Gibson Skylark GA-5T Questions

[jamforthelamb]

Hello all,

New to the forum here. I recently lucked into a couple of vintage Gibson amps. The first one I wanted to ask some questions about is a 64' (based on the transformer codes)Gibson Skylark GA-5T. The amp functions, but it has one tube the makes a scratching sound when it's even lightly touched, and will lower the amp's volume. The previous owner also put an 8 inch speaker in it with a wood adaptor (it's supposed to have a 10 inch). There also is another tube that is lit up like a light bulb. Here's a picture of the inside.

My game plan is to get the proper sized speaker in there (any recommendations for speakers?), look into the tube issues, convert it to a three prong plug, and do a "re-cap". I haven't done much of this kind of work yet, but do have some experience in electronics, so I thought I'd come on here for some experienced advice

Thanks!
-jftl

[dkevin]

GA-5T

Welcome to the land of Gibson amps! You have a Crestline series amp there and there are a few things you can do to make it sing.
Whenever I get a new amp, like to test the tubes (or have them tested so I know what I am dealing with. I will take the chassis out of the cab and give it a visual inspection looking for burned resistors, broken wired or dead spiders. The electrolytics I will replace if they are original. At the same time, I throw on a grounded cord and I am set for the listening test.
With correct schematic in hand I will compare the existing layout with the schematic looking for surprises. When you find them ( notice, I did not say "If you find them") make note on the schematic for future reference. I also take a multimeter and read the values of the resistors and note them on the schematic as well. This comes in handy when I am diagnosing problems later.
If the amp sounds healthy but a bit thin, you are starting in the right place. Crestlines are known for being thin and even a bit screechy when driven hard. A 10 inch Alnico helps ( like a Fender Blue alnico). This is about as far as you can go without making changes to the original schematic. Some people are very adamant about not modifying the original design. I am not one of them.
There are small changes that can be made to this amp to help it overcome its thin- trebly tone. I would be happy to share my experiences with you if/when you decide that modifying this amp is something you want to do.
Whatever your decision, this amp is point-to-point wired and all the components are easy to see, reach and replace. These amps are relatively inexpensive and a great place to learn about tube amps.
Enjoy!!

[jamforthelamb]

Hello dkevin,

Thanks for the tips! I'll look into that fender speaker. I know a guy that may be able to test the tubes for me, otherwise I'll have to look into my options. I'll likely just get it stock functional first, and then see if I want to mod it or not. I also have a Falcon from the same era, so I have some options

-jftl

[dkevin]

More GA-5T

I purchased my Fender Blue Alnico from Angela Electronics. At the time they were selling for $59 plus shipping. While you are checking the amp over pull a copy of the GA-5T and the EA-50T schematics to refer to. I mention this because my GA-5's seem to wander all over the map when it comes to bypass caps and grid resistors and the dreaded Sprague network. There is a website called "Paleo-Electronics" and he has worked with Gibson GA-5's too.
Happy amping!

[lasikukko]

Hello everyone! I bought a GA-5T too and when I got it running (replaced filter caps and new power transformer for 230 VAC use in europe) it was very very thin-sounding. I upped the coupling caps a bit, removed the sprague network and replaced the stock speaker with Fender blue alnico. Now the amp sounds great, very "alive", and responds to guitar controls and pick attack marvelously. Next I'm adding reverb to it, so I can play smaller gigs with it instead of my vibrolux.

[dkevin]

GA-5T

Hello!
Glad to hear about your GA-5T. You did pretty much what I have done to this amp to make it reach its potential. What version of the amp do you have? There is a tweed version and a Crestline version and a white panel version and finally, a medalist version. This helps us follow your mods because the schematics are all different.
I love these amps for their affordability and 10 inch speaker. They are a really great way to get into amp repair/modifying without ruining a "priceless gem". As is, they have the reputation of being thin and screechy. I also experimented with different interstage caps and a new speaker. They make all the difference in the world!

[lasikukko]

My amp is the crestline version. It's bit different from the schematic i have.
It has 100k series resistor after the volume pot, i intend to Make it 470k and
build the reverb circuit around it. And i will use a switched pot in place of the
rotary on-off switch for the reverb control so the amp will retain its original
appearance. If anyone has modified skylark it would be nice to hear what improvements
you have made...

[GibsonLover]

My Crestline is a '62 and also had a 8" Radio Shack speaker in it. I bought a 10" Jensen from 1966, and it really sounds good without even touching the caps. The amp sounds good, but I'd really like to change the Tremolo. I feel it is almost worthless on this amp because there is no intensity control. Since the Tremolo only has a speed control, and is labeled "frequency", I thought it would seem appropriate to rewire the Frequency control into a Tone knob and it would still have meaning. An amp with Volume and Tone is more useful to me than one with Volume and a Trem that I can't use. It beats drilling another hole somewhere to add a Depth control that I still probably wouldn't use.
I have two Gibson GA-75 projects ahead of this one (1950 and 1952) but I'll post my changes when I finally get back to it.