GA-5 head

[shwnicus]

Greetings all!

First timer here.  Actually, even greener than that, longtime drummer just starting into bass / electric guitar!  So bear with me, please!

I'm in a jump blues / vintage R&B band.  Our guitarist/lead has a beautiful but very aged GA-5 amp that he absolutely loves.  However, he has two wishes: more low end, and just a little more power (8-10w).  He also has no interest in mod-ing the amp he has now.  He loves his vintage stuff to stay vintage I guess.

Enter me, and my new interest in guitars.  I have an 8 yr old son with some interest in guitar, so I went and traded some of my old drums in for a decent epiphone les paul and a little fender mod amp.  But I'd love to have that valve sound.  I'm think of building an equivalent tube amp to Rob's GA-5 in a stand-alone chassis to see if we can get the sound he wants using different cabinets.  And as a bonus, my son and I get the fun of building our own amp together.

I've done a lot of reading, and have the GA-5 schematic.  But I also looked into the "The Schedule 40 GA-5 amp" project and like what I see there, but I have a few questions:

1) switching the 6v6 to el84, if I have read correctly, is not just a "use this power tube instead of that one", correct?  And won't that drastically change the tone from the warmth of the GA-5 to a more "British" sound?

2) is it even possible to mod this schematic to produce just a little more power (5w -> 8w)?  I see the GA-6 schematic adds a second 6v6 in parallel with a beefier output transformer, but it also adds things like tone control.  which, again if I'm following, somewhat destroys that tone the GA-5 makes.  having the tone pot even present will flatten the signal.

thoughts and advise are much appreciated, thanks!

[kagliostro]

A 6L6 GA5 ?


K

[shooter]

You can do anything....with enough info and experience, that said, I've fixed a couple of these and they were each different.  I believe the "older" one used 6CU7 pre-amp tubes and 6AQ5s in the PA.  The amp should be about a 8-10W already, so if your friends sounds anemic it could have issues, caps, tubes, etc.

Even if you copied your friends exactly, it won't sound exactly the same, may not even be close, that's where the "magic" of the builder comes in.  Is his a 5T (tremolo) or just a 5?  If you're gonna try one, I would do the trem version, personal choice. 
both the 6AQ5 and 6V6 want around 8-10k impedance depending on your B+, the big difference is the socket, 9pin for the 6AQ5, 8pin for the 6V6.
I built a PSE GA-8 using 6L6s that the players I know liked a lot, so 1st thing, settle on a schematic, insure you understand enough not to leave your youngin fatherless because you got zapped, try and find a competent builder locally and ask lots of dumb questions here til you're ready.  Buy the parts and start melting solder

[eleventeen]

Sounds like you should look at a 5E3 Deluxe if you want a "tweedy" sounding amp with more power than your "Champ" GA-5. The 5E3 amp uses dual 6V6 in a push-pull configuration. You could alternatively check out a 6L6-fired Champ. Since you've spoken of building a naked chassis and do not at this moment seem to be that worried about the implications of having no box for it, you can buy a 5E3 from any of a number of sources that would vastly simplify your sourcing of parts, which would be an outstanding way of beginning your amp-building career. You probably cannot source the parts for such an amp any cheaper than buying a kit, and it would have instructions and pictures etc; etc;


Many of us began building amps from scrounged parts or old hi-fi amps or radios long ago and far away. That was quite a different time and those things were routinely thrown out or available very cheap as junk. Those conditions do not prevail today. Now, that stuff is "vintage" and valuable and almost nobody gives it away. Tube sockets and pots that used to cost 25 cents each are now $3. Unless you have the means of bending metal and accurately drilling and working it (eg; sheet metal) not to mention the time and space to do so, these are not easy tasks. There are 75-100 parts in even a simple amp and getting them all together is no longer a trivial task.

Please also consider carefully the implications of having an open chassis with over 400 volts in it near your 8 year old.



There's not a hellacious amount of difference between an EL84 and a 6V6. The EL84 can run (but does not have to) on somewhat lower voltages than 6V6 and requires a little less drive.

[shwnicus]

thanks!

a little more (hopefully useful) information:

1) my 8 yr old will come no where near the electronic guts.  learn by view if he's interested.  at this point exposure is all I'm interested in

2) I'm looking at the schematic for the "first" GA-5.  no tremelo, no tone controls.  the Fender equivalent would roughly be the 5c1 (which used the 6SJ7 preamp tube).  Fender eventually ditched this tube in favor of the 12ax7 in the 5e1 and 5f1 schematics, but also made subtle but significant changes along the way.  I'm no expert, but the Fender tweeds have a different tone than the "original" GA-5.  the skylark, 5T, and on generally fall in line with the Fenders in the tone catalog, but there's just something about that GA-5 that I'm hoping to resurrect.

I found this article, which kind of started me down this path
http://www.premierguitar.com/articles/21037-undervalued-gear-the-gibson-les-paul-junior-ga-5-amplifier?page=1

and between the schedule 40 project here:
http://el34world.com/projects/Schedule40.htm

and this on youtube (pretty good example of the tone I like, at least as much as you can get from youtube audio)
https://www.youtube.com/watch?v=c3aApZ2G7_0
though, I do find it interesting that this example added a tone control.  from my reading, I've gathered that the very existence of the control pot will "flatten" the signal, thereby losing some of the character of these more simple schematics.

[kagliostro]

The GA5 and 5F1 looks to be the same amp with variation on components value (and an added 20uF bypass capacitor on V1)





You can easily find on the net a layout of the 5F1 and use GA5 values to obtain your amp

A good choice is Doug's layout that follows the original 5F1 schematic (someone else add filtering to the PS)

http://el34world.com/Hoffman/files/Hoffman_5F1.pdf



this for an amp that has the same power of the original

if you want a bit more, you can build a versions of the 5F1 with a 6L6 power tube instead of 6V6

here you can find documentation about building a 5F2 (that is very similar to 5F1) with a 6L6 power tube (Voltages and OT spec)

http://www.thallenbeck.com/blog/posts/d-i-y-5f2-champprinceton-clone/



An alternative will be to parallel two 6V6 tubes and to connect or disconnect from ground the cathode of one

as to have the standard power or doubled power amp

K

[sluckey]

Do you have any electronic knowledge/experience? Soldering skills?

The GA-5 is a great first time project. I suggest to build it exactly IAW the original schematic. If you need more power, put a mic in front of it or build a bigger version for your second amp. 

As K said, the GA-5 is very nearly identical to the 5F1 except for component values. I would use Doug's 5F1 layout or just buy a board from him if you don't want to mess with that part of the process. Load it with GA-5 component values.

A lot of people here have built a 5F1 (or GA-5) and will be happy to answer questions or give you hand. We all love to see pics too. Good luck.

[Willabe]

2) I'm looking at the schematic for the "first" GA-5.  no tremelo, no tone controls.  the Fender equivalent would roughly be the 5c1 (which used the 6SJ7 preamp tube).

[sluckey]

I'm looking at the schematic for the "first" GA-5.  no tremelo, no tone controls.  the Fender equivalent would roughly be the 5c1 (which used the 6SJ7 preamp tube). 

The 5C1 uses a pentode preamp tube and is very different from the GA-5. The 5E1 is almost identical to the GA-5.

[Willabe]

From Gibson Amplifiers, 1933-2008;

GA-5, Les Paul Jr., '54-'57.  1x6SJ7, 1x6V6, 1x5Y3.  1x5"x7" or 1x8".

GA-5, Skylark, version 1, '58-'62.  1x12AX7, 1x6V6, 1x5Y3.  1x8" Jensen.

GA-5, Skylark, version 2,  '63-'65.  1x6EU7, 1x6C4, 2x6AQ5, 1x 6X4.  1x10".

GA-5, Skylark, version 3, '66-'67.  1x6EU7, 1x6C4, 2x6AQ5, 1x 6X4.  1x10"CTS, Jensen upgrade option.

GA-5,T, Skylark, version 1, '60-'62.  2x6EU7, 1x6AQ7, 1x 6X4.  1x8" Jensen.

GA-5,T, Skylark, version 2, '63-'65.  2x6EU7, 1x6AQ7, 1x 6X4.  1x10". 

GA-5,T, Skylark, version 3, '66-'67.  2x6EU7, 1x6AQ7, 1x 6X4.  1x10".

Edit; Book typo, should be 1x6AQ5 instead of 6AQ7.


                        Brad       

[Willabe]

Gibson GA-5 Les Paul Jr.


              Brad   

[kagliostro]

Champ 5C1 



 

Franco

[sluckey]

Oops! Sorry, I missed that.

[Willabe]

Well the Gibby schemo says 'Les Paul Jr.' and not GA-5, so......... it's confusing.


            Brad   

[shwnicus]

this is the schematic I'm going off of, straight from the source (so to speak)

http://www.gibson.com/Files/schematics/GA-5.pdf

[sluckey]

Looks like a 5E1, except all the tubes have fallen down. Looks nothing like a 5C1.

[Willabe]

The schematic you just posted is like Sluckey and K said, very much like a Fender 5F1.

As per your description and the sound you want, the 1st GA-5, is the Les Paul Jr. with a single pentode preamp tube, not a dual triode 12AX7. And the link you posted from Primer guitar is on the GA-5 Les paul Jr, single pentode preamp tube, not the GA-5 Skylark, dual triode preamp tube. (12AX7 is V1 and V2 in the schematic you just posted. It is 2 preamp tubes in 1 glass bottle.)

2) I'm looking at the schematic for the "first" GA-5.  no tremelo, no tone controls.  the Fender equivalent would roughly be the 5c1 (which used the 6SJ7 preamp tube).   

but the Fender tweeds have a different tone than the "original" GA-5.

the skylark, 5T, and on generally fall in line with the Fenders in the tone catalog, but there's just something about that GA-5 that I'm hoping to resurrect.

I found this article, which kind of started me down this path
http://www.premierguitar.com/articles/21037-undervalued-gear-the-gibson-les-paul-junior-ga-5-amplifier?page=1

I posted the whole GA-5 (and GA-5T) line up to compare the tube line ups and the schematic for the GA-5 Les Paul Jr. which is very much like the Fender 5C1 you mentioned.

You need to find out the tube line up in your friends GA-5 so you go after the right amp model. Although they both can/do sound very good, depending on your taste, they will sound different.


                   Brad   

[kagliostro]

You need to find out the tube line up in your friends GA-5

ditto

Franco

[shwnicus]

Thanks for the clarification there.  That's very interesting how the schematics differ over time ;)  I know specifically that his amp is not a skylark, but we'll pull the back cover and look at the pre-amp closely.  By most accounts, its this preamp in the GA-5 that gives it that "early breakup awesomeness" but clearly that's all relative.  the more you know!

Thanks all!

[DummyLoad]

GA-5,T, Skylark, version 1, '60-'62.  2x6EU7, 1x6AQ7, 1x 6X4.  1x8" Jensen. GA-5,T, Skylark, version 2, '63-'65.  2x6EU7, 1x6AQ7, 1x 6X4.  1x10".  GA-5,T, Skylark, version 3, '66-'67.  2x6EU7, 1x6AQ7, 1x 6X4.  1x10".

really? a dual diode and hi-mu triode output? perhaps 6BQ5 is what's meant?   

--pete

[Willabe]

really? a dual diode and hi-mu triode output? perhaps 6BQ5 is what's meant?

It could be a typo in the book.


              Brad   

[Willabe]

That's very interesting how the schematics differ over time ;)

The schematics change as the company changes the models circuit.

but we'll pull the back cover and look at the pre-amp closely.

You don't need to pull the back cover, just pull the 1st preamp tube, furthest from the power transformer (PT) and closest to the input jack. Look and see if it has the tube # on it. And if it has 8 pins it's a pentode, if it has 9 pins it's a dual triode.


                   Brad   

[sluckey]

perhaps 6BQ5 is what's meant?

My source shows 6AQ5.

[kagliostro]

I've found this schematics for the skylark version, are this other versions ?





EDIT: A page with Fender correspondence to Gibson amps

http://www.rru.com/~meo/Guitar/Amps/Gibson/gfmap.html

Franco

[Willabe]

I've found this schematics for the skylark version, are this other versions?

Yes, see below, but I think he want's the   - 1st -  GA-5 (Les Paul Jr.) with the pentode preamp.
 

From Gibson Amplifiers, 1933-2008;

GA-5, Les Paul Jr., '54-'57.  1x6SJ7, 1x6V6, 1x5Y3.  1x5"x7" or 1x8".

GA-5, Skylark, version 1, '58-'62.  1x12AX7, 1x6V6, 1x5Y3.  1x8" Jensen.

GA-5, Skylark, version 2,  '63-'65.  1x6EU7, 1x6C4, 2x6AQ5, 1x 6X4.  1x10".

GA-5, Skylark, version 3, '66-'67.  1x6EU7, 1x6C4, 2x6AQ5, 1x 6X4.  1x10"CTS, Jensen upgrade option.

GA-5,T, Skylark, version 1, '60-'62.  2x6EU7, 1x6AQ7, 1x 6X4.  1x8" Jensen.

GA-5,T, Skylark, version 2, '63-'65.  2x6EU7, 1x6AQ7, 1x 6X4.  1x10". 

GA-5,T, Skylark, version 3, '66-'67.  2x6EU7, 1x6AQ7, 1x 6X4.  1x10".

Edit; Book typo, should be 1x6AQ5 instead of 6AQ7.

       


                         Brad   

[shwnicus]

Rob's amp has the 12ax7 preamp tube, so look's like this is the winner:

GA-5, Skylark, version 1, '58-'62.  1x12AX7, 1x6V6, 1x5Y3.  1x8" Jensen
so much for me being sure it wasn't a skylark ;)

So as plans go, I'll probably follow kagliostro and slucky's advise and start with Doug's 5f1 layout but use GA-5 schematic specific components, and go from there.  I'm intrigued by the sound in that GA-5/Princeton hybrid thing from youtube and the schedule 40 project.  But if Rob's amp is the sound I'm chasing, I'll start as close to that as I can get.  As others have said, I can always build another 

thanks, ya'll!

[shooter]

Did you find a schematic when you opened it up, one of the skylarks I fixed used a tranny for the PI instead of a tube, that "may" be part of your "tone/sound" - fwiw

[shwnicus]

the schematic was not inside, but all three tubes listed above were identified.  If I can get any information off of the transformers, I'll list them.