I’m finally going to write about the fun I’m not having debugging a 60′s Gibson Skylark. To be specific, this is a GA-5T Crestline. The Crestlines differ from the 50′s Skylarks in that they are almost a completely different circuit, using two output tubes in a push pull configuration. Here is the schematic.
The first step, of course, is symptoms. When I unpacked the amp, I cleaned it out a bit, just to be sure no packing material would cause shorts or anything like that. I turned all knobs down all the way, and plugged it in. It wasn’t turned on, just connected to the power. Again, just checking for shorts. I listened for anything at all and sniffed around. No obvious problems.
The next step is power. Still nothing plugged in to the inputs. Powered up and after about twenty seconds (tube warm up) I got a loud hum. So much for the “Sounds great!” description from the asshole on eBay. Oh well, I had not really expected a whole lot of truth there.
Turned up the volume, and the hum got just a bit louder, but not really as much as you’d expect from being turned from 0-11. So.. bummer but at that point I knew I was going to have to go in.
Love the site.
I am a big fan of single-ended Gibson amps. I have 4 GA-5′s and an Epiphone EA-50. None of them is exactly the same.
As I traced the circuits I discovered several of the dreaded Sprague networks and several undocumented resistors and capacitors added to combat oscillation.
The more closely I attempted to restore the circuit to the original schematic, the more I enjoyed the amp’s tone. As originally wired, they are thin and piercing in tone. As designed (with one or two minor corrections) they are smooth and rage when dimed.
Thanks again for the site!
Dwight
Ahh fantastic. That makes my decision so much easier. I suspected that at complete rebuild would be my best bet on that one and now I’m gonna do it.
I won’t rebuild the Tremolo, but I might stick a tone circuit in there. I had just been wondering about a new speaker to combat that reediness.
As for the tremolo circuit, take a close look at the Gibson GA-5T schematic and compare it to The EA-50T tremolo. There are some significant changes there. I am not quite knowledgeable enough to understand why the changes are there but I find them intriguing nonetheless.
BTW, my best sounding amp of the bunch is an EA-50 with a Fender Blue frame Alnico speaker purchased from Angela. That thing howls!
Interesting. I’m not a fan of tremolo in general, so that’s why I was going to bag it. Since both versions (T and no T) are 5 tube deals, it won’t really matter much.
I’ve been dying to compare a bone stock Crestline (PP) to a bone stock SE Skylark. Looks like I’ll have to build them to do it though, since the market is really taking off on the Gibsons. I paid exactly 202.50 and 203.50 with shipping for the two crestlines I have and now they are never going under 300.
I just got a epiphone ea-50t off ebay. The amp is in really great shape, 4 tube with diode rectifier and a real hum. My thought is the filter caps but i’m looking at this blue cylinder attached to the chassis that has no means of support and reads 264 volts or so and i’m not real keen on reaching in and grabbing it to see how to remove it. i know that i can discharge it using a resister but haven’t done it yet. The amp looks like the skylark whitepanel and my thought was to change out the three big filter caps and see if that stops the hum. The other option is to replace the 6C4 and 6EU7 tubes and see what that does. As you can probably tell i’m electronically challenged so please forgive my lack of knowledge. Tube amps really fascinate me so I can’t help playing around with them.