Gibson GA-100 Information

[Structo]

So Jon, are you going to use this for guitar or bass?

What attracted you to this circuit?
Have you played through one before?

[jon]

BASSSSSS!!!!


I could understand why people would think that the amp would be uninspiring for guitar, with only one gain in all. I also bet that the reason that people feel that the compressor is not effective is that with guitar the input signal is not enough to really utilize the feature. With a bass I think it will work nicely.

[Cliff Schecht]

Something I noticed tonight while staring at the schematic was the lack of a NFB loop around the output stage. I think this is part of the reason why this amp sounds so good with an acoustic guitar. The little acoustic Champ-style amp I built for a friend had a switchable NFB loop and I found that with the NFB disabled, acoustics sounded great and with it on, the high end was neutered. There is a sparkle that is lost when you connect the NFB loop and reduce the gain at high frequencies. The negative feedback linearizes the frequency response and reduces gain overall, something that is not necessarily desirable for acoustics. I'm betting the GA-100 sounds so good with acoustics for the same reason.

The amp is fun with bass but sorta wimpy through the single 1x12 cab. The speaker that comes stock seems much more suited for guitar. I bet this amp would pack a good wallop through a more modern bass cab. The compressor is a neat feature but IMO is sort of a one trick pony because of the lack any available knobage to tweak with. Maybe I can figure out a way to add a threshold and attack/release to get a bit more functionality out of it. Is a ratio control going overboard?

Sorry no gutshots of the real one yet, hopefully I remember to do this tomorrow before I head back up to school..

[jon]

The amp is fun with bass but sorta wimpy through the single 1x12 cab. The speaker that comes stock seems much more suited for guitar. I bet this amp would pack a good wallop through a more modern bass cab. The compressor is a neat feature but IMO is sort of a one trick pony because of the lack any available knobage to tweak with. Maybe I can figure out a way to add a threshold and attack/release to get a bit more functionality out of it. Is a ratio control going overboard?

I will be using an EV TL606 cabinet with it. If you could figure out how to add some adjustments to the compressor that would be great.

[Cliff Schecht]

Got pics taken, just waiting for them to get emailed and I'll post them. When the bottom chassis cover was pulled off I was pleasantly surprised by two eyelet boards and a well organized building scheme. Was not the rats nest I was expecting..

[jon]

Any ideas on how to add controls to the compressor?

[Cliff Schecht]

I'm pretty sure adding a knee knob is as easy as putting some resistance in series with the diodes. This reduces the current through them as the resistance increases and makes for a softer knee. Attack and release would I think be adding in some series resistance where the 6FM8 plates feed into the diodes (increasing RC time constant). This would limit how fast the 0.1uF cap gets charged and determine the attack/release times (although not independently). Not sure about threshold yet. This is all speculative still, I need to build the circuit in simulation and see what I can achieve without adding too much complexity.

What is your timeline looking like on this build? Are you trying to get it done as quick as possible or are you in no rush? Mines going to take a bit as I tend to spend a lot of time planning out the details so I don't hit any surprises while building. Plus figuring out the circuit mods we want I think will take some time. I'd love for others to chime in with their thoughts on the compressor circuit and how to modify it too. There are guys here who are probably much better versed than I in what would be useful with the compressor. I understand the design and how compressors work but I don't do much recording or ever really use compression ever so I'm not sure what additional features are actually merited. For the modifications I think it's best to focus on making the best bass compressor possible so again, I'd love for others to chime in to help me/us understand what makes a good bass compressor.

[jon]

I am beginning to layout my chassis and finalize my iron. My tubes are on the way.

If you could get me those gut shots it will help with my chassis planning.

[Cliff Schecht]

Ask and ye shall receive .

[Cliff Schecht]

cont..

[Cliff Schecht]

It hit me last night that I can finally use the chassis I've been trying to repurpose as a 5E3 but never got around to (lack of iron and caring). It originally had some weird bank of 5 counting switches with inductors all over it and BNC's for input/output. Really not sure what it was past some sort of tuning network. It's the perfect size for this Gibson. I'm eyeballing it at 17x4x3 which gives me more room to play with inside than an original.

The one thing I'm doing differently is I'm mounting my pots and jacks inside the chassis instead of floating over on the control panel. My chassis is deep enough to allow for this and IMO it's not very safe having the 120V lines being so easy to touch.

[jon]

I'm gonna start punching eyelets on Wednesday.

[jon]

Here are my GA-100 Boards. I'm gonna replace the orange drops with CD150's and I am going to change the power supply resistors. I did goof a little and I added too many eyelets on each board. I'll fix that tonight.

[IMG:600:800]http://i82.photobucket.com/albums/j250/ ... G_1010.jpg[/img]

[jon]

Interestingly, I was reading the Gibson catalog from 1962, it claims that the GA-100 would have come with a "Special Imported" 12" speaker in a bass reflex enclosure.

What do you think it was a Goodman's, or a Fane, maybe something else.

[bluefireamps]

It was a Norelco. The one I worked on had the original speaker. Unfortunately it was damaged so I never got to hear it. The P12N I installed sounded great.
Dave