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I just removed the metal grills and cord box, jumpered the 22pF cap/1m resistor, disconnected the 25uF bias bypass cap and added a 47k NFB on my GA5.
The amp sounds awesome! I was hesitant to modify the amp because I wasn't sure that I was going to keep it. I liked it, but wasn't completely impressed with the tone, My 5E3 clone was killing it and wasn't so much louder that the GA5 would be useful to me.
For me, removing the rear metal grill and cord box was mandatory. They were easy to remove and it makes the amp easier to move, store, swap tubes on and connect speakers to.
Removing the front grill was a pain in the butt! Don't bother unless you have a condition where you can't leave well enough alone!!! It came out fine and I really do believe that it improved the tone, but not nearly as much as the electronic mods.
I'm very impressed with the electronic mods. I did them one at a time starting with jumpering the 22pF cap/1m resistor, disconnecting the 23uF byass bypass cap and adding the NFB.
I like the first two mods more than the NFB. I'm not sure of that one yet.
The amp is vastly better with a Telecaster now. In fact, it's a match made in heaven.
It's a mix of Champ tones and Vox chime. I still prefer the amp for clean to semi dirty tones. My 5E3 still kills it on songs like Funk #49 though!
The amp is chimey and rich where it was boxy before. It's much more responsive to the guitar's volume control. As much as my 5E3 clone.
The NFB seems to make the amp a tad too middy with my Les Paul and PRS Soapy.
Cranked way up the amp is still too out of control for my liking. That might be the EL-84. That's fine the clean and semi dirty tones more than make up for it. Maybe it could be even better tuned with a few more mods, a 820 to 1K5 cathode bias resistor on V1 as dansamp suggested, for instance.
I'm impressed!
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Don
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