Gibson GA-100 Information

[jon]

Thinking about building one. It has a few cool features that I would like to experiment with, tube compressor, anode drivers, Voltage regulators , and a simple preamp. Has anyone worked on one or does anyone have any gut shots. Any insight would be great.

[Cliff Schecht]

Ooohhhh man! I've been wanting one of these ever since I plugged an acoustic into one at my buds shop. These amps have some SERIOUS cool factor and are super sounding clean amps. Don't expect much distortion from it but it will take pedals well. I'd use it like I used to use my 59 BMRI, tube it up for cleans all the way up the volume knob and use effects to color the sound (although I'm not big on effects anymore). The whole amp just screams cool once you study it up close. The ancient looking stock speaker in it's roomy cab makes for a nice dimensional sound too. This thing was designed as a bass amp but I shit you not it makes a fantastic sounding acoustic amp which is why I'm so interested in cloning one.

I feel like we are letting a secret out here, they only made maybe 600 of those amps and I'm having the hardest time finding another one. This is the other reason I want to clone one, I even have a cool old unlabeled alnico speaker that I unfortunately tore the old paper to shreds playing my Express too loud through it a few years back.

If you are interested I'd be down to work on this project in parallel. I just finished up my 5F6A and since I have a somewhat predrilled chassis in mind for this amp I can easily bump it up to the top of my build list. FWIW it has a really goofy layout that I don't mind deviating from and probably will do so. I can also get some gutshots of my buddies original as well as some pics of the cool cab and whatever else you want pics of. Also the tubes for these guys (the not common ones at least) are super duper cheap, maybe a few bucks a pop if you are splurging.

[M Fowler]

http://www.el34world.com/Forum/index.php?topic=11906.0

Oops wrong model GS-100 still looking. But wait the photo show GA100 and the schematic listed is GA-100 so the gut shots are GA-100.

Mark

[martin manning]

Like this?

[bluefireamps]

I restored a GA-100 for a friend. Sorry, no pictures and he no longer owns it. I never liked the compression circuit with a guitar. You could leave it out. His had a reissue Jensen P12N in it. It was a great sounding amp. Good headroom and, yes, it did take pedals very well. I have over the years thought about building one. The simple preamp and pentode drivers are very unique. Keep us posted if you go ahead with it.
Dave

[jon]

How much HV current do you think one would need for this build? I know I need a minimum of 3.6A of 6.3 Heater current. Going by the numbers I think 200mA would work maybe 175mA

[bluefireamps]

I'd go with 250ma, 200ma minumum. I remember the PT being pretty decent sized. It was supposed to be a bass amp. That should keep it running cool and keep the B+ from sagging too much.
Dave

[jon]

Who's got a 700v CT transformer rated for 250mA? I've been looking and I can't find anything.

[Cliff Schecht]

I'm going to use an 800VCT transformer and knock down the voltage a bit with a amplified Zener. I still need to find an OT for this guy though, something beefy. I will probably get a Magnetic Components 50W or 60W OT with M6 laminate upgrades and multiple output taps.

[Phil_S]

Who's got a 700v CT transformer rated for 250mA? I've been looking and I can't find anything.

No 5V winding, but if you are willing to get off the beaten path a little bit, this one is a great bargain. http://www.antekinc.com/details.php?p=59
You wire the two 350v windings in series so the join becomes the CT. This will give you at 350-0-350 at a minimum of 250mA and plenty of filament supply. If you really want a 5V rectifier, you have plenty of money left over for a 5V transformer.

You can get 5V @ 2A from this one running the secondaries in parallel: http://www.antekinc.com/details.php?p=1

[jon]

I've got an OT (hammond 1650R 100W 5K) I might just use the 273BX 700 CT @ 201mA, or this Edcor http://www.edcorusa.com/products/589-xpwr002_120.aspx

[jon]

I got a PT (Thanks Chris G) 330-0-330 @ 200-300mA. it was scavenged from an old Stromberg Carlson AU-57 PA head. I am also going to use a Hammond 1650R. With this PT and OT I should be able to run 6L's or 6550's and choose between 50W or 70W.

The game is on.

[Cliff Schecht]

I can't figure out which chassis I'm going to use but I've got a few options that I'm staring at. Since the real heads have a lot of crap crammed into a small space, I'm going to do the same. One option is the chassis I used for both my first TW build and my first 5F6A build. Since both of those projects are now gutted out of the chassis, I can easily fit in a smaller PT (those Antek toroids are pretty expensive real estate wise), drill in a few more holes and PTP the whole build without too much fuss. I still want to see the insides of the original though so I can at least get an idea of what we're up against.

I found a suitable PT and OT (both Stancor of course ) but the only choke I have that can handle plate current filtering I think is too big (I measured close to 30H earlier). I'll probably find something cheap online or just use a resistor instead. Also this way I can up the stock value a bit, I'm thinking 5H minimum and at least 150mA current handling but it doesn't need to be ridiculously large (inductance-wise that is). While one might be tempted to shove more capacitance on the output stage to tighten things up, I think part of the charm of this amp is that the output stage does sag some when pushed.

Never thought about using other tube types for the output stage but the 6550's would be great with this amp, I love them in bass amps. The OT I found was a Stancor A3800 which does I think 85mA per side but is only rated at 30W. Don't think I'll be throwing any 6550's at this OT, even if it can handle the current it might not sound so great when it's pushing too much audio power and overheating! I still have those two 100W OT's that Millett gave me and I was going to do either a 100W 6550 Marshall or some sort of bass amp with these (or both, those hi-fi guys always order things in pairs hehe). So many options, so little time!

[Phil_S]

Here, this one might go for a bargain price. It was running 4x 7189, powering 11 tubes total including one 5AR4, and should be 360-0-360.
http://www.ebay.com/itm/Scott-222C-Powe ... 43a9eb304a

[jon]

Cramped....

I'm going to try and get mine into a 12x8x2. Looks like I might just make it.