I recently restored a Gibson GA20T to working order and although it gets a pretty nice tone, the 12AX7 channel is very noisy and has NO sustain even when it's getting a little crunchy.
I'm thinking of changing the input stage to common cathode bias, anyone done this or have any comments/recommendations?
I've read in a couple of places that those yellow astrons are usually leaky so I'll probably try replacing those too.
What I'd really like this amp to do is be like a tweed Deluxe, so any ideas or comments are welcome.
GIBSON GA20T GRID LEAK BIAS-CHANGE IT?
Moderators: pompeiisneaks, Colossal
Re: GIBSON GA20T GRID LEAK BIAS-CHANGE IT?
Grid leak bias tends to give a kind of ratty distortion - I'd probably change it, but some people do favor it for the vintage Chess Records nasty sound. Have you tried a 12AY7 in V1?
Re: GIBSON GA20T GRID LEAK BIAS-CHANGE IT?
I'm waiting on a 12AY7, there's a 12AU7 in there now.
i'm going to go over all the caps for leaks and probably replace a few resistors too to see if I like it better with more parts to spec.
i'm going to go over all the caps for leaks and probably replace a few resistors too to see if I like it better with more parts to spec.
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Funkalicousgroove
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Re: GIBSON GA20T GRID LEAK BIAS-CHANGE IT?
Leave it alone!!!!!! build or buy something else but don't make that cool unique amp sound common!! JMHO, do what you will 
Owner/Solder Jockey Bludotone Amp Works
Re: GIBSON GA20T GRID LEAK BIAS-CHANGE IT?
OK, so I am going to leave this amp as stock as possible, I agree that's the best way, so I've been measuring parts and replaced a couple of leaky Astrons, etc, compared values with a schematic from www.freeinfosociety.com, etc. www.freeinfosociety.com/electronics/sch ... nga20t.pdf
And there are a couple of parts that vary from the schematic by a factor of 10. The grid leak resistors on the 12AY7 are listed as 10 meg but the ones in the amp are 1 meg, brown-black-green, and measured a little over 1 meg. Early Fender amps with similar circuits have 5 meg grid leak resistors, I believe that kind of bias is called 'contact bias.' One of the 1 meg resistors had broken at one lead, anyway, I put 10 meg resistors in and will have to see how it works when it's all put together again.
The schematic lists a 47k resistor between a pair of 220k's but the amp has a 4.7 in that spot-yellow-violet-red.
What gives? Anyone have similar experiences with older Gibsons?
I'm guessing the difference might be fairly subtle with the 4.7k but the difference between 10 meg and 1 meg in must be fairly stark-the amp sounded nice when I did use it but seemed to lack sustain, maybe the grid leak resistor is the factor...
And there are a couple of parts that vary from the schematic by a factor of 10. The grid leak resistors on the 12AY7 are listed as 10 meg but the ones in the amp are 1 meg, brown-black-green, and measured a little over 1 meg. Early Fender amps with similar circuits have 5 meg grid leak resistors, I believe that kind of bias is called 'contact bias.' One of the 1 meg resistors had broken at one lead, anyway, I put 10 meg resistors in and will have to see how it works when it's all put together again.
The schematic lists a 47k resistor between a pair of 220k's but the amp has a 4.7 in that spot-yellow-violet-red.
What gives? Anyone have similar experiences with older Gibsons?
I'm guessing the difference might be fairly subtle with the 4.7k but the difference between 10 meg and 1 meg in must be fairly stark-the amp sounded nice when I did use it but seemed to lack sustain, maybe the grid leak resistor is the factor...
Re: GIBSON GA20T GRID LEAK BIAS-CHANGE IT?
I think you're right about the grid resistors - the examples I've seen have all been 5-10M. The 4.7K/47K resistor sets the amount of gain that's sent back to the grid of the second phase inverter triode. Actually, that resistor and the 220K form a divider that sets the signal level. You could experiment with the value to get the PI balance right.