Guytron GT100/FV Schematic needed to fix problem!

Discussion in 'Amps and Cabs' started by NateTheGreat, Jan 10, 2008.

  1. NateTheGreat

    NateTheGreat Member

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    Hi folks,
    I've seen many posts here and on the web asking for a schematic, but no actual schematic. So, here I go:

    Does anyone have a schematic for a GT100/FV?

    I've e-mailed Guytron but haven't heard back.

    I have an amp guru friend that's going to take a look at it. If anyone is interested in the nature of the problem....

    My GT100 took a dump the other day. The AC light comes on, but the channel lights do not light up, no sound out of the speakers (unless you tap the resistor wires in the power section inside the amp), and it appears that there's no power getting to any of the 12ax7's as they are all dark and cold.

    OH -- just in case, I swapped out the EL34's, then the El84's, then replaced the 12ax7's one by one with a known good tube. Didn't chagne the nature of the problem.


    The heaters for the EL84's and the El34's seem to be working at least. My first guess is a blown power transformer, then an output transformer.

    The strange thing -- a leg off from one of the 1ohm resistors for biasing (R90) had come unsoldered. Vibrated loose -- or heated up (ouch)? Don't know - just one piece of the puzzle....

    Thanks,
    -Nathan
     
  2. beej

    beej Supporting Member

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    I'd call Mick- his cell # is on the site.
     
  3. cochese

    cochese Supporting Member

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    If the bias resistor came loose it's possible you may have had a very badly shorted output tube which can take out that resistor. That should be the first thing to fix.

    Before you fix it though is your amp guru friend a hand wired tube amp repairman or does he have experience with both hand wired circuits as well as printed circuits.

    I doubt they will send a schematic and a good tech shouldn't need it for this problem. My suggestion is contact Guy first http://guyhedrick.com/Welcome.html then proceed. If you mess with something that is beyond you or your friend's experience you could muck up a really expensive amp when a small repair is all that's needed.
     
  4. JackD

    JackD Gold Supporting Member

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    If you can't get any help from the company (Mike), email me at home. Try Guy at the email that cochese listed above. If you are having any issues reaching him, let me know. I talk to Guy often. He is doing work on his own and does design, restoration, modificaiton and custom build work. He just did some amazing mod work for me on an amp and he is in the process of doing a custom build as well (you gotta love Guy!!)

    My email address is:

    jfdifranco@comcast.net

    Thanks
     
  5. NateTheGreat

    NateTheGreat Member

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    Thanks much folks. I've e-mailed guy at his personal web site.

    RE: the tubes -- they check out. I put them in another amp of mine and they sound fine. I also resoldered the resistor, but that didnt' fix the problem. I was wondering if the resistor could have failed and, if so, was it in the circuit path that would cause the type of behavior I'm seeing.
     
  6. cochese

    cochese Supporting Member

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    This may sound stupid but did you check both fuses after you resoldered the resistor?
     
  7. NateTheGreat

    NateTheGreat Member

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    Yes, I did that. They were fine, but I checked with known good one's two.
    Guy got back to me, didn't offer a schematic (though he said they were available at the 'patent office'). He offered his phone # should I need it. Wow. Considering that he no longer is affiliated with Guytron, that says a lot about him as a person.
     
  8. beej

    beej Supporting Member

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    Huh- how about that. In fact, they are- quite a few useful schematics there.
     
  9. Ray Gianelli

    Ray Gianelli Member

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    I'm guessing you have a problem in the low voltage power supply since the channel indicator lights aren't on. The heaters for the 12AX7's may be DC regulated; that would also point to LV power supply issues, although you state the the heaters in the power tubes are OK. I'd have your friend trace the heater supply back from the power tubes... that may lead you back to the trouble. Unfortunately the Patent Office schematics don't' show the power supply.

    Let us know what you find!
     
  10. NateTheGreat

    NateTheGreat Member

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    Stopped by to post an update. Ray -- you're hot on the trail, as it turns out.

    The problem is in the DC heater circuit. This is the circuit for the 12ax7's only. The ac is converted to dc to eliminate hum.

    This circuit is dead. My friend thinks its a cap. T03 was also mentioned as a possibility, but my friend said it shows a lot of gain when tested without load....

    Unfortunately, messing with that cap will require that the board is pulled... all of those connectors and tie wraps... My friend doesn't want to get into that. So, its off to a tech. Any recommendations in the Cleveland area? I'm thinking Empirical sound right now....
     

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