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Dirty-South Blues Harp forum: wail on! > Gibson GA-6 amp
Gibson GA-6 amp
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276 posts
May 23, 2021
9:49 PM
The guitar player in my band let me use his '57 Gibson GA-6 amp at rehearsal today. I love the sound. Its got a lot of low end, and breaks up early and smoothly. Not the ratty and raspy distortion that a lot of amps have. But a smooth compressed over-drive that sounds great.

The problem is feedback. I just can't get it under control using all the usual techniques. My other amps are well behaved with simple tube swaps (usually 12AT7 in place of 12AX7). I also use a Lone Wolf Harp Shield which really helps in a loud situation to get more volume out of the amp before feedback.

I can get the amp loud and playable by watching where I stand, keeping the tone all the way down, turning up the mic volume on the amp a little (the mic volume affects the instrument volume even if nothing is plugged into mic input), and using the Harp Shield set to about 80%. But its too close to the edge of feedback and sometimes gets away from me. I can usually get my amps setup much better than this.

There are various versions of the GA-6. This one is claimed to be a '57 and it has two pre-amp tubes, one 12AY7 and one 6SL7. I tried a 12AU7 in place of the 12AY7 and it was no good - low volume, bland tone, weak.

So now I'm looking at the 6SL7. I'm not familiar with these tubes, having never owned an amp that used one. Is there a lower gain version of the 6SL7 that is a drop-in replacement?

I'm looking for someone with experience with this or similar amps, or some knowledge about 6SL7 tubes. Thanks!
79 posts
May 23, 2021
11:16 PM
All I can really ad is these are great amps! Greg Izor in Austin (if you know of him) had 2-3 of these and I played one several times sitting in with his band (with Mike Keller guitar you may have seen with Kim Wilson ((F Thunderbirds gigs)) and Mark Hummel O'Dell's videos) sounded great but, they were all using smaller amps. Dennis Gruenling I believe has a couple too but, could likely only get info if you know him. I wonder if maybe you are trying to get more volume that it can do? Seems to be a few real amp experts here so maybe someone will add advice which I cannot help with. Great amp though!
Dougie Blue
14 posts
May 24, 2021
5:53 AM
As I understand it, swapping tubes will not give you any additional volume before feedback - it will only allow you to turn the volume up further before hitting the volume level where feedback begins. Now, if you want to be able to just turn the amp up louder and have a volume control that is not so quick from off to feedback, then here's a trick I've just discovered - check out the Mosky "VOL" pedal. It's basically a volume knob "pad" that can go from fully off to full on - does not boost volume, only attenuates it. It does not color the tone - it's an active pedal, so needs 9V (I've done basic tests on that - will do more tomorrow). It only costs $28 on Amazon. Just pop it into your signal chain, turn it on about 1/4, and see how much you can get the amp volume knob up.

Again, this won't give you more volume, just let you turn volume of the amp more by reducing the gain hitting the first preamp stage, and thus pushing the later stage(s) harder.

If you need more volume, I think you will need to mic it and run to a PA or powered speaker. My understanding is that, while many love the tone of these vintage amps, they don't have the volume of later models that have the same power rating.

Others with more experience may have better advice. Good luck!
391 posts
May 24, 2021
7:20 AM
My records show that the 6SN7 is lower gain than the 6SL7.

Last Edited by TetonJohn on May 24, 2021 7:20 AM
278 posts
May 24, 2021
7:57 AM
Thanks everyone.

@ Gabriel
It does sound good. I agree! I'm not trying to get more volume than possible with this amp. Just trying to tame it to get more headroom and volume before feedback. I actually did talk to Dennis G about the GA-6 before I tried it. He confirmed they're great for harp, but said he sold his and didn't currently have one. I didn't ask him anything about the tube configuration.

Thanks for the info. I know how pads work, and they do work well, but I'm not interested in going that route at this point. I prefer to reduce the gain through tube swaps rather than with an external pad. And I understand that reducing the gain through a pad or tube swap won't give you more volume. But it will give you more "usable volume" before feedback. That's what I'm trying to accomplish.

Thanks, this is exactly what I was looking for! I did some searching and was able to find the following:

gain = 70
heater current = 300mA

gain = 20
heater current = 600mA

My concern with swapping them is the heater current. I have no way to know if the 6VAC winding on the Gibson transformer can handle the extra current. I'd hate to blow it. Has anyone done this tube swap 6SL7 --> 6SN7 before? In this amp or any other?

Jim McBride
Bottle 'O Blues microphones
184 posts
May 24, 2021
11:50 PM
I looked at the schematic for that amp. From what i see, its already had tube swaps. I'd put two quality 12ax7's in v1 and v2, then solder in a 1 meg resistor inline to the middle leg of the instrument volume pot, then only use the instrument 1 input. I'd also change the resistor to ground from v1 from 820 ohms to 2.2k ohms. That should dramatically change the gain of the amp, but still have good tone and punch.
121 posts
May 25, 2021
11:46 AM
It may be a silly advice, but did you tried different inputs? As you noted, the mic and inst volume interact with each other input.
You could try AT7 tubes too.
Here's a video of some work on a GA-6 (ttps://youtu.be/jxo5K8nDANg)
392 posts
May 25, 2021
11:48 AM
I've seen easy plug-in tube socket adapters that will let you use 12ax7 type tubes in a 6SL7 socket -- might be fun to experiment with -- but I'm not techy enough to tell you if there would be any possibility of damage.
279 posts
May 25, 2021
7:23 PM
No, it has the original tubes. Gibson made various versions of the GA-6 with different tubes. Mos tof the schematics you find on-line are for the version with 2 x 12AX7's. But mine has a 12AY7 and 6SL7. If you do a little digging you can find the older schematics showing this config.

The three instrument inputs are all the same. The mic input is significantly lower output, and doesn't sound good. But you can plug into an instrument input, then turn up the mic input to reduce the signal on the instrument side in order to get a little more volume out of the amp before feedback.

There would be no reason to use a 12AT7 tube since it already has a lower gain 12AY7.

I talked to someone that knows vintage amps, and has owned the GA-6 and knows it well. His advice was that they have great tone and any tube swap will kill the tone on this amp. So I tried a lower output mic with it and it still sounded good and I had a lot more room on the volume knob to work with. Not a lot more overall volume, but more adjustment to work with on the volume to find a spot with good tone, volume, and not on the edge of feedback. I don't particularly want to use that particular mic,so I plan to go the "pad" route and use my regular mic. I ordered a Lone Wolf Mojo Pad. This should let me use my regular mic, no tube swaps on the amp, and find a sweet spot to play without having to continually fight feedback. The pad was @DougieBlues original suggestion when I first posted about the GA-6. He mentioned an active one, I prefer passive - that's why I ordered the LW Mojo Pad. Once I get it and test it out I'll post my results. Thanks for all the advice.
122 posts
May 26, 2021
7:09 AM
Long time ago when I tried different preamp tubes, I liked 12AT7 over 12AY7 because to my ears the 12AY7 sounded more treble. Since you got Mojo Pad, you may had soolved you problem. Let us know your impression when it arrives to you.
393 posts
May 26, 2021
7:24 AM
That's one hip lone wolf -- to have a pad instead of a den. (couldn't resist)

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