Loud harsh distortion/crackling when amp it turned up

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by To Here Knows When, May 31, 2015.

  1. To Here Knows When

    To Here Knows When Member

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    So, after repairing and converting an old Stromberg Carlson Signet 22 EL84 PA amplifier, I noticed a terrible sounding distortion when is turned up loud, and the tone control is set to cut no treble. I tested this on several speakers to make sure they weren't the culprit. The amp has all new tubes, a JJ 12ax7, JJ EL84s, and a NOS Raytheon 6u8. The EL84s are biased around 80% dissipation. I would go lower, but I'll have to get a higher value resistor. Originally it was biased at absolute max... I let the two preamp tubes have more voltage by lowering the 33k resistor that goes to the filter cap. I did this to increase headroom a bit. All voltages are still well within a safe level.

    This is not my first experience with tube based electronics, but I've never repaired a guitar amp. I was wondering is crossover distortion is what is occurring. I have a sound sample of me hitting a few chords


    Thank you!
     
    Last edited: May 31, 2015
  2. pdf64

    pdf64 Member

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    No, I don't associate that with crossover distortion, rather just regular old unpleasant distortion.
    It may be blips of oscillation, ie parasitic; have power supply caps been replaced?
    Do you know that the tubes are good, eg tried them in another amp?
    That they are new is no guarantee that they're fit for purpose.
    Schematic?
     
  3. To Here Knows When

    To Here Knows When Member

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    I did not replace the can capacitor, no. It filters the AC well, however the values have drifted quite a bit. I replaced all other capacitors and many resistors though, and changed the preamp section up a bit to be like the one in the Marshall 18W Lite II schematic. I have also removed the .033 input caps and added 68k input resistors. The guitar signal goes through both of these resistors into both halves of the 12ax7 simultaneously. There is a 1M resistor from the hot input to ground. There are 2 volume knobs, so each half can be adjusted. I wanted to raise the voltage to the 12ax7 plate, since it was only around 55-60v, so I changed a few resistor values. R26 is the one I changed to a 15 K. I made sure the 6u8 voltages were still high, and they were.

    I have not tested the tubes in another amp. I have other 12ax7 tubes i tried in the preamp with no change, and I also tried the original 6u8 out with the same results. The EL84s are brand new and matched so I can't imagine they have issues.
     
  4. zenas

    zenas Member

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    I don't trust an old cap can. They can test out pretty close to spec and act ok (no hum) but still not preform right at all.
    Part of the problem is most testing involves reading the capacitance on something with a 9 volt battery. Not even close to what a filter cap sees.
    In fact few old amps I work on have the "tell tale" signs associated with bad filter caps the unfiltered ac hum.
    They do have tend to have odd noises and weak bass response.

    Long story short. Messing with old electrolytics is right up there with pissing in the wind and trying not to get it on your self.
     
  5. pdf64

    pdf64 Member

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    There don't seem to be control grid stoppers or screen grid current limiting resistors; suggest to fit ~5k6 1/2 watt in series with each terminal 2, and 470 ohm 1 watt in series with each terminal 9, of the power tubes, at the tube socket terminals.

    Regarding the can cap, I wouldn't trust it.
    If you've got some suitable single caps, then take the can cap out of circuit and try them instead.

    Plus for guitar use (ie liable to be overdriven), an additional B+ node (eg 1k 3 watts, 22uF) for the screen grids is highly beneficial, especially for high gain pentodes, to avoid ghost notes.

    Were the power tubes 'burnt in' prior to being matched?
    Even if they were, something could have displaced during shipping, so they're still likely suspects.

    Opening the feedback loop (eg lift one leg of R24) may help to give a smoother transition into overdrive.
     
    Last edited: May 31, 2015
  6. To Here Knows When

    To Here Knows When Member

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    I didn't mention that I did add a 2K (I also have a 1K to use if needed) resistor before pin 9 on the output tubes and I replaced the negative feedback resistor with a 100K. When I first got this amp, I tested it out, as the old owner said it worked, and it did. It, of course, needed fixed up, but I remember it being very loud and the volume controls were more sensitive before. I think this is simply because the output was running ridiculously hot, but I am not sure. It still sounds good after my repairs, other than the distortion issue...

    I will definitely add stoppers where needed and would like to replace the can cap as well. I did replace C13, which is the elec cap right after the rectifier, and I replaced c6, the low voltage 50uf cap.

    The tubes were not burned in when I got them.
     
  7. To Here Knows When

    To Here Knows When Member

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    Could this be a result of hitting the 6u8 with too much signal? Maybe the inverter side of the 6u8. I just thought of this, since my preamp design is more powerful than the original one in the amp.

    If this is the case, maybe a blocking resistor would help. I'll have to try that.
     
  8. pdf64

    pdf64 Member

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    Probably not, or at least the pentode common cathode gain stage.
    Perhaps the cathodyne though, so maybe consider a big ~470k grid stopper, see http://www.valvewizard.co.uk/cathodyne.html
    After you've eliminated the B+ caps etc.
     
  9. To Here Knows When

    To Here Knows When Member

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    Thanks for the comments! I'm learning as I go haha!

    I made a bit of a "breakthrough". My goal while I modified it was to get the voltage on the 12ax7. Well, one thing I did was change the plate resistor, which in the schematic is a 220, to a far lower value. Big mistake, it's the reason my volume dropped and the volume knobs became unresponsive! After some research, I changed the resistor to a 100k, which is common in amps, and its great! Got the volume back. The plate voltage is 72 now too, which is much better than the 55 it had way earlier. The schematic actually says there is 72 volts on the plate. I also removed the NF resistor. I tried a pot on it to test which value I liked, and the change was minimal, so I just removed it.

    So now I only have the distortion issue very slightly when the amp is extremely loud. I still have to add the stoppers too. I'm sure after I do that, and replace the can cap, this problem will be fixed.


    As of now, I added 100K resistors before the output tubes and the horrible sound seems to be gone completely! I cranked it up to max, it sure had a very bright distortion, but not the destructive sound like heard in the sound clip.
     
    Last edited: Jun 2, 2015
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