Problem with first fire of GA-1 RVT clone

Discussion in 'Shock Brother's DIY Amps' started by NSB_Chris, Sep 17, 2019.

  1. NSB_Chris

    NSB_Chris Tele-Meister

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    I am trying to build a clone of an old GA-1 RVT for fun. My latest posts of the build are here:
    https://www.tdpri.com/threads/help-to-fenderize-reverb-on-ga-1-rvt.960334/

    There is trouble in paradise! It works but not completely. I will benefit from those that might recognize what is going on here.

    When the volume is rolled up off of zero it sounds good even on the crappy test speaker I have for first fire.
    The vibrato and the reverb work and sound really good, but at about 2/3 volume it just cuts out and there is a really high frequency tone for the rest of the volume pot range. The tone is steady but not very loud. I switched the secondary wires on the OT to be sure it was not a feedback oscillation.

    If I did not make some mistake in the circuit or assembly, could this be related to the bastardized reverb circuit that I shoehorned into it? Somehow interacts as the input load increases. The same sound occurs at any volume pot setting if I unplug one of the wires to the reverb tank which makes me think it might be related to the reverb circuit.

    Or did I just get a bad tube? The issue does seem to be a little inconsistent and seems to be affected by the volume control on the guitar. Hard to tell but it might be cutting out right as the input tube is starting to distort.

    Any help or insight would be greatly appreciated!
     
  2. NSB_Chris

    NSB_Chris Tele-Meister

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    I am going to order another set of tubes to at least rule out that I didn't get a bad tube. Could be tube oscillation that I am hearing.
     
  3. NSB_Chris

    NSB_Chris Tele-Meister

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    In researching something else, I happened to see Rob's AB763 Mods section on reducing reverb oscillation. He talks about adding a capacitor across the 220k reverb recovery grid leak resistor "to filter out unneeded high frequencies which helps cut noise and prevent oscillation in the sensitive reverb circuit." Does anyone think that is maybe what I am experiencing? Would the oscillation being prevented be a super high frequency oscillation and be sensitive to the signal level going into the reverb input?
     
    Last edited: Sep 20, 2019
  4. NSB_Chris

    NSB_Chris Tele-Meister

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    Here is the schematic as it is now:

    GA-1 RVT V02B (1).jpg
     
  5. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    sounds like oscillation of some sort - try moving around the wires while the symptom is occurring to see if that changes the behavior.
     
  6. NSB_Chris

    NSB_Chris Tele-Meister

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    I chopsticked everything I could inside the chassis and wiggled the wires on the outside. I switched out the cables to the reverb tank and the instrument cable just to be sure, but the same behavior persisted. Moving the volume on the guitar changed the volume pot setting where the signal would cut out and the high pitch sound started. Also it seemed to change a little as the tubes heated up which made me think it was a bad tube. Those are on order so will have those soon enough. Wish I had read Rob's tip about the capacitor before I put that on order because I could have purchased the cap at the same time.
     
  7. tubeswell

    tubeswell Friend of Leo's

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    Did you swap the OT primary winding leads around?
     
  8. NSB_Chris

    NSB_Chris Tele-Meister

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    I switched the wires on the OT secondary to make sure that the negative feedback was not the issue. I don't think it was as switching the wires gave the typical squeal from the wrong OT orientation and I switched it back.
     
  9. NSB_Chris

    NSB_Chris Tele-Meister

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    Purchased a 0.0022uF capacitor and put it across the 220k recovery grid leak resistor. It helped but did not solve the issue. I then switched out the tubes one by one and that did not do the trick either. I must be driving the reverb too hard and that is why it cuts out at about 2/3 volume. Any ideas what I should do to alter the reverb circuit?

    I know the B+ voltage is a good bit higher than the original, but I never measured the original amp at the plate, screen and cathode pins. Probably should do that.
     
  10. NSB_Chris

    NSB_Chris Tele-Meister

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    I tried one more thing before calling it quits for the evening. I put a 220k and then a 1M resistor between C11 and V1B pin 9 to create a drop the voltage going into V1B grid. The amp got lauder but I still can’t get the volume all the way up. Maybe I need to do something on the recovery side.
     
  11. Wyatt

    Wyatt Tele-Afflicted

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    Positive feedback?

    Try reversing the primary wires on the output transformer.
     
  12. jtcnj

    jtcnj Tele-Holic

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    he did the OT secondary swap and back in post #1 and #8.
    Have you used the divide and conquer troubleshooting method to try an isolate the source?
    if you ground / disable the reverb and / or the vibrato circuits can you crank it?
     
  13. Wyatt

    Wyatt Tele-Afflicted

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    But I missed that the OT wires were checked, and I assume OP does mean they swapped the primary. Another easy way to check for positive feedback is to disconnect the NFB from the OT entirely, NFB is lost for the test, but the oscillation will stop. But psiotive feedback should affect the signal strength.

    I wonder if the triode stage in the 7199 is being crushed byt the current demands of the reverb driver stage? I think it should be fine considering it's driver/PI status in Dynacos. The 7199 is not an affordable tube to build a guitar amp around.
     
  14. jtcnj

    jtcnj Tele-Holic

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    correct; I misspoke.
     
  15. NSB_Chris

    NSB_Chris Tele-Meister

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    I will definitely try grounding the input to the reverb and see if it goes away. I did leave an extra socket hole in case I need to get closer to the Fender reverb circuit, but hopefully that wont be necessary.
     
  16. NSB_Chris

    NSB_Chris Tele-Meister

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    Good suggestion. Grounding the input to the reverb driver stage pin 9 allowed volume to go to 10. The test speaker did not like it, but it went there. So now I know the culprit is the input side of the reverb circuit.

    So with the 1M resistor I put between C11 and pin 9, and the 1M grid leak resistor (R12), I have the input signal voltage halved. Should I lower the R12 resistor value to see if the amp will still crank and I preserve at least a little reverb?

    I think you are correct! Would it help to lower the B+2 voltage to the RT?
     
  17. NSB_Chris

    NSB_Chris Tele-Meister

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    First I tried lowering R12 to 470k ... oscillation still occurs at high volume setting.
    Second I changed the B+2 dropping resistor R45 from 1k to 10k. B+1 went up by about 20V and B+2 went down by almost 80V ... oscillation still there.

    I am all out of ideas. I am getting closer to coming to grips with having to put in a 12AT7 tube like the Fender reverb design and just let the triode of the 6U8A go unused.
     
  18. Wyatt

    Wyatt Tele-Afflicted

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    Have you isolated the oscillation to the reverb circuit yet? Can you bypass the verb and the oscillation is gone?

    Have you tried another 7199? Are you using NOS or Russian?
     
  19. NSB_Chris

    NSB_Chris Tele-Meister

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    I purchased a second set of tubes and tried them one by one. No change. The original 7199 type was NOS and the second was Russian, I think.

    Pretty definite the issue is in the input section of the reverb.
     
    Last edited: Sep 25, 2019
  20. NSB_Chris

    NSB_Chris Tele-Meister

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    I am guessing that there is no good way to get the existing triode on the 7199 type tube to drive the reverb circuit, so I am going to purchase parts to install a 12AT7 tube as a reverb driver like the fender circuit calls for. What is the best way to leave the unused triode on the 7199 type tube? Just leave pins 1, 9 and 8 open or should I ground some of them? I have never faced this before.
     
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