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Thread: Help me ID this old Gibson GA amp (and schematics request)

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    Help me ID this old Gibson GA amp (and schematics request)

    The amp just says "GA-" and the rest of the model name has been rusted off. I've went through all the schematics from The Tube Amp Book CD and none match this configuration, which seems to have an extra tube. There's a pair of 6V6 output tubes and then a single 6L6 before the rectifier tube. No idea what's going on there... any ideas?

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    Bent Member Chuck H's Avatar
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    I think the 6l6 is a regulator. Gibson did this sort of thing on a few amps. The GA86 used a 6V6 and the GA100 used an OC2. Your amp doesn't appear to have the right tube compliment for either. But they also made impromptu circuit changes, sometimes dramatic and amps that differ from any known schematic are common. They also recycled names a lot. I don't even know how many different "Les Paul" amps they made. Run a *oogle search with something like: vintage gibson ga "regulator tube" :and see what that turns up.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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    Have you tried a black light on the model number?

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    First model Les Paul GA-40 amp.
    Early mid-50s

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by sgelectric View Post
    First model Les Paul GA-40 amp.
    Early mid-50s
    A *oogle search reveals a perfect match cosmetically and the same tube count as a 1952 GA40! Good call

    http://www.vintage-guitar.de/detailsitem_2164.html

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

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    Bent Member Chuck H's Avatar
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    Sweet! :thubsup:

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "If you build it, it will hum..." Justin Thomas

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    Thank you, fantastic! Next question: What's a regulator?

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    Quote Originally Posted by waspclothes View Post
    Thank you, fantastic! Next question: What's a regulator?
    Some amps have screen grid supply regulation. This can be done using a a tube or transistor arrangement.

    However, in the case of the GA40, the extra 6V6 shown on the schematic linked above is not being used for a regulator. It is part of the tremolo oscillator circuit. The 6L6 in your amp could be the same (if your amp is a GA40). On the other hand, if your amp is not a GA40, then the 6L6 in your amp could be anything, including a screen grid regulator. No way of verifying except by detailed inspection of the circuit in your amp.

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    Bent Member Chuck H's Avatar
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    Quote Originally Posted by tubeswell View Post
    Some amps have screen grid supply regulation. This can be done using a a tube or transistor arrangement.

    However, in the case of the GA40, the extra 6V6 shown on the schematic linked above is not being used for a regulator. It is part of the tremolo oscillator circuit. The 6L6 in your amp could be the same (if your amp is a GA40). On the other hand, if your amp is not a GA40, then the 6L6 in your amp could be anything, including a screen grid regulator. No way of verifying except by detailed inspection of the circuit in your amp.
    Thank you tubeswell! I think I see what they're doing now. Pretty slick. The 6v6 IS sort of being used for a regulator and the 6sn7 oscillator is controlling the voltage being passed to the power tube screens by manipulating the 6v6 bias at it's grid, right? I wonder how it sounds/feels/behaves? Seems like a smart way to do it.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

    "...less ear-friendly but handy for jazz." Leo_Gnardo

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    "If you build it, it will hum..." Justin Thomas

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    Quote Originally Posted by Chuck H View Post
    Thank you tubeswell! I think I see what they're doing now. Pretty slick. The 6v6 IS sort of being used for a regulator and the 6sn7 oscillator is controlling the voltage being passed to the power tube screens by manipulating the 6v6 bias at it's grid, right? I wonder how it sounds/feels/behaves? Seems like a smart way to do it.
    I think it is manipulating the 6v6 bias at the cathodes. The trem 6v6 turns on and dumps current into the shared cathode resistor, pulling it up, thus cutting off the main 6v6 push-pull outputs.

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    Supporting Member tubeswell's Avatar
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    Quote Originally Posted by octal View Post
    I think it is manipulating the 6v6 bias at the cathodes. The trem 6v6 turns on and dumps current into the shared cathode resistor, pulling it up, thus cutting off the main 6v6 push-pull outputs.
    Yes, the GA-40 trem oscillator uses a combination of grid voltage change and screen grid voltage change in the 6V6 to pull the 6V6 cathode up and down, so it functions as a low impedance driver (cathode follower) for wiggling the bias on the output tubes.

    Quote Originally Posted by Chuck H View Post
    ... The 6v6 IS sort of being used for a regulator and the 6sn7 oscillator is controlling the voltage being passed to the power tube screens by manipulating the 6v6 bias at it's grid, right?
    In the GA40, that extra 6V6 forms a cathode follower to drive the bias wiggle. If it was intended to be a proper regulator, the output voltage would be constant.

    An example of use of a tetrode as a simple screen supply regulator is attached (the triode-connected KT61 - which is a 'tetrode equivalent' to an EL33 pentode). The 66.6% voltage divider sets the grid voltage in the KT61 at 2/3 of the HT voltage. If the screen voltage in the KT66s drops, it causes a plate-to-cathode voltage increase in the KT61, which the fixed KT61 g1 voltage instantly corrects*, (and vice versa) keeping everything constant at the KT66 screens. Using a tetrode (or pentode) in this way is the best way of regulating output tube g2 voltage IMHO (assuming you have real-estate and heater winding juice to spare, and relying on the overall power supply voltage being constant, and that you have factored in adequate protection for the h-k voltage rating of the regulator tube, and the regulator tube you choose being sufficient to cope with the total current demand of the output tube screens, at the voltage you are dropping across the regulator tube).

    * because, relatively speaking, the KT61 grid-to-cathode voltage is tending to increase, which tends to make the KT61 conduct more, which acts to pull the plate-to-cathode voltage of the KT61 back into line.
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    Last edited by tubeswell; 10-23-2017 at 03:32 AM.
    Building a better world (one tube amp at a time)

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    Bent Member Chuck H's Avatar
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    The 6v6 elevation at it's cathode is constant via R28 in the above diagram (when switched). But the 6sn7 plate is modulating the grid of the 6v6 and this changes it's bias relationship with the 6v6 cathode and therefor the 6v6 cathode current and voltage. I think the oscillator (6sn7) is manipulating the 6v6 bias at the grid and the 6v6 cathode response is manipulating the power tube bias.

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    "I'm just going to perform a bit more scientific investigation, turn it up to 11 and rip of the knob." überfuzz

    "...less ear-friendly but handy for jazz." Leo_Gnardo

    "A pedal, any kind, will not make a Guitar player more dangerous than he already is." J M Fahey

    "If you build it, it will hum..." Justin Thomas

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