1965 Gibson GA40T tone control

[fredcapo]

I picked up a 1965 Gibson GA40T (replaced power transfirmer with one onhand that privides lower B+) with single tone pot ...lacks midrange...bass and treble very clean. Can someone tell me if the circuit design is the reason? If so, can you suggest a component tweak that might ooen up the midranges? Or a total rebuild of the tone stack. Or could the lower B+ be the issue? Thanks.

[Enzo]

Are you playing Fender single coils or Gibson humbuckers through it?

For what it is worth I have never heard one of those old Gibsons I thought sounded worth a damn.

[fredcapo]

Enzo thanks for responding. Been testing it with all sorts ofbguitars. Just has a notch that does not enhance the tone to my ears. I have been finding misc anecdotal bits and pieces online about Gibson tube amps...runs the gamut from killer to crap...there is a lot of variation in the designs. I just dont have thebknowledge to analyze the circuit.
To me it has potential with hand wired chassis to be a great amp once tweaked. Very well built.

I put in an odd speaker that could be part of the complaint, since the original ceramic jensen was crackling, so need to correct that. I found some tone stack info online and will try to jumper in a few tweaks.

[Pedro Vecino]

I do not know that model, but the filters after first triode (220K-220K-500pf and 0.0033) should collaborate with it.

[Enzo]

We have said it a million times - there is no one thing that will have more affect on your tone than the speaker. It might be as simple as a change there.,

[drewl]

Bypass those stupid pi filters and they sound great.

[J M Fahey]

Won´t talk about taste, to each his own, but those bridged T filters definitely cut midrange.
Just disconnect one leg of C4 and C9 and test again.

[fredcapo]

Juan, others,

Thanks. Following that up with a search, it seems Gibson saw the notch as a feature...maybe once I find the appropriate speaker, i will drop in a switch to bring it in or out.

[Steve A.]

Won´t talk about taste, to each his own, but those bridged T filters definitely cut midrange.
Just disconnect one leg of C4 and C9 and test again.

I think that Gibson added the bridged-T filter to their mid-60 amps to make them sound more like the blackface Fenders without having to make major design changes. Before the big EVJ mod craze*** I bought one and rewired it to Gibson Les Paul Jr GA-5 specs... the Gibson amps were selling for $500+. That was cool but I ran across the bridged-T circuit in a friend's amp and added it to my EVJ/GA-5 to make it sound more like a BF than a tweed. As you said to each his own...

Someone here suggested interrupting the path of the capacitors to ground with a pot to be able to dial back the effect of the bridged-T filter.

Steve A.

*** I suspect that I was the only person who modded an Epiphone Valve Jr to get a *cleaner* and less powerful sound from the amp...

[bob p]

Someone here suggested interrupting the path of the capacitors to ground with a pot to be able to dial back the effect of the bridged-T filter.

Could have been Fender: