$299 Gibson tube amp - Made in USA!

[androo]

Got it. Thank you so much for reminding me, and everyone. I don't know much about the innards at all, but I do know they can be very, very dangerous.

I will do as you said Rob, a quick and dirty drain. Is there any other things I should do to stay safe?

#1 quick and dirty drain (don't throw the power switch to turn it off, but pull the cord out of the wall while playing and keep playing, yes?)
#2 be careful not to touch much when I take out the chassis
#3 don't touch the blue cap or anything that looks like it's connected to it.
#4 unsolder that lead from the post, bend it out of the way a bit.

Anything else?

droo

[Guitarslinger1]

Originally Posted by Sengal Coyle

I don't want to derail a good tech discussion...but I got pulled into buying one of the GA-5s from MF through this thread....what an awesome little amp. I absolutely love it. Knowing that I'll never get huge bass response from a 5w amp in a tiny cab, is there anything I can do to maximize/tighten the bass a bit?

Go back in this thread to page 6, or further back, and read forward.

[Rob DiStefano]

Originally Posted by Sengal Coyle

I don't want to derail a good tech discussion...but I got pulled into buying one of the GA-5s from MF through this thread....what an awesome little amp. I absolutely love it. Knowing that I'll never get huge bass response from a 5w amp in a tiny cab, is there anything I can do to maximize/tighten the bass a bit?

Perform all three mods that specialty guitars has outlined - these tweaks will really open up the GA-5.

For maximum bass response, install a beefier ceramic speaker such as a Weber 8F150, or AlNiCo such as the Weber 8A150.

[Rob DiStefano]

Originally Posted by androo

Got it. Thank you so much for reminding me, and everyone. I don't know much about the innards at all, but I do know they can be very, very dangerous.

I will do as you said Rob, a quick and dirty drain. Is there any other things I should do to stay safe?

#1 quick and dirty drain (don't throw the power switch to turn it off, but pull the cord out of the wall while playing and keep playing, yes?)

YES.

#2 be careful not to touch much when I take out the chassis
#3 don't touch the blue cap or anything that looks like it's connected to it.
#4 unsolder that lead from the post, bend it out of the way a bit.

I didn't even bother unsoldering the cap - just clipped one side off ... the green arrows show the red jumper wire added and the clipped cap lead ...



Anything else?

ADD THE 47K NFB RESISTOR - YOU'LL LOVE IT!



droo

....

[big jimmy]

where can i get that last resistor Radio Shack?

[Guitarslinger1]

Originally Posted by big jimmy

where can i get that last resistor Radio Shack?

Sure. A 47k 1/2 watt of any type will be fine.

[Dad Gad]

i tried putting a new tung sol ri 12ax7 and it wont fit! thats my most favorite pre tube and it wont fit! i tried two of em. anyone have luck with that tube?

[Rob DiStefano]

Originally Posted by big jimmy

where can i get that last resistor Radio Shack?

Yep, 99 cents for five of 'em (47K).

[Rob DiStefano]

Another "mod" I just made was to add a tube shield for the 12AX7 - believe this does help with noise, dunno why Gibson didn't supply one in the first place, prolly cost them 10 cents.

[Rob DiStefano]

Recap of my GA-5 mods to date ...

CIRCUIT (as of 3/7)
* 1m resistor and 22pf cap removed and jumpered
* 1st stage 25uf cap lifted
* 47k NFB added
* 1/4" external 8 ohm speaker jack added
* add tube shield to the 12AX7 preamp tube
* add HD alum foil to inside cab top and sides, down into the cab's chassis slots
* replace the 2-way power switch with a Carling 3-way for off/standby/on - this will help with tube life since the rectifier is solid state - this mod is in progress ...

SPEAKER
* Weber 8F150 - awaiting arrival next week (I hope)

CAB
* remove the power cord box
* remove the rear grille
* rebaffle with 1/2" birch ply and cane grille - awaiting the speaker

[Guitarslinger1]

Originally Posted by Rob DiStefano

Any more thoughts on the ground buss?

The 47K NFB resistor did more to lower the 60hz hum, than any hum reducing trick I could remember. So if you like that mod, it will help the hum that these types of amps are prone to.

I tried all sorts of things to reduce noise and hum, with limited success. I isolated the preamp grounds from the power amp grounds. Didn't seem to make a difference. I moved the 100 ohm resistors that make and artificial ground tap for the heater wires, from the preamp area of the board, to the pilot light, tried grounding it at the power amp ground buss, and to the top of cathode bypass resistor for the power tube (this is an old trick that works sometimes). Not sure if any of this went any more forwards than backwards.

The thing that made the most difference in noise was shielding the top and edges of the cabinet with heavy duty aluminum foil. Amps spray a lot of noise, that tends to wreak havoc with single coils. Shielding the cab completely seals the electronics electrically, which reduces that crap quit a bit.

I'll post some pics later.

[Rob DiStefano]

Originally Posted by specialty guitars

The 47K NFB resistor did more to lower the 60hz hum, than any hum reducing trick I could remember. So if you like that mod, it will help the hum that these types of amps are prone to.

Yep, the noise suppression was immediately noticed when the NFB was added.

I tried all sorts of thins to reduce noise and hum, with limited success. I isolated the preamp grounds from the power amp grounds. Didn't seem to make a difference. I moved the 100 ohm resistors that make and artificial ground tap for the heater wires, from the preamp area of the board, to the pilot light, tried grounding it at the power amp ground buss, and to the top of cathode bypass resistor for the power tube (this is an old trick that works sometimes). Not sure if any of this went any more forwards than backwards.

The thing that made the most difference in noise was shielding the top and edges of the cabinet with heavy duty aluminum foil. Amps spray a lot of noise, that tends to wreak havoc with single coils. Shielding the cab completely seals the electronics electrically, which reduces that crap quit a bit.

I do exactly that to all my amps and was thinking about that yesterday, then forgot about it. I use Reynolds heavy duty aluminum foil, spray on some 3M spray adhesive cement, fit to the cab top - in the case of the GA-5, the foil should run down the sides and into the chassis slot to complete the ground. Alum foil makes for a good noise shield.

I'll post some pics later.

....

[Guitarslinger1]

I'm afraid it's not pretty. Did this LATE last night. I opted to leave the tracks clear, and bring some foil around the edges where the chassis screws on to ground the shielding.

[Guitarslinger1]

Originally Posted by Rob DiStefano

Recap of my GA-5 mods to date ...

CIRCUIT (as of 3/7)
* 1m resistor and 22pf cap removed and jumpered
* 1st stage 25uf cap lifted
* 47k NFB added
* 1/4" external 8 ohm speaker jack added
* add tube shield to the 12AX7 preamp tube
* add HD alum foil to inside cab top and sides, down into the cab's chassis slots
* replace the 2-way power switch with a Carling 3-way for off/standby/on - this will help with tube life since the rectifier is solid state - this mod is in progress ...

SPEAKER
* Weber 8F150 - awaiting arrival next week (I hope)

CAB
* remove the power cord box
* remove the rear grille
* rebaffle with 1/2" birch ply and cane grille - awaiting the speaker

Rob, what did you think of the 22pF across the volume control?

[Rob DiStefano]

Originally Posted by specialty guitars

Rob, what did you think of the 22pF across the volume control?

First, good job on the foil shield - it can be a pain to do.

I didn't particularly like the 22pf bleeder cap (across the input and wiper lugs) - I like the smoother, darker response of the amp's tone as its vol pot was cranked. IOW, I feel the tone gets fatter and meatier as the amp vol is increased. Now I get a great, greasy blues tone at 2pm on dial (using a Strat loaded with Keystones or a Tele with Rio Tallboys) ... to my ears, this tone is just perfect. At slightly lower volumes 11 to noon, it cleans up enough for crisp fingerstyle ragtime or blues pickin' Wouldn't have happened without yer 3 killer mods - thanx again!

I think I'll work on some A/B mp3 demos 'tween the two amps before #1 gets the speaker mod.

[denfelo]

This is my first post on this forum even though I have lured for years. This amp loves P-90's, Open G, and slide. I am liking it a lot so far. I am going to give it a few weeks until the honeymoon is over before I review it. It has been a while since I have used the guitar controls this much, but that is one of the keys to this amps tone.

[Rob DiStefano]

Originally Posted by denfelo

This is my first post on this forum even though I have lured for years. This amp loves P-90's, Open G, and slide. I am liking it a lot so far. I am going to give it a few weeks until the honeymoon is over before I review it. It has been a while since I have used the guitar controls this much, but that is one of the keys to this amps tone.

Yes, I totally agree - in stock format the the GA-5 begs to be controlled by yer guitar's vol and tone pots. However, after the 3 main circuit mods the amp behaves a lot tamer and works even better to allow yer guitar's tone to shine through. IMO.

[Guitarslinger1]

Originally Posted by Rob DiStefano

First, good job on the foil shield - it can be a pain to do.

I didn't particularly like the 22pf bleeder cap (across the input and wiper lugs) - I like the smoother, darker response of the amp's tone as its vol pot was cranked. IOW, I feel the tone gets fatter and meatier as the amp vol is increased. Now I get a great, greasy blues tone at 2pm on dial (using a Strat loaded with Keystones or a Tele with Rio Tallboys) ... to my ears, this tone is just perfect. At slightly lower volumes 11 to noon, it cleans up enough for crisp fingerstyle ragtime or blues pickin' Wouldn't have happened without yer 3 killer mods - thanx again!

I think I'll work on some A/B mp3 demos 'tween the two amps before #1 gets the speaker mod.

Thanks.

Right on.

Yeah, I'm 50/50 on the treble bleed cap. My amp is dark when down low, which is what that mod helps. As you turn up the volume knob, the cap has less and less effect. The amp brightens naturally from the resistance being decreased (same as on a guitar), at the same time, the cap is having less and less effect on the circuit because the resistance on its side of the pot is increasing as the volume is turned up. In theory, with the right pot value, cap value etc. the tone should stay even. But as with guitar volume controls, there are all sorts of treble bleed arrangements, and to my ears, it never quite works perfectly with any of them.

Those A/B demos would be a kick!

[dansamp]

I have been following this thread with some interest
I missed out on getting one of these from MF
wondering if anyone has tried lowering the cathode resistor on v1
down to 1k5 or 820 ohms
I would think that would give you a little more voltage and make the distortion not so fizzy
I believe the stock resistor is 2k2 or 2k7 ??

Dan..........

[Guitarslinger1]

Originally Posted by Gee

These are highly tunable little amps. You could use the mods that are well documented in the FAQ section of the VJ part of SEWATT.com. There are some very simple mods, i.e. changing one or two resistor values and cap values to fine tune the amp into a Marshall, Vox, Fender or many more exotic sounding amps. See attached file for some basic mods.

Dan,

Gee posted a link that describes that and more that can be done to "tune" the circuit.

But losing the cathode bypass cap on the first 2.2K does wonders to clean the amp up. But, nothing wrong with going to 1.5K or any of the other possible mods.

[red57strat]

I just removed the metal grills and cord box, jumpered the 22pF cap/1m resistor, disconnected the 25uF bias bypass cap and added a 47k NFB on my GA5.

The amp sounds awesome! I was hesitant to modify the amp because I wasn't sure that I was going to keep it. I liked it, but wasn't completely impressed with the tone, My 5E3 clone was killing it and wasn't so much louder that the GA5 would be useful to me.

For me, removing the rear metal grill and cord box was mandatory. They were easy to remove and it makes the amp easier to move, store, swap tubes on and connect speakers to.

Removing the front grill was a pain in the butt! Don't bother unless you have a condition where you can't leave well enough alone!!! It came out fine and I really do believe that it improved the tone, but not nearly as much as the electronic mods.

I'm very impressed with the electronic mods. I did them one at a time starting with jumpering the 22pF cap/1m resistor, disconnecting the 23uF byass bypass cap and adding the NFB.

I like the first two mods more than the NFB. I'm not sure of that one yet.

The amp is vastly better with a Telecaster now. In fact, it's a match made in heaven.
It's a mix of Champ tones and Vox chime. I still prefer the amp for clean to semi dirty tones. My 5E3 still kills it on songs like Funk #49 though!
The amp is chimey and rich where it was boxy before. It's much more responsive to the guitar's volume control. As much as my 5E3 clone.

The NFB seems to make the amp a tad too middy with my Les Paul and PRS Soapy.

Cranked way up the amp is still too out of control for my liking. That might be the EL-84. That's fine the clean and semi dirty tones more than make up for it. Maybe it could be even better tuned with a few more mods, a 820 to 1K5 cathode bias resistor on V1 as dansamp suggested, for instance.

I'm impressed!

[boris bubbanov]

Now the sons of guns at MF offer me a 10 percent off coupon, now that the blessed things are gone.
Ah, it ain't no fun unless it is a game; I couldn't just pay the three bills and be done with it.

[Rob DiStefano]

I'm thinking of tweaking/testing the NFB further - a 100k or 200k trimpot on one side of a DPDT switch. The trimpot would allow finding the NFB resistance sweet spot and the switch would allow removing the NFB completely.