Gibson GA 18 Explorer Guitar Amp

[CJ]

somebody robbed dad's basement and brought in a Gibson amp plus a Showbud pedal steel,

these guys get big bucks on the vintage market so the challenge is to fix it without changing any components,  ???

this one is very clean, altought it is missing the logo which is quite common for these amps,

 

[CJ]

there is a loose Sprague dry lytic rattling around in there with one of the wires taped up,

the guy wanted his wife to plug it it in to see if it was working, 

notice that the back is on upside down,

 

[CJ]

this top panel could use a little rub down with some alcohol but we do not want to remove any silk screening so just make sure the jacks and pot nuts are tight and that's it,

 

[CJ]

small tag board, this thing was probably pretty quick off the assembly line,

 

[CJ]

close up

 

[CJ]

tube socket wiring,

we might suck that old crusty solder out of there but replacing the sockets would requring drilling out those rivets,

maye we will just retension the sockts and try it as is,

 

[CJ]

5y3 and 6v6 sockets,

 

[CJ]

don't know about that color banded cap, sits right on the line,  could explodeat any minute, 

is explodeat a word? 

 

[CJ]

got a Mallory allclmped upand ready to shoe horn into the chassis,

ithinkthiskeyboard needs avacuumjob,  ;D

notice the puny output, replace that with a bigger one and you lose your vintage Jensen,

 

[CJ]

that 47k is probably a feedback resistor,

 

[CJ]

speaker code one one side...

 

[CJ]

and the part number on the other side,

 

[CJ]

here is a pdf of the manual>

http://www.acousticmusic.org/userfiles/file/pdfs/historical-data/Gibson/Gibson%201959%20GA-18%20Amp.pdf

with the extraction we want>

 

[CJ]

non aged schematic

 

[CJ]

related links>

http://truevintageguitar.com/inventory/1960-gibson-ga-18-explorer/

http://djlservice.com/AudioGibsonGA18wPhotos.htm#_Toc142400908

http://www.tdpri.com/forum/amp-central-station/82159-love-gibson-anyone-own-ga-18-ga-18t.html

http://forum.gibson.com/index.php?/topic/68024-gibson-explorer-ga-18t-amp/

http://www.0rigami.com/gg/amps1.html

other Gibson schematics>

http://www.classictubeamps.com/SchemGibson.html

http://www.gibson.com/Support/Schematics.aspx

here is a borrowed shot showing the foot pedal>

 

[CJ]

here a a schematic featuring 6BQ5/EL84 power tubes>

 

[CJ]

redraw shemo and modified version, looks like more  filtering has been added,

 

[beatnik]

in your opinion, would the added filtering in the modified schematic really bring an improvement ?

maybe the amp is going to be less noisy, but what about the sound and sensitivity ?

 

[CJ]

we will have to see how bad this thing hums, push pull amps tend to cancel a lot of hum,

i am using 30 uf instead of 20 uf for the first section,

and 20 uf instead of 10 uf for the second stage,

a gutar amp modulates the power suppy, so when you over filter, you lose some of the funk, mainly from the fist section,

so using 30 instead of 20 will make it run with less hum and not affect the tranfer function too much,

over filtering could also cause the speaker to get a little extra thump and we don't want that as it is 56 years old,

 

[beatnik]

usually the cap on the center tap of the transformer is bigger than the ones to the preamp stage and phase inverter isn't it ?

in the modified schematic it is the opposite

is there a rule of thumb in choosing value for those caps or it's more a listening test ?