Gibson GA 40T Effect Loop?

[shades]

Hi! I got this wonderful Gibson GA 40T Amp and love it!
But i'm a greedy man, i wan't reverb!
so i thought i would make a effect loop (between the preamp and the poweramp) to have the reverb effect after the pre amp.
But i'm just not experienced enough to read the schematics.
Can anybody guide me though this mod ? Or talk me out of it if it's not a good idea ?

Simon.

[Jazz P Bass]

Gibson Reverb

I have attached a 20 RVT schematic, just to give you an idea of what is involved.
Looks like you need 1 1/2 12AX7 tubes.

[shades]

Huh ?

I don't get it, i thought it would be cutting the signal between the pre and power amp ?
Like a line-out but with a return as well.

Simon

[Jazz P Bass]

Gibson Reverb

What don't you get?
The 20VT schematic shows the signal being shared at the Volume pot.
What are you trying to do?
Are you thinking of attaching an external digital reverb effect?
I was thinking you wanted a tube circuit.

[shades]

Oh

Oh no i just wan't to cut the signal before the power amp so i can connect something like a Fender Reverb Tank or a digital reverb yes.
Nothing fancy with a volume or a mix knob, just a "send" and a "return"
Can it be done ?

Simon

[Jazz P Bass]

Reverb circuit

First off: you can't just hook up a reverb tank to the signal.
Look at the schematic for the 20RVT.
At the bottom is the reverb circuit.
The tank has to be "driven" & then the signal has to be received.
Thus, the reverb circuit.
Sorry, I can't advise an effects loop circuit for you.

[shades]

Okay...

i did not understand the thing about driving the Tank, i'm danish so maybe it's a gramma thing.
So is there another way to have reverb in my Gibson amp ?
Or in a Marshall 100w i was planning to do the same thing to?

Simon

By the way thank you!

[Jazz P Bass]

Tube Driven Effects loop

There was a post today that has a schematic of an amp with an effects loop.
Here is the schematic.
It is a goofy drawing, but I think it will tell you what you need.

[luthierwnc]

Shades -- you're just talking about an insertion point for a passive loop, right? If you absolutely must do it, it should probably go right after C16 where you would run a shielded wire to the send (shorting) jack, wire the shorting arm to the input of the return jack and have another shielded wire going to the phase inverter (V4A).

That said, you certainly don't want to hack-butcher the amp. Just put a pedal in front. Without a buffer (either in the amp or on the pedal board) whatever you are using for a reverb would need to handle line signal strength. You can get them but a stomp-box is a lot easier and won't get you blacklisted by the vintage preservation league.

FWIW, Skip

[shades]

Yeah but...

Yeah a passive loop... I just don't like the sound of the reverb through a pre-amp compared to pre-amp through the reverb!
Can't i just use a line driver before the reverb ?
I was planning to use a Electro Harmonix Holy Grail but you know it's just not the same thing when the reverb is "in front" of the amplifier.
The amp is from around '55 and sounds like that, so i wan't the reverb to sound like it's "build" in.

Simon

[luthierwnc]

it might work OK anyway. Your problem is that there isn't a lot of room in one of these things to make the changes. Maybe you could rewire some of the unused input jacks but then you have to cross power and heater wires with a vulnerable signal wire. I wouldn't do it -- and I've made dozens of scratch amps. Whatever you do, don't drill any holes. sh

[shades]

Hm

Okay. Well i'm not going to do it then..
So there really isn't anyway to get a good reverb in a vintage amp ?


Simon

[luthierwnc]

do it like Larry Carlton. He plays a Dumble head over a single Thiele 12 EVM12L. Tha amp works more-or-less as a stage monitor. He mics the speaker into a small PA board and adds his effects from there. You can see it pretty well at about 3:30 on this link:

[ame="http://www.youtube.com/watch?v=jBS7BEPb1Jk"]YouTube- Larry Carlton w/ Robben Ford[/ame]

Obviously, he is playing to a room where most of the audience is getting front-of-house sound. Jam night at the Brew and Sub might call for a different stage rig. sh

[pdf64]

It was funny how the sound guy leaves Carlton all but inaudible until 4:27.
The Gibson G40T schematic and picture is from the 1960s, not 50s.
It would be much better to find an external pedal that sounds good in front of the amp. Almost any modifications will compromise the sound of the amp to some extent, unless you are prepared for things to get complicated.

[shades]

Well

It just that i love the way the pre-amp clip and that would not work with a reverb in my pedal chain.
So maybe i should leave this amp be and find another amp for the reverb stuff. The thing is that the 4 amps in my life is a Marshall Bluesbreaker, Marshall 100w Plexi, my Gibson G40T and the Fender 59 Bassman and non of them have reverb.

Simon