Weak tremolo in Gibson GA-40

[Thndr]

capnjuan,

New member, first post, I have a GA-20T I am rebuilding, I layed out the preamp it's using a 12AY7, I am having trouble with understanding grid-leak biasing. How is it getting voltage? There is no wire going to it.. pin 3 and 8 are jumpered then going to ground. Just trying to understand the circuit, it's just not making sense.
I would appreciate any help.

Thanks
Thnder

Hi Mike: finishing my thought here. This is the GA20T;s channel 1 preamp:



Each input jack of channel 1 feeds one half of the 12AY7, the tube relies on grid-leak biasing, and the cathodes of the 12AY7 are jumpered together. If you want to experiment with the amp, it might make more sense to un-jumper the cathodes, add cathode bypass resistors and caps, get rid of the 10meg resistors and input caps, and use a more reasonable value like 470K-1meg.

The front end of channel 1 bogs down pretty badly; if you wanted something out of the ordinary, the trick would be to create separately biased tube halves and take the output of the 1st half of V1 and feed it to the other half.

Anyway, good luck with your amp! John

[Thndr]

capnjuan,

New member, first post, I have a GA-20T I am rebuilding, I layed out the preamp it's using a 12AY7, I am having trouble with understanding grid-leak biasing. How is it getting voltage? There is no wire going to it.. pin 3 and 8 are jumpered then going to ground. Just trying to understand the circuit, it's just not making sense.
I would appreciate any help.

Thanks
Thndr

Hi Mike: finishing my thought here. This is the GA20T;s channel 1 preamp:



Each input jack of channel 1 feeds one half of the 12AY7, the tube relies on grid-leak biasing, and the cathodes of the 12AY7 are jumpered together. If you want to experiment with the amp, it might make more sense to un-jumper the cathodes, add cathode bypass resistors and caps, get rid of the 10meg resistors and input caps, and use a more reasonable value like 470K-1meg.

The front end of channel 1 bogs down pretty badly; if you wanted something out of the ordinary, the trick would be to create separately biased tube halves and take the output of the 1st half of V1 and feed it to the other half.

Anyway, good luck with your amp! John

[tubeswell]

capnjuan,

New member, first post, I have a GA-20T I am rebuilding, I layed out the preamp it's using a 12AY7, I am having trouble with understanding grid-leak biasing. How is it getting voltage? There is no wire going to it.. pin 3 and 8 are jumpered then going to ground. Just trying to understand the circuit, it's just not making sense.
I would appreciate any help.

Thanks
Thnder

Grid leak biasing relies on a large grid leak resistance to create a bias voltage - (You'll note those grid leak resistors are 10M). Its not a great way of biasing a tube.

[Thndr]

Tubeswell,

Thank you for the reply, I should have made a new thread, I have questions.. on this amp there is no physical wire going from the grid leak resistors (this version they are 1megs) to the cathode. So is this leakage occurring within the tube its self? It's probably a silly question for those who know the answer, I am just frustrated enough to ask.

When I get home today I going to look again.

Thanks again,
Thndr

[Enzo]

You might look up "grid leak bias" in a tube manual like the RCA or even a book like the RHD4 - Radiotron designers handbook. You can get a more detailed explanation.

The grid leak resistor connects to ground. The cathode connects to ground. They are thus connected together, whether there is a wire or not.

The hot cathode in a tube boils off electrons forming what they call the space cloud. With the very high resistance to ground from the grid, it sits there more or less in empty space. The space cloud electrons, well some of them will land on that grid and accumulate there. This causes the grid to build up a small negative charge from all those electrons. That negative sharge, even if it is only a volt or less, is still bias for the tube. SO I guess yes, the leakage does ocur inside the tube.

[Thndr]

You might look up "grid leak bias" in a tube manual like the RCA or even a book like the RHD4 - Radiotron designers handbook. You can get a more detailed explanation.

The grid leak resistor connects to ground. The cathode connects to ground. They are thus connected together, whether there is a wire or not.

The hot cathode in a tube boils off electrons forming what they call the space cloud. With the very high resistance to ground from the grid, it sits there more or less in empty space. The space cloud electrons, well some of them will land on that grid and accumulate there. This causes the grid to build up a small negative charge from all those electrons. That negative sharge, even if it is only a volt or less, is still bias for the tube. SO I guess yes, the leakage does ocur inside the tube.

Thank you for answering, I made a new thread for this awhile ago. The drawing is deceiving to me (newbe), I'll take your advice and do somemore reading. My worry among otherthings is the person who worked on this before me has wired the tubes to pc board incorrectly by accident. I made a partical layout diagram of what I am seeing in the new thread 1959 GA-20T.

Back to the books
Thanks again
Thndr

[LPJ57]

I know that this is an old post. Could you please tell me or show me on the pictures were the .0047 and .047 caps are in this amp? I am new at the rebuild thing just try to find these caps in the amp. GA 20 T. Thanks