$299 Gibson tube amp - Made in USA! - Page 7

Guitarslinger1 · March 5th, 2008, 11:27 PM

Here's a pic of the 25uF bypass cap "lifted" from the first gain stage, to reduce gain.

IMO, this mod in conjunction with the bypass/removal of the 22pF/1 meg resistor mod sounds fantastic. The amp is still bright, gainy, and lively, but fuller and less "crispy" sounding.

If you decide you like the lower gain tone with the 25uF bypass cap out of circuit, it is best to go ahead and remove it completely. It is also best to go ahead and remove the 22pF cap/1 meg if you decide to keep your amp that way.

Rob DiStefano · March 6th, 2008, 05:51 AM

That's what I'm talkin' about - sounds like just the circuit mods I'm looking for - thanx!

What expected difference by adding the 47k negative feedback resistor instead of lifting the bypass cap - or what results by both adding the NFB resistor and lifting the bypass cap?

Looks like the NFB resistor would go from the hot side of the OT speaker lead to pin 3 of the preamp tube?

Gee · March 6th, 2008, 06:32 AM

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.

Sidney Vicious · March 6th, 2008, 08:06 AM

Anybody know if the earlier modern version - the one without the cord hole and the "safety grate" over the speaker hole - likewise is lacking that overly heavy duty speaker grate behind the grill cloth?

Rob DiStefano · March 6th, 2008, 08:14 AM

Just made the mods as suggested by specialty guitar ...

... yeah man! MUCH better tone - thicker, more presence, breaks up a tad later, still Loud but less jangly. I'm gonna make the mods permanent.

red57strat · March 6th, 2008, 08:42 AM

Quote:

Originally Posted by Rob DiStefano

Just made the mods as suggested by specialty guitar ...

... yeah man! MUCH better tone - thicker, more presence, breaks up a tad later, still Loud but less jangly. I'm gonna make the mods permanent.

Nice.

I figured the 1m resistor in parallel with the 22pF cap had to go when I first saw the schematic for these amps, before I even ordered mine, but I didn't know about the bias bypass cap.
The original schematic for the 5F1 Champ didn't have this cap though the 5E1 Champ did. They both had negative feedback loops.

The schematic for the original GA-5 calls for a 47k NFB resistor between pin 8 (the second cathode) of the 12AX7 and the speaker.

Rob DiStefano · March 6th, 2008, 08:54 AM

NFB to pin 3 of the 12AX7 ?

stephent2 · March 6th, 2008, 08:55 AM

"I'm gonna make the mods permanent."

is there any compelling reason to do so? why not leave the mods (as is in your picture) reversible?

Rob DiStefano · March 6th, 2008, 09:08 AM

Quote:

Originally Posted by stephent2

"I'm gonna make the mods permanent."

is there any compelling reason to do so? why not leave the mods (as is in your picture) reversible?

The difference to my ears was quite apparent and I see no reason to go back to the original circuit. specialty guitars did advise to make the mods permanent if they were liked - dunno why ....

I'm gonna add the NFB (once I know for sure exactly where the 47k resistor goes) and see how that sounds. If need be, the NFB can easily be wired in with a mini switch on the chassis bottom to allow the use of both modes.

red57strat · March 6th, 2008, 09:21 AM

Quote:

Originally Posted by Rob DiStefano

NFB to pin 3 of the 12AX7 ?

That schematic is correct in layout, though it's numbered differently than I'm used to seeing.

You want to put a resistor from the cathode of the driver side of the 12AX7 tube to the speaker for a negative feedback loop, as that schematic shows, just don't count on the numbering scheme to match the socket in your amp.

BTW, I'm not an expert, maybe specialty guitars will chime in and clarify.

red57strat · March 6th, 2008, 09:22 AM

Quote:

Originally Posted by Rob DiStefano

The difference to my ears was quite apparent and I see no reason to go back to the original circuit. specialty guitars did advise to make the mods permanent if they were liked - dunno why ....

I'm gonna add the NFB (once I know for sure exactly where the 47k resistor goes) and see how that sounds. If need be, the NFB can easily be wired in with a mini switch on the chassis bottom to allow the use of both modes.

Maybe use a 50k pot to dial it in?

Guitarslinger1 · March 6th, 2008, 10:08 AM

Quote:

Originally Posted by Rob DiStefano

The difference to my ears was quite apparent and I see no reason to go back to the original circuit. specialty guitars did advise to make the mods permanent if they were liked - dunno why ....

The only reason why is that having half connect components and superfluous parts running about can make the circuit unstable and cause parasitic oscillations and noise. It's not imperative that the components be removed, just recommended. And even if removed, those parts or new parts, can be reinstalled to reverse the mods.

You want to go to pin 8 on the 12AX7 with the 47k from the speaker output.

I know my picture is lame.

Guitarslinger1 · March 6th, 2008, 10:11 AM

Quote:

Originally Posted by red57strat

That schematic is correct in layout, though it's numbered differently than I'm used to seeing.

You want to put a resistor from the cathode of the driver side of the 12AX7 tube to the speaker for a negative feedback loop, as that schematic shows, just don't count on the numbering scheme to match the socket in your amp.

BTW, I'm not an expert, maybe specialty guitars will chime in and clarify.

On this current amp, it's pin 8 on the 12AX7.

I'll go back over this thread and see what you guys think of the various mods. I have the problem that almost all variations and combinations of variations sound good, so it's hard to decide.

Guitarslinger1 · March 6th, 2008, 10:14 AM

Quote:

Originally Posted by red57strat

I figured the 1m resistor in parallel with the 22pF cap had to go when I first saw the schematic for these amps, before I even ordered mine

Same here. when I saw that on the scheme I went "wtf?"

Gerald Weber mentioned that Gibson amps almost always had some sort of "remedial tone shaping circuitry" and I think that bass cut/treble pass filter would clearly fall under that description!

red57strat · March 6th, 2008, 10:27 AM

We need to get all these amps, stock and modded, and all the owners into one room for a giant comparison!

I mentioned to someone else, that although I like the stock speaker, I won't rest easy until I've heard it with an 8" alnico magnet speaker.

Guitarslinger1 · March 6th, 2008, 10:28 AM

Here's where I'm at with it, and I'd appreciate hearing all the feedback from others who are trying these mods.

1. Removed the bass cut/high pass filter. Removed the 22pF and the 1meg and replaced with a straight wire. To my ears, this is a keeper, no matter what.

2. Installed a 47K negative feedback resistor. This tamed the high end, so I...

3. Installed a brighter ribbed cone 8" speaker. In this case, the one I had on hand that is broken in is a Kendrick, but most any brighter sounding ribbed speaker would work like Weber's Sig 8S or a Jensen Ri.

4. At this point, I can't decide if I want the 1st stage 25uF bypass cap in our out. Both ways sound really sweet, just different.

5. Installed a NOS Tungsram 12AX7 I didn't know I had.

And that's it so far.

IMO this thing sounded great out of the box, but is a real head turner now.

I'm kind of pissed, it now sounds much better than my 5F1 which was my living room "pride and joy". (maybe it's just time for a new 6V6...)

I agree with Rob, that the amp is just fine out of the box, and is a real winner for a new tube amp at that price.

I believe I was wrong about the cab being press board. It's not as heavy I seemed to think originally. (it was late)

I suppose one could try different EL84s, but the JJ sounds very nice. I'm not going to bother.

EDIT: I forgot. I also took the 22pF cap, and installed it across the hot and the wiper of the volume pot. This acts as a treble bypass to keep the treble there when the volume is turned down low. It's the same thing the 47uF cap in a Deluxe Reverb on the vibrato channel volume pot is doing. This keeps the treble there and a little "zingy", when the volume is turned down, and is in IMO, what Gibson should have done in the first place instead of that lame low cut filter.

On step 2, you may or may not want to do this, and you may want to try different resistor values. The way it is from the factory with no negative feedback (NFB) is raw, gainy and zippy, which is very cool. The NFB, tames it down and evens the frequency response. A higher value resistor will give it less NFB, so you can experiment and decide.

Guitarslinger1 · March 6th, 2008, 11:03 AM

Quote:

Originally Posted by red57strat

Maybe use a 50k pot to dial it in?

You could install a 250K or 500K pot, and or a switch, or even a no-load pot in place of both.

Rob DiStefano · March 6th, 2008, 12:44 PM

specialty -

Is there a reason why you mentioned earlier to completely remove the 1m resistor/22pf cap instead of leaving the jumper in? Ditto's for completely removing the 25uf bypass cap ... ?

red57strat · March 6th, 2008, 12:49 PM

He mentioned concern over parasitic noise.

You could always add switches for them.

The amp could have a power switch, volume control and a row of mini-switches that you can't remember the purpose of!

Guitarslinger1 · March 6th, 2008, 01:06 PM

Quote:

Originally Posted by Rob DiStefano

specialty -

Is there a reason why you mentioned earlier to completely remove the 1m resistor/22pf cap instead of leaving the jumper in? Ditto's for completely removing the 25uf bypass cap ... ?

Quote:

Originally Posted by specialty guitars

The only reason why is that having half connect components and superfluous parts running about can make the circuit unstable and cause parasitic oscillations and noise. It's not imperative that the components be removed, just recommended. And even if removed, those parts or new parts, can be reinstalled to reverse the mods.

Red 57 is right. You could add a switch to switch the 25uF cap in and out of circuit. In fact, this is exactly what the "FAT" and "boost" switches are doing on Blues Jr.s and many other amps.

Guitarslinger1 · March 6th, 2008, 01:12 PM

Quote:

Originally Posted by red57strat

He mentioned concern over parasitic noise.

You could always add switches for them.

The amp could have a power switch, volume control and a row of mini-switches that you can't remember the purpose of!

Sure thing! You could turn this poor little amp into a hellish nightmare of switches, knobs, pull-chords, rip-chords, knee levers, trap doors, whistles, flashing LEDs, stealth cloaking, Ronco Veggimatics, and secret compartmants. Or just set it up to sound sweet and play it!

Blues Hurler · March 6th, 2008, 03:33 PM

Quote:

Originally Posted by specialty guitars

Sure thing! You could turn this poor little amp into a hellish nightmare of switches, knobs, pull-chords, rip-chords, knee levers, trap doors, whistles, flashing LEDs, stealth cloaking, Ronco Veggimatics, and secret compartmants. Or just set it up to sound sweet and play it!

That's exactly what I've been looking for! (slaps forehead) Do you have a schematic?

Guitarslinger1 · March 6th, 2008, 03:44 PM

Quote:

Originally Posted by Blues Hurler

March 6th, 2008, 05:51 AM	#242 (permalink)
Rob DiStefano Friend of Leo's Join Date: Mar 2003 Location: Park Ridge, NJ Age: 62 Posts: 4,674	That's what I'm talkin' about - sounds like just the circuit mods I'm looking for - thanx! What expected difference by adding the 47k negative feedback resistor instead of lifting the bypass cap - or what results by both adding the NFB resistor and lifting the bypass cap? Looks like the NFB resistor would go from the hot side of the OT speaker lead to pin 3 of the preamp tube? Last edited by Rob DiStefano; March 6th, 2008 at 07:26 AM.

March 6th, 2008, 08:55 AM	#248 (permalink)
stephent2 Friend of Leo's Join Date: Jun 2003 Location: Atlanta, GA. Age: 57 Posts: 2,321 20 Photos	"I'm gonna make the mods permanent." is there any compelling reason to do so? why not leave the mods (as is in your picture) reversible? __________________ "and the women,.... great googly-moogly". --Howlin' Wolf

March 6th, 2008, 10:27 AM	#255 (permalink)
red57strat Tele-Afflicted Join Date: Oct 2003 Location: Massachusetts Posts: 1,118	We need to get all these amps, stock and modded, and all the owners into one room for a giant comparison! I mentioned to someone else, that although I like the stock speaker, I won't rest easy until I've heard it with an 8" alnico magnet speaker. __________________ Don

March 6th, 2008, 10:28 AM	#256 (permalink)
Guitarslinger1 Banned Tele-Holic Join Date: May 2005 Posts: 966	Here's where I'm at with it, and I'd appreciate hearing all the feedback from others who are trying these mods. 1. Removed the bass cut/high pass filter. Removed the 22pF and the 1meg and replaced with a straight wire. To my ears, this is a keeper, no matter what. 2. Installed a 47K negative feedback resistor. This tamed the high end, so I... 3. Installed a brighter ribbed cone 8" speaker. In this case, the one I had on hand that is broken in is a Kendrick, but most any brighter sounding ribbed speaker would work like Weber's Sig 8S or a Jensen Ri. 4. At this point, I can't decide if I want the 1st stage 25uF bypass cap in our out. Both ways sound really sweet, just different. 5. Installed a NOS Tungsram 12AX7 I didn't know I had. And that's it so far. IMO this thing sounded great out of the box, but is a real head turner now. I'm kind of pissed, it now sounds much better than my 5F1 which was my living room "pride and joy". (maybe it's just time for a new 6V6...) I agree with Rob, that the amp is just fine out of the box, and is a real winner for a new tube amp at that price. I believe I was wrong about the cab being press board. It's not as heavy I seemed to think originally. (it was late) I suppose one could try different EL84s, but the JJ sounds very nice. I'm not going to bother. EDIT: I forgot. I also took the 22pF cap, and installed it across the hot and the wiper of the volume pot. This acts as a treble bypass to keep the treble there when the volume is turned down low. It's the same thing the 47uF cap in a Deluxe Reverb on the vibrato channel volume pot is doing. This keeps the treble there and a little "zingy", when the volume is turned down, and is in IMO, what Gibson should have done in the first place instead of that lame low cut filter. On step 2, you may or may not want to do this, and you may want to try different resistor values. The way it is from the factory with no negative feedback (NFB) is raw, gainy and zippy, which is very cool. The NFB, tames it down and evens the frequency response. A higher value resistor will give it less NFB, so you can experiment and decide. Last edited by Guitarslinger1; March 6th, 2008 at 10:42 AM. Reason: Forgot something

March 6th, 2008, 12:49 PM	#259 (permalink)
red57strat Tele-Afflicted Join Date: Oct 2003 Location: Massachusetts Posts: 1,118	He mentioned concern over parasitic noise. You could always add switches for them. The amp could have a power switch, volume control and a row of mini-switches that you can't remember the purpose of! __________________ Don

March 5th, 2008, 11:27 PM	#241 (permalink)
Guitarslinger1 Banned Tele-Holic Join Date: May 2005 Posts: 966	Here's a pic of the 25uF bypass cap "lifted" from the first gain stage, to reduce gain. IMO, this mod in conjunction with the bypass/removal of the 22pF/1 meg resistor mod sounds fantastic. The amp is still bright, gainy, and lively, but fuller and less "crispy" sounding. If you decide you like the lower gain tone with the 25uF bypass cap out of circuit, it is best to go ahead and remove it completely. It is also best to go ahead and remove the 22pF cap/1 meg if you decide to keep your amp that way.

March 6th, 2008, 08:06 AM	#244 (permalink)
Sidney Vicious Tele-Afflicted Join Date: May 2003 Location: New Orleans, Louisiana, USA Posts: 1,070 6 Photos	Anybody know if the earlier modern version - the one without the cord hole and the "safety grate" over the speaker hole - likewise is lacking that overly heavy duty speaker grate behind the grill cloth?

March 6th, 2008, 08:14 AM	#245 (permalink)
Rob DiStefano Friend of Leo's Join Date: Mar 2003 Location: Park Ridge, NJ Age: 62 Posts: 4,674	Just made the mods as suggested by specialty guitar ... ... yeah man! MUCH better tone - thicker, more presence, breaks up a tad later, still Loud but less jangly. I'm gonna make the mods permanent.

March 6th, 2008, 08:54 AM	#247 (permalink)
Rob DiStefano Friend of Leo's Join Date: Mar 2003 Location: Park Ridge, NJ Age: 62 Posts: 4,674	NFB to pin 3 of the 12AX7 ?

March 6th, 2008, 12:44 PM	#258 (permalink)
Rob DiStefano Friend of Leo's Join Date: Mar 2003 Location: Park Ridge, NJ Age: 62 Posts: 4,674	specialty - Is there a reason why you mentioned earlier to completely remove the 1m resistor/22pf cap instead of leaving the jumper in? Ditto's for completely removing the 25uf bypass cap ... ?