Gibson GA9 amp

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Gibson GA9 amp

Postby Jaybird803 » Sat Dec 02, 2006 7:42 am

I own a Gibson GA9 amp that i use for harp. It is great except that the Jensen CQ10 I have in it seems way too bright. When I use a GE-7 Equalizer, it is the best sound I could want. Is there a speaker that might change the tone without having to use th GE-7/
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Postby jeffl » Sat Dec 02, 2006 1:28 pm

I've got the same issue with my little Champ,and I've just resigned myself to either puttin' up with the tone (there's worse things in life),or pre-ampin' it with the Harp Commander. Your post is stirring a flicker of a memory about a review I've seen online where a guy does exactly what you're talkin' about: he reviews the same amp with about 4-5 different speakers in it... Have you searched the Harmony Central reviews to see if anybody else has screwed around with one of those Gibson's speaker setups. BTW, is the GA-9 a Skylark...?
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Postby songdog » Sat Dec 02, 2006 3:39 pm

Hey Jaybird, those Gibsons are cool looking amps!

I have a vintage tweed Fender Champ that I use for harp. Since the value of these old tweeds have been steadily creeping upwards I decided it might be a good idea to replace the speaker for a new one thereby preserving the original should I ever decide to sell the amp. On Brian Ps suggestion I replaced it with a new model from Weber. This new speaker did indeed smooth out just a little bit of the brightness and improved the bass too.

However I've been so busy lately that I haven't had the time to play much through the amp so I'm guessing the speaker hasn't fully broken in yet. Usually the low frequencies of a speaker will get better as the cone loosens up. But so far it's been just a little bit of improvement over the original speaker.

If you are just tweaking your EQ a few dB here and a few dB there then a replacement speaker might just be the ticket. However if you are EQ'ing some frequencies 4-6dB or more, to get the sound you like, then I would think it's not likely that a speaker swap will result in those kind of differences.
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Gibson amp

Postby Jaybird803 » Sat Dec 02, 2006 5:04 pm

Thanks for the good advice. Seeing as how I am looking for more than a few db's in change, I probably am looking at some sort of mod to make that . The amp has no tone control so maybe i should look into adding something in the amp itself. The amp is not a Skylark, but a lap guitar amp I am told. It has no name other than the model #
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Postby Jaybird803 » Sat Dec 02, 2006 6:02 pm

I am Posting the schematic, in case anyone has some ideas as to mods or tone changes.Image
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Postby t bone bruce » Sat Dec 02, 2006 10:51 pm

Sweet, 2x6V6 in parallel single ended operation..pushing about 9-10 watts I'd guess. There are very few amps using that design -Gibson GA8 Gibsonette, GA9, and the THD bivalve are the only ones I can think of. I see it has a field coil speaker too, might make speaker replacement tricky. You could increase the value of C3 to 0.1uf, which will increase the bass response, but I think in terms of collectable value you'd be better off leaving it stock. If you get the sound you want with the EQ pedal why risk damaging a beautiful vintage amp?
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Postby ricochet » Sat Dec 02, 2006 11:33 pm

Yeah, that's a lap steel amp. It's supposed to be bright. Bleeding off a bit of treble with a small capacitor on the order of 100 pF between the grid of the 6SJ7 and ground, or from the "top" of the pot R1 to ground (or both) would reduce its brilliance. I agree that keeping the amp stock and using the EQ pedal to get the sound you want is preferable to modding the amp. If it's got a Jensen CQ10 in it, the field coil speaker is already history. So it's not original, anyway.

In replacing a field coil speaker with a permananent magnet one, the field coil really should be replaced. One option (which Hammond recommended in replacing the field coil speaker on the M3 organs) is to use a power resistor of similar resistance to the field coil. In this case that'd be a 1K ohm 10W resistor. Another option would be to substitute a choke. This amp used the field coil as a choke input to the power supply. By doing this, you'll lose some voltage on the power supply and a bit of power output compared to running the rectifier output straight into the first filter capacitor, but you'll have much tighter bass response. If you put the resistor between the rectifier and first filter cap, you'll have lots of voltage sag when the amp's pushed hard (out of its normal Class A operation), and the bass output will suffer. I'd vote for a choke.
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Postby Jaybird803 » Sun Dec 03, 2006 4:22 am

Thanks for the good tips both T-bone and Ricochet. I think that I will give them a shot and see what happens. The amp has been worked on by previous owners and is no longer stock. The field coil was totally trashed when I got it. I have put in a 15 henry choke rather than the 10 watt resistor route. The amp runs very silent. There was a new Kendrick transformer in the amp when I got it, it was put in rather sloppy and it took a while to properly route the wires. I don't think that adding a couple of parts is going to damage the value of the amp. If it doesn't play good what is it worth anyway. I don't have my amps as museum pieces, but at the same time I do appreciate the concept of trying not to hack something up into an unrecognizable piece of junk, and keep them clean and sounding as good as possible. These changee you both have recommende are reversible if desired in the future.
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Postby ricochet » Sun Dec 03, 2006 5:59 pm

Sounds good, Jaybird. I think you did right with the choke.
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Postby Jaybird803 » Sun Dec 24, 2006 10:38 pm

It has taken a number of trys and what seemed like an eternity, but even a blind man can eventualy thread a needle. I ended up removing one of the input jacks to make room for a tone pot. I installed a tone stack like the one in a Princeton Brown 6G2 circuit, and it works like a champ(mixing metaphoric amp terms ha ha) and sounds fantastic. Thanks for the encoragement for me in this project.
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Postby ricochet » Mon Dec 25, 2006 12:10 am

Glad you're happy with the results, Jaybird!
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