Problem with first fire of GA-1 RVT clone

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

  1. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    Your reverb circuit has a close resemblance to the one I have been using on my last few builds. Actually, the architecture is probably close enough that you could rewire it to do it without any new lugs. Here’s the schematic I’ve been doing.
    D4949F5B-D42F-45B1-A6CB-FBE83004B7A8.gif
    it uses a fender transformer and tank. I’ve been using a 12AX7...I think you need to rewire the tube socket for this.
     
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  2. NSB_Chris

    NSB_Chris Tele-Meister

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    Thank you. Can you update with a higher resolution picture or attach the file, I am having trouble reading it.
     
  3. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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  4. NSB_Chris

    NSB_Chris Tele-Meister

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    Thanks! The half of the 12AX7 you are using on the driver side must be able to handle a lot more than the triode in the 7199 type that this circuit has. If I have to add another tube, and since I have prepared for he worst and added an extra tube socket hole in the chassis, I will probably just use the entire 12AT7 tube on the driver side and keep the recovery stage the same as I don't have reason to believe that it is part of the issue.

    My wife has "asked" that I build an amp for her niece that has started playing guitar and your circuit will come in handy since my current plan is to add reverb to a simple champ style circuit. Adding a single tube to cover the reverb section has significant advantages!
     
  5. NSB_Chris

    NSB_Chris Tele-Meister

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    I added the 12AT7 for the reverb driver like the normal Fender arrangement and the oscillation went away. It goes to full volume now. The triode section of the 7199 type tube just couldn't handle that reverb transformer/tank I guess.
     
  6. tubeswell

    tubeswell Friend of Leo's

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    So is everything working fine now?
     
  7. NSB_Chris

    NSB_Chris Tele-Meister

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    The initial problem with the severe oscillation and cutout at higher volume setting is definitely gone. The reverb is decidedly stronger, which is not surprising with both halves of the 12AT7 driving it. I hope it is a 12AT7 because the box I got from Tube Depot was completely unmarked and the tube has no markings on it whatsoever. I purchased the least expensive version, because I did was not sure if it was going to be the solution and I guess I got an extreme generic.

    Anyway, it does have some "crackling breakup" at high volume. If you dig in on the pick attach it seems to have "momentary cracks". Not sure how else to describe it. Its not a fuzzy or harsh regular breakup, just intermittent. I did not test very long as it was late. I will probably work in the other tubes I have and see if that has an effect. At first I thought it might be that my B+ voltages were higher than the original, but they are only higher by the standard 115 to 120VAC change.

    I have not done a head to head comparison to the vintage amp, but first impression is that it sounds really good with the volume pot below about 8. However, the appeal of this amp is the pentode breakup at full volume. I could get the warm tone at lower volume from a champ circuit without the exotic tubes etc. This continues to be an challenging experiment and a work in progress.
     
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