp81sc

    jmp81sc Tele-Meister

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    I've spent a fair amount of time looking. I redid my ground bus using better wire, changed out some suspect looking resistors. Reflowed solder joints. Rewired my volume pots and checked them. Checked layout and wiring. Put in all new tubes. no change I did all of this one change at a time and checked the amp operation after each change.

    Basically the trem only works if the volume on the channel not being used is turned all the way down. I am not hearing any interaction between the volume controls typical for a tweed amp.

    I get a scratch or pop when the volume controls hit 0 volume or full volume, in between is normal.

    Other than that the amp sounds fine.
     
  18. tubeswell

    tubeswell Friend of Leo's

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    Is your pre-amp filter cap grounded properly? (Meter the cap ground lead to chassis with your DC meter - you are looking for DC-continuity and almost no resistance)

    Also check your screen filter cap ground)
     
  19. Wally

    Wally Telefied Ad Free Member

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    That cathode voltage on the V3P3 cathode is still unknown to us. Sort of no a stone unturned kind of thing I have. The scratchiest on those volume pots could be a bad cap...maybe the coupling cap to V2? Maybe a tone cap? f
     
  20. jmp81sc

    jmp81sc Tele-Meister

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    Hi Tubeswell and Wally

    Resistance to chassis on the filter caps 0.2 ohms. This has been consistent for all of my grounding points throughout the amp.

    V3P3 is 2.28 volts vs 1.7 on the layout and schematic. V3P8 swings from 281 to 303 unless the speed and depth pot are maxed out then it is a steady 316 volts [260 on schematic].

    I am getting some DC voltage at the volume pot side of V1 coupling caps, 2-3mv. I have -0.17 volts where the right hand lug of the Mic volume connects to the .02 coupling cap.

    I am leaning towards poor workmanship or improper layout/wiring, but it is possible that it is a bad component. I hate to just start replacing stuff, I already did some of that with no change. I will spend more time following the signal path with the scope to try to pinpoint the issue.

    Thanks for looking. Could the higher B+, 408 vs 370, be the source of my problems with a cascading effect as it goes down the line? wrong cathode resistors for the voltage?

    Thanks for helping me with this.
    John
     
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