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'Organ Donor' - Hammond AO-29 to 6V6 Plexi Conversion

Discussion in 'Shock Brother's DIY Amps' started by kleydejong, Jan 3, 2017.

  1. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    That tester has a limit of 20mfds at the top of the range.
     

  2. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Well, a moment of glory followed by shame. I swapped the OT leads, removed the NFB entirely, double checked some ground connections to have continuity to the chassis, and fired her up to test. Played a chord to check if it was on and WOW. Super loud and definitely had 'that' Plexi sound. Grabbed my camera, turned on my computer, and hit record. Captured this:



    I'm running the speaker output into my load box. Basically two 4 ohm 50 watt resistors in series with a line out going to my interface. Applied a speaker sim using Impulse Responses in my DAW. Sounded great for a minute. Got me really excited!

    Then for a reason I have yet to understand, the signal dips out for one second. Plays good for a few more. Then cuts out entirely... Crud.
     
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  3. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I put this project on the back burner for a while. Coming back to it now, hopefully with fresh eyes. Since that last video I posted immediately above where you can hear it cut out I am no longer getting any output.

    I took the layout and went through to double check all the proper connections are made. I took some voltages and did some continuity checks.

    I found something I think is odd. The speaker output jack is presenting continuity to ground on the hot terminal. Further the entire secondary side of the OT is grounded. The OT is pictured below on the top left with the brown paper covering. The secondary side is on top. There is a yellow and green wire which are going to a switch to select 4 or 8 ohms. Then a black wire which is grounded. All of those wires are presenting as shorted to ground on the continuity meter.

    [​IMG]

    Is this a bad sign for this OT?

    I also did some quick voltage checks. The power tubes are seeing 436 vdc on the plates. Heaters are 3.2v.
     

  4. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    Welcome back Kley!
    Do you get any sound when probing the grids checking voltages(pop test).How about a new voltage chart. Having continuity to ground at the connections you mentioned doesnt seem far fetched to me. Are they 0.0ohms direct short?speeker voice coil is one wire connected to gnd at the other end, same with ot winding.
     
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  5. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Thanks! Had a couple of crazy months.

    I am getting some very light popping when probing pin 5 on both output tubes.

    Took some voltages.

    V5

    1 - -44
    2 - 3.2
    3 - 435
    4 - 436
    5 - -44.9
    6 - 436
    7 - 3.22
    8 - 0

    V4

    1 - -44.9
    2 - 3.2
    3 - 434
    4 - 435
    5 - -44.8
    6 - 437
    7 - 3.2
    8 - 0

    I then continued to probe on the tube pins. The phase inverter tube sounds on pretty much every pin. But neither V1 or V2 responds to any kind of tap to any of the pins. Can I use that to infer that the problem is somewhere in the preamp? Likely V2?
     

  6. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    Testing voltages from the output tubes back in the chain toward the input should reveal louder pops when contacting pins...especially the plates....as one moves closer to the input. The pops at the input stage should be the loudest since that signal is amplified through the entire circuit. This is an old test...as old as electronic circuits, and is valid. Start at the output as you did and end up at the input. When you encounter a stage that does not yield a pop upon contact, you have found a problem area.
     
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  7. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    More troubleshooting tonight. I continued with the tap test throughout the preamp. I now discovered that I can get a signal all the way through - however it is extremely faint. If I dime the master and push both preamp gains to 75% I get a very quiet guitar signal through the speaker. Past 75% it screeches into a high pitched wine (guessing lead dress). Still relatively quiet. Like below talking level quiet.

    The preamp controls seem to be working fine. If I dime all controls except the two preamp gain controls at 75% I get a nice distorted tone like you'd expect, just at whisper volumes.

    I went through V1 and V2. They seem okay to me. Higher than the schematic, but I believe it would be within acceptable range for a 12AX7.

    v1

    1 - 164
    2 - 0
    3 - .961
    4 - 3.2
    5 - 3.2
    6 - 211
    7 - 0
    8 - 1.91

    v2

    1 - 172
    2 - 0
    3 - 1.09
    4 -
    5
    6 - 300
    7 - 171
    8 - 172

    V3 is the Phase Inverter. Really weird stuff is happening here. I believe this is my problem area. I'll come back to this.

    I took voltages for the two power tubes. I was getting 445v on the plates, so I popped in a set of JJ 6V6's as I was getting nervous about high voltage on a set of Electro Harmonix 6V6's I had in previously. I then biased based on the reading on the cathode. I set it to 21 mv, which seemed a little low to me - and I was hitting maximum on the bias control. Weird. But better. Output increased slightly. From whisper to maybe a medium talking level.

    Then I went back to V3.

    1 - 316
    2 - 42
    3 - 290
    4 - 3.2
    5 - 3.2
    6 - 316
    7 - 264
    8 - 291
    9 - 3.2

    Heaters seem fine. Everything else seems way high. The grids and cathodes seem really far off.

    I seem to remember this thought that I was having some trouble with the negative feedback and presence circuit before, so for some reason I ripped that out entirely. However in reviewing the schematic it seems like the PI grids and cathodes follow a path through the Presence control to ground no? Is it possible that without this ground these higher voltages are failing to discharge properly to ground and the PI is getting way out of whack as a result?

    So it seems to me that I must restore the presence and NFB circuit to stock.
     
    finom1 likes this.

  8. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    +1 on the PI voltages. Something is incorrect in that area.
    Re:that tap test....note that when the pop! is not louder than the stage just downstream from that test
    You have found a problem. The pops should get progressively louder at each stage moving from the output to the input....with the input stage yielding the loudest pop.
     

  9. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Progress! I added a ground wire from the 10k resistor / .1 cap. Presence and NFB are still removed, but with this path to ground I'm getting much more normal guitar like sounds and output levels!

    [​IMG]

    I captured a quick recording. It was really late, so I just ran the amp into my Weber Mass attenuator. Then the DI output into my audio interface. Then applied a simulated guitar cabinet EQ filter using an Impulse Response of a Marshall 2x12 with a G12M speaker. Sounds like this:



    It is a little noisy, and the normal channel does that squealing thing past 75% on the volume - but it is a step in the right direction.

    I'm hearing a little bit of a ratty / fizzly quality, which I may attribute to voltages and bias still needing to be tweaked. But I'm happy to have a functioning amp again.

    I'm still very confused at what exactly happened and why the amp stopped working, but it appears that some of my prior troubleshooting caused more harm than good. Sometimes a little break and a fresh perspective can be the remedy.
     

  10. Wally

    Wally Telefied Ad Free Member

    Mar 17, 2003
    Lubbock, TX
    And what happened with the voltages??? For my money, those voltages in that area would have been the first to check after that change. Fwiw, I suspected that you had not established that circuit when you removed the NFB loop.
     
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  11. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Hoping to get a thorough set of voltages tonight, but I'm definitely moving in the right direction. Had some short time over lunch just now. I reinstalled a 47k NFB resistor. I put in a pair of 6L6GC tubes and biased pin 8 to 29mv. I'm maxed out on the dial, which seems to be a bit of a problem given that I'm still a little cool I believe. But I'm close enough for now.

    I also remembered that I had put in a set of cheap Ruby 12AX7's. They're fine for testing, but I remember now that they are probably the cause of that nasty, ratty distortion character I mentioned previously. I popped in some fresh Tung Sol 12AX7's instead.

    Turned her on to take some voltages. Was surprised to get a very nice loud responsive signal. I was tempted to plug in a guitar. I succumbed to that temptation. I played for 20 minutes with a big grin on my face, and didn't have any time left to take voltages. One of my daughters was sleeping, so I kept the master at noon and the normal volume on about 1 or 2. Was treated with a nice warm, bouncy clean tone. I captured a tone clip and will share as soon as it is rendered.
     
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  12. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Tone clip featuring a nice warm clean tone. Very dynamic to play. Plexi's have a very underrated clean tone IMO.

     
    Zepfan likes this.

  13. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    At first reading of the title, I thought it was gonna be about Lorena Bobbitt generosity (wink,wink).
     

  14. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I took voltages this morning. I'm amazed at how many of the voltages have fallen into place now that everything is dialed in properly.

    Rectifier

    1 -
    2 - 374
    3 -
    4 - 389vac
    5 -
    6 - 385vac
    7 -
    8 - 374

    V5

    1 - -36
    2 - 3.2
    3 - 367
    4 - 372
    5 - -36
    6 - 373
    7 - 3.2
    8 - 30.1mv

    V4

    1 - -36
    2 - 3.2
    3 - 367
    4 - 372
    5 - -36
    6 - 373
    7 - 3.2
    8 - 29.6mv

    V3

    1 - 158
    2 - 9.1
    3 - 15
    4 - 3.2
    5 - 3.2
    6 - 266
    7 - 12
    8 - 14
    9 - 3.2

    V2

    1 - 135
    2 - 0
    3 - .806
    4 - 3.2
    5 - 3.2
    6 - 266
    7 - 135
    8 - 136
    9 - 3.2

    V1

    1 - 116
    2 - 0
    3 - .810
    4 - 3.2
    5 - 3.2
    6 - 163
    7 - 0
    8 - 1.4
    9 - 3.2

    Another demo. This time running speaker out into a Weber attenuator. Then running a DI out into my audio interface and applying a speaker simulation in the computer.

     

  15. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Regarding bias, at 367v on the plate and 30mv on the cathode it would appear I'm very cool for 6L6's. According to Rob's calculator I'm at 11 watts which is 36%.

    Problem - I'm at max turn on the bias control. Curious if it is possible to tweak some components in the bias circuit to get more max plate current?

    Or easier solution, I could go to 6V6's again with the new voltages and rebias. With 6V6's and bumping it back to about 20mv it should settle in nicely.
     

  16. D'tar

    D'tar Tele-Afflicted

    Jan 11, 2013
    WNY
    You can lower the value of R37 (47k), there is defenitely some math to know in advance what value, or trial and error, maybe a 33k ? Could even parallel in another 47k.
     
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  17. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I did a little experimenting with 6V6's. I could get my plate current to a nice 20mv - but with the 6V6's my voltages jumped up. With 6L6's I was getting about 367 on the plates. With 6V6's it was more like 436.

    Is that to be expected? Is it related to current? Perhaps related to the power transformer being designed to drive quite a few more tubes? I'm confused...

    I ran some more tone tests. I suspect that the bias issues are leaving something to be desired.

    I also noticed that when I capture a recording in my DAW, the audio wave form appears to be asymmetric. Like the negative part of the wave form isn't working properly. A couple of screenshots:

    [​IMG]

    [​IMG]

    In listening to the audio clips there is a 'blatty' or almost farty character. Loose and lacking some definition and clarity.
     

  18. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    I reviewed a Marshall 1959 schematic and it appears that bias circuit connects a 47k resistor with a 27k variable resistor. Whereas the Hoffman features a 47k resistor and a 50k pot. So 33k seems more in line with the larger tubes.

    I swapped in a 33k resistor. I was able to get my plate current up to 51 ma. Gets me up to 18 watts at 61% plate dissipation. I recorded a tone test very briefly, but my first impression is that it helped.

    At really high distortion levels I noticed a little bit of that asymmetric wave form. Now I'm starting to wonder if that's part of the 'cold clipper' v1 bias thing going on. Hmmm, off to Rob's website to study up.
     

  19. kleydejong

    kleydejong Tele-Holic

    591
    Aug 30, 2010
    Orange City, IA
    Alrighty, calling it a success. Massive thanks to all of you for helping me get this up and running. A dream amp for me now sits in my basement!

     
    jsnwhite619 likes this.

  20. finom1

    finom1 TDPRI Member

    13
    Feb 26, 2016
    Maryland
    Fantastic, thank you!!!
     

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