Home Forums Design and Building 2 Stroke Amplifier Design and Building 2nd two stroke – oscillation issues

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  • #5115
    Rohbiwan
    Participant

    Build one is working swimmingly

    #2 has not fared as well. Upon initial power up I had a HF oscillation. By initial troubleshooting it appeared to begin between the triods, in the volume and tone pot, but eventually I replaced the entire assembly.

    I did discover however that removing the feedback loop, specifically the 68k resistor, leaving it open, the oscillation diminished and the amp sounds fine until….

    opening the tone control all the way causes the amp to either shut down output, or oscillate – HF, I would guess over 6k, closer to 8k as it is pretty piercing. The volume works but is limited by the tone setting. The volume at 10% OR so, the tone can be cranked. If the tone control is centered then the volume can be cranked.

    So far: Replaced all signal caps and resistors (they all tested ok after being removed). I have not tested the PS caps, but all the voltages are good to the grids.

    PT is a Weber 022722 and the Weber WSE15

    Checked wiring several times, running out of ideas.

    Thanks
    Bill

    #5808
    Paul
    Participant

    http://www.tubeampnetwork.com/forum/effects-design-and-construction/653-squeal-and-hum-on-2stroke

    Check my long winded thread on squeal issues. [MODERATOR – that thread should be moved to 2 strok from Effects…]

    You have what sounds a bit similar to what I had. Maybe this helps, maybe not, but note that the weber OT you’re using has the blue and brown primary leads swapped VS what the Dave Hunter spec’d hammond has – so if you put blue and brown where DH says with that Weber, you’re going to get squealing at some combinations of higher knob settings.

    Took me forever to figure that one out, since I was checking my wiring and trying all sorts of other things.

    #5809
    Rohbiwan
    Participant

    Dangit! I suspected that possibility, but avoided switching the leads as I kept thinking “naaa, that can’t be right”.

    I’ll test that first – thanks.

    #5810
    Andy
    Keymaster

    Thanks Paul for the follow up. I went ahead and moved the topic, your link should redirect though.

    #5812
    Rohbiwan
    Participant

    Problem sorta solved

    It was not the OT, I had it right the first time.

    The problem was due to the totally different layout I created. This amp had to look different as it is for my daughter and her budding guitar interest. Due to the re-arrangement and my inexperience in routing wire for low noise, it was inductive in nature. The 2nd triode output, when rerouted, solved all the squeal problems. I suspect I may rewire the entire unit, simply to practice the skill and find the least noisy arrangement.

    Thanks for the help
    Bill

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