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5F10 Build distorting excessively

Discussion in 'Shock Brother's DIY Amps' started by milocj, Apr 16, 2018.

  1. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    Okay, I probably can't give as much info as needed, and I can probably eventually track this down on my own, but right now I'm a little stumped. I bought this as somebody else's project that they completed (built chassis) and knew it had a distortion problem when I purchased it. Right off the bat I found some wiring issues but nothing that would have caused the distortion problem (mostly ground and lead dress). I also have the parts on hand to make a complete new board and drop it in, but part of me wants to track down the problem.

    So, the amp was built with 5E3 transformers and appears to be pretty well made (except for no center tap of any type for the filaments) and I believe it's actually louder than my own personal 5E3 build when run through the same cabinet. That seems odd alone given that I used to own a 1960 Harvard and know what to expect. The amp more or less seems to function as it should, except there is an excessive amount of distortion and almost no clean even with the volume down and trying to tame it with pick attack. There is the underlying clean tone that is equally as loud as the distortion on top of it (and it seems to be musical distortion as opposed to noise).

    The details that I have been able to nail down are that the voltages are all about 20% high, most likely due to the power transformer from a tweed Deluxe, and I haven't been able to find any bad solder joints anywhere as of yet. Plate voltage is 380-392 depending on bias, the plate voltage on the 6AT6 is about 200V, and I can bias it anywhere from a low point where crossover distortion is definitely noticeable all the way up to 100% dissipation. I haven't been able to take any other tube voltages because I have serious back problems and right now with the back pain my hands are a bit too shaky to feel comfortable going for the pins that are more hidden.

    All in all, it really sounds like it simply has way too hot of an input signal that just can't be tamed, but I get the same results with all guitars and all have relatively weak pickups. Any ideas on where to look before I tear it apart and rebuild? I do have an oscilloscope but don't really know how to use it. I've located problems in amps with no sound multiple times before using a signal tracer, never had to figure out how to tame this type of distortion.

    Thanks..
     

  2. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Would you be more comfortable probing the board eyelets, where there is quite a bit more room to work? That would collect much of the same info.
     
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  3. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    I can try to get the rest of the preamp voltages tomorrow. Need to lift it to a better location to get at everything while its plugged in.
     
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  4. dan40

    dan40 Tele-Holic

    924
    Aug 19, 2015
    Richmond Va
    Check the plate resistors on the first two preamp stages to be sure they are the correct 100k and not 220k by chance. How about the cathode resistor on the second gain stage...any chance the builder added a bypass cap on this that the original circuit did not use?
     

  5. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    Plate resistors are correct...100k, 100k and 56k...with no added bypass caps.

    I had done a visual check for the proper resistors and wiring when I got it, especially after having no center tap. Some of the resistor colors were a bit difficult to read, but so far I think I have verified them all.

    Hopefully I can get at the rest of the voltages tomorrow. Oh! I did replace the pots with 3/8" shaft versions with lock washers and added a preamp ground buss to guarantee that there wasn't a grounding issue with the smaller import bushing size and lack of a buss even though there weren't other noise issues.
     

  6. jsnwhite619

    jsnwhite619 Tele-Afflicted Ad Free Member

    Age:
    34
    Sep 10, 2013
    Georgia
    I have built a couple Harvards and though I used a 12ax7 in V1 and only wired half of it, I can say that both were louder than my 5e3. Ran one at 305v, and one at 370v, both were louder than my Deluxe. But, the zero clean is definitely an issue -- I love the cleans on those things.
     

  7. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    I always expected my original Harvard to be louder than a Deluxe given the fixed bias, but mine wasn't. It started to break up around 7-8 o'clock and the cleans were wonderful. I tend to play relatively loud at home so I used to mic the Harvard and run the mic into a Bassman to boost the clean volume. :lol:
     

  8. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    So, I may have found the area to attack with a soldering iron even though all tapping and pressing on the board make no difference.

    Here are the numbers: Aside from the high plate voltages, the 6AT6 looks about correct with 1.53 volts on the cathode. One pin of the PI is where the problem appears to be. The second plate of the PI is about proportionally high with the others at 262vdc. I get 21vdc on pin 7 (schematic says 43) and 57.7 on pin 8 (schematic says 44.9). For pin 3, however, the DMM goes from a small floating voltage to dead-on 0v for pin 3. I also get 0vdc at the junction of the 1500 and the 56k NFB resistor so I assume this is where the problem is.

    Given this info, is the actual issue with the distortion that the amp is only using one half of the PI because the second gain stage isn't conducting? I sometimes get lost as to how this section works because I tend to only learn it when I spend a lot of time with amps, then forget it when I don't have any reasons to keep up on the more technical aspects. I would have expected lower volumes if I had a gain stage issue.

    Thanks again, folks.
     

  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    Yeah, a malfunctioning PI would not sound good. One power tube would idle, and the other would go in and out of cutoff as signal rose.
     

  10. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    Found and fixed what was the main problem, and then created a couple of small ones in the process. The board used was similar to a Vox type of tag board so the original builder didn't have room in a couple of places to put two resistors end to end (like off of the PI) so he turned them sideways at the top or bottom of the board. In this case he grounded the wrong end of the resistors and was sending the NFB and the cathode. I missed this when I did the original visual because there was loud sound.

    Grounding the correct junction of resistors gave me a pretty strong initial signal, but he had used hair thin braided wire to shield the original run for the second gain stage and that broke. Replacing it with just a piece of solid core for now has created terrible screeching oscillation and distortion on top of the signal. This board doesn't have any holes to feed the signal wires through so the lead dress is pretty bad with all the wires crossing between the board and chassis wherever they find a way. I suppose I can try to wire up the two grids above the board with some better shielded wire in the meantime, though I think rebuilding on a neater eyelet is in the very near future since I'm only missing a few resistors.

    At least we tracked down the issue first and that always feels better.
     
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  11. milocj

    milocj Friend of Leo's Ad Free Member

    Aug 21, 2005
    Michigan
    So, quick update. I remembered that one of the first things I had tried when receiving the chassis was to disconnect the negative feedback just in case the leads were reversed. Since that made no difference I had reconnected it when fixing the wiring mistake. I went back inside, snipped the lead across the two speaker jacks, and no more oscillation. A quick reversal of the two OT wires on the tube sockets and all the distortion (and most noise) is gone. Now the amp plays fairly clean a good part of the way up.

    It's still at least as loud as the 5E3, and definitely cleaner. Even stays cleaner much higher on the dial than my old Harvard did, but that may be due to the pots as I think I replaced these with both linear pots.

    Thanks for all the help.
     

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