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Vintage Tube Amp Questions

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(@elbandito)
Active Member
Joined: 16 years ago
Posts: 6
Topic starter  

Hey everybody. I posted this in the guitar section before I realized that there was an amp section. :?

I own a 1956 Gibson GA40 tube amp and I've got a few questions about usage. Firstly, there are two channels to this amp, with two inputs each: One Instrument & One Microphone each. Each channel has its own volume control and there's one "master" tone knob that goes from 'Bass' to 'Treble.' I think the Microphone input is a little hotter than the Instrument one, but I haven't got any proof other than my ears.

My first question is this: I'd like to split my guitar signal (after my effects chain) and send one output into Channel One and the other into Channel Two. I'm hoping that this will allow me to mix a little grit into the clean sound and get a natural overdrive sound. Is this a good idea, considering the volatility of tubes?

My second item is this: In order to utilize a stereo setup, is it absolutely necessary to have two amps, or can i somehow feed each of the channels of my GA40 into separate speaker cabinets? I once saw a pedal by Radial that allowed the user to selectively switch between speaker cabinets in a similar way to the way an A/B/Y pedal would work for amps. Would something like that be likely to work?

Also, I'd like to remove the head of the amp and put it into it's own cabinet. This amplifier has already been modified sometime over the course of it's life, so if you're a vintage purist, don't freak out on me! How would I go about making a head cabinet? Are there any places that I can order one from? The amp utilizes a vertical design, so a Fender or Mesa cabinet wouldn't do.

Any help on these topics would be greatly appreciated!
Thanks very much!


   
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(@greybeard)
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Joined: 21 years ago
Posts: 5840
 

Hi, I've just deleted the double post, so don't wonder where it went. :D

I started with nothing - and I've still got most of it left.
Did you know that the word "gullible" is not in any dictionary?
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(@prndl)
Reputable Member
Joined: 18 years ago
Posts: 199
 

I fixed a Gibson GA-50T last Friday! It's a cool amp.

The microphone input does have more gain that the instrument inputs in the Tremolo version, so it's not surprising they did the same with yours.

A quick look at the schematics shows the instrument inputs have 47K into a .05µ and 100K to ground. Removing the .05 and replacing the 100K with a 1M will most likely improve the gain and make it sound better for guitar. A more extensive mod would be to rewire those stages in series, which would push the amp into distortion.

The pedal you describe can work, although it's made for Fender amps where the two channels are out of phase. This amp doesn't have that problem, so you can put the same signal into both inputs. It's also possible to jumper the instrument and mic inputs with a little patch cord.

The chassis is mounted on the bottom of the cabinet with screws into a piece of wood. It is possible to make a custom cabinet for it, but they'd need the chassis to make everything fit right. The hard part would be the chassis angles in on the top, which would look odd.

I have a nice tweed cabinet it might fit into, or I could build a custom cab. It's the cab in this web page
http://www.naturdoctor.com/Chapters/Amps/LivingRoomAmp.html

1 watt of pure tube tone - the Living Room Amp!
http://www.naturdoctor.com/Chapters/Amps/LivingRoomAmp.html
Paper-in-oil caps rule!


   
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(@elbandito)
Active Member
Joined: 16 years ago
Posts: 6
Topic starter  

"A quick look at the schematics shows the instrument inputs have 47K into a .05µ and 100K to ground. Removing the .05 and replacing the 100K with a 1M will most likely improve the gain and make it sound better for guitar."

Thanks for the tip! I'll definitely look into that. I'm not really looking to push the amp too far into the distortion range, just looking for a touch more 'edge' outta the ol' girl.

"It's also possible to jumper the instrument and mic inputs with a little patch cord."

So I'd just plug a mini patch cord into say, the channel one mic input and the channel two instrument input (Where would the output from my guitar go)? I suppose if that worked, then I could simply vary the volume of each channel to suit my needs, right? Does that not dump too much signal onto the valves?

"The chassis is mounted on the bottom of the cabinet with screws into a piece of wood. It is possible to make a custom cabinet for it, but they'd need the chassis to make everything fit right."

The chassis is actually mounted to the top of the cabinet with the tubes hanging down. This is the reason why I can't really use a generic Fender or Mesa-type head cabinet. If I can figure out how to attach pictures to my posts here, then I'll show you what I mean. Incidentally, will it damage the tubes or any of the other hardware if I were to mount the chassis sideways into a Fender-style head cabinet?


   
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(@prndl)
Reputable Member
Joined: 18 years ago
Posts: 199
 

So I'd just plug a mini patch cord into say, the channel one mic input and the channel two instrument input (Where would the output from my guitar go)?

Yes. Plug the guitar into channel two, which has three instrument inputs.
I suppose if that worked, then I could simply vary the volume of each channel to suit my needs, right? Does that not dump too much signal onto the valves?

This is a low gain amp to begin with, so it's unlikely there will be a problem. Also, the amp was designed to have a microphone and instrument (not necessarily a guitar) at the same time.
The chassis is actually mounted to the top of the cabinet with the tubes hanging down.

The GA-50T was mounted on the bottom with the tubes up, like the photo below. The idea at the time was to put the amp in front of the musician.

The problem is the face slants opposite to that of a Fender, which makes it difficult to put in a standard cabinet.
The face would angle into the cab, which wouldn't look that bad if you put a wide top edge that hangs over the knobs for a recessed look. Another solution would be to mount it in the rear of the head cab and put blank speaker cloth in the front.

How about making a cabinet with two 6" Weber Signature speakers ... a mini 2x6!

The other hidden issue is the output transformer is mounted on the 12" speaker, which makes it a bit difficult to make a head (without speakers) since there are no speaker connectors.

1 watt of pure tube tone - the Living Room Amp!
http://www.naturdoctor.com/Chapters/Amps/LivingRoomAmp.html
Paper-in-oil caps rule!


   
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(@elbandito)
Active Member
Joined: 16 years ago
Posts: 6
Topic starter  

My amplifier is not a GA50T. It is a GA40 with tremolo. My chassis is mounted from the top of the amplifier and the tubes hang down.

Hey PRNDL, how'd you put that pic up in your post?


   
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(@ricochet)
Illustrious Member
Joined: 21 years ago
Posts: 7833
 

As on most boards, you have to put the picture on a hosting site and link to the URL of the picture in your post. I use: http://photobucket.com/ There are lots of other hosting services, and you can always put them up in the free space your ISP gives you as well.

"A cheerful heart is good medicine."


   
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(@prndl)
Reputable Member
Joined: 18 years ago
Posts: 199
 

It's a standard tweed chassis.

For two speakers, you can have someone build a 2x10 tweed cabinet, or you could add an external speaker cabinet. To match the impedance, the easiest is to parallel two speakers at double the impedance of the original speaker.
I haven't seen a head cab for a tweed chassis.

I found a photo of the amp using a Google search.

1 watt of pure tube tone - the Living Room Amp!
http://www.naturdoctor.com/Chapters/Amps/LivingRoomAmp.html
Paper-in-oil caps rule!


   
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(@hyperborea)
Prominent Member
Joined: 17 years ago
Posts: 827
 

I haven't seen a head cab for a tweed chassis..

Weber has them - https://taweber.powweb.com/store/wccabs.htm

So does Mojo Musical Supply - http://www.mojomusicalsupply.com/cabinets.asp?id=44972

Pop music is about stealing pocket money from children. - Ian Anderson


   
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(@prndl)
Reputable Member
Joined: 18 years ago
Posts: 199
 

Sorry about all the misleading info ... sometimes I get "amp dyslexia" and confuse numbers.

Hyperborea is correct - you can use a standard tweed head cabinet, as long as it's the correct size.

1 watt of pure tube tone - the Living Room Amp!
http://www.naturdoctor.com/Chapters/Amps/LivingRoomAmp.html
Paper-in-oil caps rule!


   
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