Any ideas on how to make my GA6 brighter?

MrBoZiffer

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I love this amp, but it's pretty dark. Neck pickups get lost in it. Here's the schematic...

GA-6_2.jpg


I haven't worked on any amps in a while so I'm a little rusty. I guess the first place to start might be the tone control cap. It looks like a .01. If I switch it a .02, will that brighten it up? It looks like Gibson wired the tone cap a little different than Fender, but I could be wrong.

I suppose I could also use one of the three instrument inputs. Although I tried that once before and the variations weren't that great.

Cap, I know you got me on this one. :wink: 8)
 

capnjuan

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Hi Mr. Bo; okay ... I'll take Gibson amps for $300 please :wink:

Easy points first; the tone control is common to both channels; changing to the instrument jacks won't make a difference. I note on your schematic the 120K and 22K resistors on the instrument jacks in lieu of the 47K grid-stoppers on the schematic; somebody's effort to get a little tone distinction among the jacks?

The input of the microphone and instrument channels are wired like several other Gibson models including the GA20T ... capacitor input with 'grid-leak' bias ... connecting a resistor from the tube section's input across to the cathode/ground instead of 56K-68K grid stoppers, +/- 470K grid-to-ground, and a 1.2K-2.2K cathode bias resistor. The difference between the values of the input capacitors (.01 v. .05) is all you will hear by changing from the mic to the instrument channel.

Some charming chit-chat from the GA6 Owners Manual about the tone control and mic channel (from: HarpAmps)

GA6mic.jpg



Gibson's suggestion is rolling off all the treble when playing on the neck pickup and rolling off all the bass when on the bridge ... although nobody knows what in the heck Gibson was thinking when it references you and three buddies all jacked in and 'blended' ... :p

The tone cap: the principle is that small-value caps block low frequencies in the signal and large-value caps block high frequencies. The cap and pot are supposed to bleed off whatever it is you don't want so ... to diminish the bass by bleeding it off, your suggestion to go to .02 is headed in the right direction ... relatively speaking, the bass will more easily pass through the .02 and the treble, finding resistance at the cap, will turn around and head back towards the line.

In fairness to Gibson, they think you're Wally Cleaver and you and your friend Dobie Gillis are going to be entertaining at pool-side ice cream parties where clean and woody - but dark by modern standards - was the desired tone ... without any of those nasty, distorted 'wolf tones' (Ted McArty). Finally and maybe before you mess with the tone cap, the 12AY7 preamp tube has an amplification factor of 40; a 12AT7 an AF of 60 ... both have the same pinouts and are drop-in substitutes. You might try a 12AT7 there to produce a gainier tone ... won't change the amp's frequency response but might perk up those instrument jacks.
 

MrBoZiffer

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Thanks for the help Cap. :D

I've actually got a 12AX7 in there right now, but I'll experiment with the preamp tube a bit. That was me who changed out some of the resistors. I thought it would help with the brightness, but it didn't. Actually, the amp is perfect as it is for my Jazzmaster. The two seem to go together pretty well. And the distortion on it is great, so I kind of hate messing with it. But it doesn't like Franz's too much. I think the weaker JM pups are better suited for it. I just wish I could get a little more detail out of the clean sound with using neck pups. I'll play with the tubes then maybe try a new cap. That's an easy mod.

I don't think this thing will work for any pool parties! When the volume is below half my F20 might be louder. :lol: I've wondered in the past if the volume was a problem. It sounds really good when it's cranked, but it's no louder than a Champ. It's got the original 12" Jensen alnico. When I first got it had some problems. I replaced a bunch of resistors and caps and put in NOS tubes. I basically returned it to stock before I played around with those two input resistors. That fixed everything right up and it sounded a ton better. It's never been very loud, but I actually kind of like it this way. The distortion sounds more like a fuzz pedal.
 

capnjuan

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Our member Walter Broes always stresses speaker efficiency; he's after as much punch as he can get without resorting to a Marshall stack or equivalent. The vintage Jensens have 'that' sound and, other than the modern Webers, only the old Jensens have it ... but they have it at a substantial sacrifice in efficiency ... Walter's point being that he needs to jack his signal into his audience using the least gear tonnage possible. New speakers ... even Weber alnicos, are more efficient and, all things being equal, will make the amp seem louder than an older one ... especially one that was made when I was in second grade ... when this was new:

1_photo1.jpg


If it's volume or enhanced presence you want, after messing with your tone cap a little, I think a trip to the speaker store might be in order. I know about the distortion in capacitor-input circuits; sounds dumb when you say it but it's good if you want it ... as they say, one size does not fit all ... I mean it sounds great if you're after a certain something but gets in the way other times.
 

MrBoZiffer

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That's all good advice. However, I don't play outside the house much. On the rare occasion that I do, I have another amp for that. In fact, the low output makes the GA6 suitable for playing at night so I don't drive the neighbors (or the wife and dogs) nuts. However, you've given me an idea... I think I'll rework the speaker connection so I can plug it in to an ext speaker. The Jensen has clearly never been removed so the OT is soldered directly to the speaker lugs. This way I can change up the speaker every now and then just for some variety.

8)
 

frankieb

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The higher value a cap is the more highs it will cut, so a .01 will be brighter than a .02. First thing I would try is lowering that .05 cap before the 12AY7 to a .022 value, if thats not bright enough drop that cap down to a .01 value and see if that brings it to where you want it.
 

capnjuan

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frankieb said:
The higher value a cap is the more highs it will cut, so a .01 will be brighter than a .02. First thing I would try is lowering that .05 cap before the 12AY7 to a .022 value, if thats not bright enough drop that cap down to a .01 value and see if that brings it to where you want it.
Hi frankieb, welcome to LTG, and thank you ... I had it backwards ... a smaller value UF will cut more bass / produce more high frequency. There's are lots of ways to screw around with these but before I'd mess with the input cap, I'd take a shot at the tone cap ... going to .005 for openers ... all things being equal, less fooling around with the original circuit which is a step closer to turning it into something that it isn't. Thanks again for the catch.
 

frankieb

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You are right Cap, I overlooked the tone cap :shock: That would definately be the first thing to try, more likely to be able to get the amp to brighten up without it losing to much of its original mojo. Changing the tone cap from the .01 to .005 would make a good difference :D
 

capnjuan

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Hi John and Frankie; a little cleanup here and a proposed change of direction. The image on the upper left is a treble cut circuit and on the right, the tone circuit from the GA6 schematic; identical except for the value of the cap. I know the idea here is to get rid of some bass ... I'll get to that in a minute. When the wiper is in the green position, the pot's maximum resistance keeps the treble in the signal. When in the magenta position, the wiper has bypassed the resistance and acts as a straight path to ground; the hf sees that .0022uf cap as a short circuit, goes around the pot, and is grounded out ... no high frequency (as defined by the cap's value) left in the signal, nothing but bass.

tonecontrol.jpg


But I don't like the idea of changing the GA6 tone cap because it might cause a loss of volume and without another gain stage between the tone control and the phase inverter, I don't know that you'd get it back. For 'integrity' reasons and to a lesser extent electronic ones, I wouldn't mess with the input cap/s either. They define the character of the amp for one thing ... in my GA20T, I found cap input/grid leak a little squishy for my tastes and changed it. Also, I'm not sure what role - if any - that cap's value plays in setting the bias of the 12AY7 ... grid-leak bias ... eh ... there are reasons why other mfrs didn't screw with it.

Anyway, I suggest changing the value of the .05uf interstage coupling caps (lower left) to .01uf .. Mallory 150s/600V (metallized polyester ... later on, if you get the tone you want, you can think about sinking a few more bucks in Sozo or Jupiter oil in paper ... exotic stuff). The cap's function is to connect two stages and block DC but its value is frequency-selective ... a cap as small as .001 will block DC however small value caps block bass and a larger values block treble. By way of comparison, the Silvertone 1471s/1481s/1482s use .01uf and most later Gibson tweed-era models use .02uf. Between the amps' and the guitars' tone controls, these aren't really woofy or bass-heavy amps.
 

bluzman

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Before messing with the caps in the amp or guitar, try a 12AX7LPS tube.
I have found these to be bright preamp tubes. Also if you have an assortment of
12AX7 tubes experiment with long plate and short plate. I have found some of the
new JJ short plates (I think they're Russian or Eastern Bloc manufactured) are a bit
brighter too. Also the power tubes play a part and newer manufactured 6V6 tubes
to my ears tend to be brighter.

A good tube company like Dougs Tubes http://www.dougstubes.com/ or
Mike K's http://www.kcanostubes.com/
will help you on the phone tube and can suggest tubes for what you want.

Also older speakers can sound darker. So if your speaker is original you may want to
pack it away anyways and replace it with something brighter A Celestion Blue is known
as a chimey and brighter speaker (expensive) the the Vintage 30 can be bright too.

Good luck!

I just landed a cream puff 1957 Gibsonette a few weeks ago myself! ;)

Gibsonettet.jpg
 

capnjuan

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No FS; 'fraid not ... I was just using the car to make the point about 'vintage' anything ... even people ... I was in second grade when that car was made ... third grade when bluzman's very cool GA6 was made ... that the parts of amplifiers that work the hardest ... power supply and cathode bypass caps and speakers ... ok .. they make stuff 'authentic' and 'original' but it's hard to overstate how much improvement can be had by updating that stuff.

Bluzman: I had a late-'50s version of that amp (threaded here). Mine was bought as a project - considerably needier than yours. Electronically simple and with a warm, smokey tone. Yours is about as mint as they come!
 

bluzman

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A guy on another type of forum saw a thread about amps so he offered
it to me. I saw a picture and didn't haggle and it was fair all around.
Mine doesn't have the 12AX7 it has the 6SJ7 and a field coil speaker.
It's anemic sounding but it works. It looks like the big cap leaked so
I'll replace that and play with tubes. The Kalamazoo Schematics KEA
match this one. It keeps my 1957 GA 40 company. I'm going to send
the GA40 out for an electrical refurb.

Thank You!... and thanks for the info you have posted in many different
threads as I lurk here and find your information very much on target and
useful!
 

capnjuan

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Hi Bluzman: I was going to ask whether it was a 6SJ7 in the preamp or a 12AX7. A fresh power supply cap ought to straighten it out nicely. Glad to be of help ... I'm a big fan of the tweed-era Gibson amps.
 
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