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How do I deal with a failed electronic project?

  1. Nov 30, 2016 #1
    My last amp design/build has failed to operate as planned. There are several perplexing issues I do not know the cause of. I started the amp up and it will play and has all the voltages I expected within a few volts.

    1. Serious amount of 60hz hum.
    2. Channel two gain and volume control interacting in a very non standard way.
    3. Every component in the preamp stage is microphonic. I have never seen a amp do that. What I mean by that is when I tap on a component I hear the tap sound in the speaker.

    What I had in mind with the design..
    I am trying to combine a channel from an old Fender Bassman with a channel from a modern high gain Fender EVH 5150. The amp is class AB push pull 100 watt.

    As I have invested a large amount of time, effort, and money in this amp, I am not very happy with the prospects of accepting failure as the outcome.

    I assume I can find and fix the hum issue. Perhaps that will not be easy but....
    I assume I can fix the gain/volume control issue...
    I have absolutely no idea what could cause a seven stage preamp section to display a microphonic condition. Obviously there is not enough information in this post for anyone to comment as to the exact causes of the above faults. I can post all the schematics and give a detailed assessment of the faults with oscope screen shots where needed.

    My question at the moment is where do I start? I assume I should try to fix the hum issue first.

    Is that the best place to start?


  2. jcsd
  3. Nov 30, 2016 #2


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    How do I deal with a failed electronic project


    "Serious amount of 60hz hum."
    I would expect 120Hz hum, from the power supply. 60Hz indicates either an open input, wire routing, or maybe a ground loop.

    Yup! That's probably the easiest one to fix. Start pulling tubes sequentially from the input stage to the output, while paying attention to the hum level and waveform.

    The microphonics problem is likely either the gain is too high or you got a bad batch of tubes. Or it may be a corroded tube socket or connector, or even a solder joint that wasn't soldered.

    A readable schematic would be useful.

    p.s. All of the problems *could* be a missing ground connection somewhere in the build.
    Last edited: Nov 30, 2016
  4. Nov 30, 2016 #3
    Thanks Tom,
    I will try to clean up the schematics with the changes I made tomorrow. No one else but me could read them at this point.
    The hum is reduced as I pull preamp tubes. It increases with gain and volume.
    The tubes are known good and tested in another amp and I have moved the tube sockets around enough to be pretty certain that is not the issue. I have disconnected things looking for issues, so I need to get back to "normal" and check every solder joint and every component connection.

    This is the first amp I have built that had any real issues. It is by far the most complex also. I think the design will work but.....
    The amp will actually play and all the voltages on the tubes are pretty much dead on and no smoke is coming out....lol....so there is some good news!!


  5. Nov 30, 2016 #4


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    yeah, definitely, you don't know what effects that may be having on the other issues

    is it really 60Hz or is it 120Hz as Tom suggested ?
    Scope the ripple on your PSU see how bad it is

    disconnect the power amp stages and scope and listen to the pre-amp output is there hum ?

    agree with Tom .... show us what you are up to :smile:

  6. Dec 1, 2016 #5

    jim hardy

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    Troubleshooting. try to draw a circle around the problem and then tighten the noose..

    on your hum....... to rule out one area, heaters a couple thoughts to ponder

    7199 is an RCA tube designed for early preamp stages to minimize hum from heater-cathode capacitance..

    i'd try lifting the preamp heater supply leads and powering them temporarily from a 6 volt battery.

    i've seen heaters grounded via a ~hundred ohm pot across heater supply with wiper grounded for adjustable hum reduction.

    remember the old trick of using preamp heaters as output pentode cathode resistors , that gives you more nearly DC heater power and prevents applying signal to not-yet warmed up drivers.

    old jim
  7. Dec 1, 2016 #6
    Here is the schematic showing the PA and the HT part of the power supply. The B+ loaded voltage is 480 VDC with 5.19 VAC ripple. There is very little ripple past the B+. This ripple is 120Hz.


    Here is the Preamp. The channel switching is not installed yet and the effects loop is not connected to the jacks.


  8. Dec 1, 2016 #7


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    I see that the FS Jack Ground is connected to Chassis Ground, not to the Power Supply common. If this isn't a problem now, it could be when the rest of it is straightened out.

    Which preamp channel is being used?

    If that means the hum progressively decreases as more preamp tubes are pulled, it indicates the hum is introduced into ALL the preamp stages.

    Next thing to look at:
    1) Using the scope, verify the frequency and waveform of hum at the output. (photo would help)

    Is this the build you presented in an earlier thread looking for comments, the one with DC Heaters from a poorly filtered full wave rectifier?

    Certainly worth the effort! Good one Jim.
  9. Dec 2, 2016 #8

    jim hardy

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  10. Dec 2, 2016 #9

    jim hardy

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    another idea to poke with

    imagine a tube big enough you could walk into. Like at some radio stations..

    The grid would be like a barbed wire fence that you could shake.


    Tubes are microphonic because moving the grid wires modulates the electric field inside.

    Take an old fashioned wood pencil that's sharp and gently tap tubes and components. I predict you'll find a particularly sensitive spot probably on the glass envelope of an individual tube.

    Is this the amp you pictured with parts so neatly mounted on turret boards ? It's gonna feel great when you get it straightened out.

    In my projects Mother Nature always makes me struggle. She is one coy mistress. Persist.
  11. Dec 2, 2016 #10
    I solved the major hum issue.
    The preamp microphonic issue went away with the repair of the bias circuit
    The bias circuit was incorrectly wired. Two of us were working on the amp. I did not check everything Dave soldered up on the board. The channel one circuit is not working correctly. It is really distorted. It is a simple circuit so I should be able to figure it out.

    Actually I have been at this too long and need a break to clear my head. There is also no guarantee at this point that this design will ever work properly.

    I have in mind to power the heaters with a six volt battery to see what the results will be. I may have something messed up in the bridge that feeds the preamp tube heaters.

    Thanks Jim for the reminder.


  12. Dec 2, 2016 #11
    Hi Tom,

    I have made some changes to the original schematics I posted. I will post some photos later today or tonight. I want to do the six volt battery test next. I have to go buy a battery.


  13. Dec 2, 2016 #12


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    Pre amp heaters as OP stage cathode resistors. Thank you, after all these years I never thought of that!
  14. Dec 2, 2016 #13
    What do I need to add to filter this bridge properly?



  15. Dec 2, 2016 #14


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    What is the load current?
    Any idea how much hum (ripple) is allowed? (Without doing the math, about 2.7VPK-PK ripple would be around 6.3VRMS.

    BTW, I get 7.5VPK as the output. (Two diode drops from peak AC input.)
  16. Dec 2, 2016 #15
    Hi Tom,
    I have the bridge built as above. Unloaded it measures 8.31 VDC RMS unloaded. The 6.3 VAC tap on the transformer is also powering the four 6l6gc output tubes (AC) which I did not indicate on the schematic above. With the seven 12AX7 preamp tubes connected the voltage is 6.3 VDC. I have around 500 mv of ripple. I think something is not correct in the bridge because when I set the scope to DC, I should see pulsating DC and not a sine wave. I don't understand how a AC signal could be coming through the bridge. For sure I have 6.3 VDC coming from the bridge loaded, measured with both the scope and a DMM. I also have a 120hz AC @ .48VAC

    In some testing I did, I hooked up a regulated power supply set at 6 VDC (connected to the 7 preamp tubes) and the current was around 1.8 amps as I remember. I was a bit surprised as I thought it should be drawing more current than that. Actually I just looked at the datasheet again and it shows .3 per tube so the 1.8mv total may be a real number. The heaters can be run AC or DC. The whole idea of the DC heaters was to reduce hum.

    In some things I have read somewhere between 4700uf and 10000uf is "normal" filtering.

    Something is messed up here!!

  17. Dec 3, 2016 #16


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    Your circuit board has many connections on the tubes side, less on the knobs side. If you ran the wires on the knob side under the board, then round and over to the appropriate point, you could hinge the board along the tube edge without disconnecting anything. That would make service easier.
  18. Dec 3, 2016 #17


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    The hum may be being caused or made worse by the style of wiring . All those loops and crossing wires could be picking up mains frequency and other stray signal inputs and feeding them back into the system .
    Last edited: Dec 3, 2016
  19. Dec 4, 2016 #18
    After a lot of investigation, I have come to the conclusion that the whole power supply needs to be redesigned. I am going back to AC heaters for all the tubes. As far as the filament supply, DC on filaments are most likely never worth it. The unbalanced DC hum finds it way into the signal more than a balanced AC filament supply does.
    I see that high gain amps like Mesa Boogie have gone back to AC filaments after trying DC. In some cases they may run DC to the first preamp tube only. Also, all these high gain designs are built on PCBs and no one that I am aware of is attempting to build these sort of designs on a turret board. Only crazy people like me...lol

    I have the amp running on both channels at the moment so I think the basic circuit design will work with some additional modification. The channel two volume control works in a very strange way which may be some error I made or perhaps is just normal for the design.....not sure.

    Complex amp...pushing me to and past my limited skill sets!!...lol


  20. Dec 4, 2016 #19


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  21. Dec 4, 2016 #20
    Hi Don,
    When the volume control is set to the zero position the sound of the amp is extremely scratchy and has a very trebly sound. As the volume is increased the sound becomes normal and when increased more the bass sounds become predominate. The volume control has little effect on volume and the volume is a function of the gain control (P4 220K Pot). There is major interaction with the gain control and the treble control. Increase treble and the volume goes up. I see this characteristic with gain and treble on other high gain amps I have played. All of this may be just the "normal" way the circuit works.

    If I understand the schematic the audio signal leaves V4B via the cathode through a 43k resistor into the tone stack. The gain control at P4 increases the the gain as it is turned clockwise arriving at the treble control which controls both gain and frequency. It then hits the volume control and exits the wiper on to grid of V6a. I do not clearly understand how the volume control changes the frequency of the signal other than as it is turned up V6 is provides more voltage to drive the output tubes.

    I have had so many other issues to solve that I have not really given this much thought. I do know from experience that high gain amps like Mesa Boogie have controls that have a serious amount of interaction and are very difficult to get set properly. This may be the case here also.

    I am considering how to redesign the whole power supply at the moment. Until I get that right, nothing else really matters much. I think that the new HT design may require a bit of complexity using zener diodes to clamp the voltage past B+ going to the grids.


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