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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Help me ID this old Gibson GA amp (and schematics request)
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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."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by sgelectric View PostFirst model Les Paul GA-40 amp.
Early mid-50s
http://www.vintage-guitar.de/detailsitem_2164.html"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Sweet! :thubsup:"Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by sgelectric View Post
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Originally posted by waspclothes View PostThank you, fantastic! Next question: What's a regulator?
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.Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
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Originally posted by tubeswell View PostSome 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."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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Originally posted by Chuck H View PostThank 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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Originally posted by octal View PostI 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.
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?
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.Last edited by tubeswell; 10-23-2017, 02:32 AM.Building a better world (one tube amp at a time)
"I have never had to invoke a formula to fight oscillation in a guitar amp."- Enzo
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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."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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