1953 Gibson GA-40 Tremolo

[Tom Phillips]

Hi All,


I have a 1953 Gibson GA-40 on the bench. This is the original Les Paul amp with octal pre-amp tubes.


Everything is now working but the tremolo sound is weak and choppy. When I scope the output and adjust to display the modulation envelope I see that the modulation depth is a little less than 50% and the duty cycle is much less than 50%. The envelope shape is pretty squared off. Not at all a nice sine shape as achieved by Fender output tube bias modulation.


Anyway, my question to the group is:
Has anyone heard one of these early 1050s GA-40 amps that had good rich sounding tremolo? I ask because the circuit checks out OK and I don’t want to try to improve something that is already working as well as it ever was. (I’ve chased that Star before and I already have enough hours in this project) I have heard good things about the rich sound of this model amp but I have never heard anything about the tremolo quality.


This is the only amp I have seen that modulates the screen voltage of the output tubes screen to create the tremolo. Note that the 1953-54 model GA-40 circuit is completely different than that of the later versions of the same model number.


I plan to keep the amp stock for the customer. It would be very helpful if someone can share some first hand experience with the tremolo for this exact model amp. Then I’ll either look deeper or call it done.


Thanks in advance,
Tom