$299 Gibson tube amp - Made in USA!

[Rob DiStefano]

Originally Posted by endzone

I just started another thread on this....MF is selling "blems" for $230. I'm sure these are the returns. Here's a great opportunity for someone to pick one up for an even better deal.

http://www.musiciansfriend.com/produ...mp?sku=481209X

Those are returns - I just knew that was gonna happen.

[Gee]

Originally Posted by red57strat

I've been thinking about messing with the filter caps since getting this amp. The schematic calls for 3 x 15uF, 3 x 22uF was used, a 5F3 Champ used 3 x 8uF and the original GA5 schematic calls for 1 x 20uF and 2 x 10uf.

Changing the first filter cap in my 5E3 clone to 30uF (from 16uF) tightened up the bass with the amp cranked quite a bit. Gerald Weber recommended not putting higher value filter caps in Champs. From what I've seen, filter cap values alter the looseness and tightness of an amp (as well as noise, of course) rather than tone.

It's a fairly cheap experiment to see how the amp responds to lower value filter caps.

I have a bunch of 16uF@475v caps including a Sprague Atom (will it fit in there???) that I can try in there. I'd be tempted to try 1 x 20uf and 2 x 10uF like the original schematic.

BTW, all of my recent 16uF Sprague Atoms measure 19uF on my capacitor meter.

Don't forget that there are many versions on the GA-5 about. The one discussed here has 22x22x22 which is pretty standard these days. Champs are different and do benefit from a 16x8x8 configuration and sound better with the early Princeton's 32x8x8 setup. Most EL84 amps actually have an extra 20 to 50 cap with a small resistor (~150R) before the first 3 caps. See attached schematic.

Also from memory the GA-5 has no 1M resistor from the input to ground. That was an error in the early Gibson schematics that got carried forward.

[Guitarslinger1]

Originally Posted by Brent Hutto

On amps without a standby switch, then would it be best to turn it on and leave it on if you're going to be away for an hour or two? Seems like having fewer cold startups might be more important to tube life than avoid an extra hour of sitting around at idling at full steady-state plate voltage.

No, that's not ideal either. Single ended amps such as this run the power tube full-on all the time, so you are wearing down your power tube by leaving the amp on when not in use, as you would be to a lesser extent with a push-pull amp as well.

[Brent Hutto]

So on balance if I'm going to play my little Champion 600 for an hour, then go eat dinner and watch TV for a couple hours and play for another 45 minutes before bed should I shut it down or leave it humming?

[Guitarslinger1]

Originally Posted by Gee

Also from memory the GA-5 has no 1M resistor from the input to ground. That was an error in the early Gibson schematics that got carried forward.

Not an error. When you plug your guitar in, it's resistance to ground takes the place of that grounding resistor. Gibson factored that in. As long as you are using a guitar, you're fine. If you plug in a mic for harmonica, it can cause big trouble with runaway gain.

So for harp players, a 4.7 meg resistor should be installed from the input to ground in vintage GA5s and in these new ones. For guitar, the input should be left as is.

[androo]

Hey folks :)
I hate to bring this awesome discussion back to elementary level (but I am a first grade teacher!) Anyway, I've gotten all the goodies I need to do the 3 original electrical mods you talked about, but now I'm balking. Chalk it up to inexperience. Could you break those 3 down fer me? What effect can I expect for each one?

The lifted cap
The jumpered resistor (1m + 22pf cap)
The NFB

Again, sorry to be so elementary! I am in love with my amp as is, and now I realize I may be getting swept away with results I'm not even sure of!
Duh ;)

droo

[Rob DiStefano]

Originally Posted by androo

Hey folks :)
I hate to bring this awesome discussion back to elementary level (but I am a first grade teacher!) Anyway, I've gotten all the goodies I need to do the 3 original electrical mods you talked about, but now I'm balking. Chalk it up to inexperience. Could you break those 3 down fer me? What effect can I expect for each one?

The lifted cap
The jumpered resistor (1m + 22pf cap)
The NFB

Again, sorry to be so elementary! I am in love with my amp as is, and now I realize I may be getting swept away with results I'm not even sure of!
Duh ;)

droo

If yer happy with the GA-5 as is, don't bother with any mods.

There is no way to fully describe what the trinity circuit mods will sound like for you - lots also depends on the kinda guitars/pickups you plug in, and what and how you play.

The trio of circuit mods will remove that shrill boxy tone, allow for a fuller 'n' fatter tone, greatly reduce hum ... this basically somewhat lessens the raunchy, overly trebly "tweed tone" to a more manageable level.

Just do all three at the same time, they compliment each other and they are way too easy to accomplish, OR to undo ...

The lifted cap - cut the jumper, as shown
The jumpered resistor (1m + 22pf cap) - solder a wire across those turret terminals, as shown
The NFB - add a 47k resistor from the yellow wire on the RCA jack to pin 8 of the 12AX7 socket, as shown

[endzone]

Rob, I'm waiting on you to post sound clips of your "modded" vs "unmodded" GA-5's. Since you've already done the mods (and like them) and have 2 amps, you're the logical choice to help some of us decide if we want to follow suit.

[Gee]

Originally Posted by specialty guitars

Not an error. When you plug your guitar in, it's resistance to ground takes the place of that grounding resistor. Gibson factored that in. As long as you are using a guitar, you're fine. If you plug in a mic for harmonica, it can cause big trouble with runaway gain.

What Gibson didn't factor in is new fangled stuff like effects pedals and tuners etc that all expect and work best with a 1M impedance.

[Rob DiStefano]

Originally Posted by endzone

Rob, I'm waiting on you to post sound clips of your "modded" vs "unmodded" GA-5's. Since you've already done the mods (and like them) and have 2 amps, you're the logical choice to help some of us decide if we want to follow suit.

Eventually I'll put up some sound bytes - but I will caution everyone that MY sound bytes have nothing to do with how an amp will sound for YOU with YOUR gear and YOUR playing.

Remember, the guitar tone path = brain, fingers, pickups, amalgam of the guitar, FX (if any), amp (circuit, tubes, speaker, etc).

I can't say this any other way - the trinity circuit mods are so easy and cheap to accomplish AND UNDO that you should just do them and see for yerself.

[androo]

Thanks again Rob :) That last drawing is great!

I read this over on the FDP,

"They're back!!!!!!! Just got off the phone with Gibson, and news is MF has bought out all of the entire GA5 directly from Gibson. No other online vendors will have these in stock as confirmed directly by Gibson."

Interesting~droo

[Guitarslinger1]

Originally Posted by Gee

What Gibson didn't factor in is new fangled stuff like effects pedals and tuners etc that all expect and work best with a 1M impedance.

With effects and all that in line, it's all the same. The resistance to ground is still there, so the circuit is fine.

With all of the circuit changes, layout and other design differences, etc. between the vintage GA5 and this new one, the likelihood that the lack of a grid return resistor could be an accidental omission, carried over from the 50s to this new design, is less than infinitesimal.

I'm certain that Gibson knew about effects pedals and tuners when they designed this new model in 2002 or whenever it was.

[Jaybird]

[quote=specialty guitars;1150915]Yours is a unique one for sure. I can't imagine lifting that 25uF cap would make it sound "spotty and gargley", when all it does is reduce gain. NFB can be a tricky issue. I do wonder if the .1uF caps made it unstable. Yours is the only one that's had those problems (that we know of), and it's the only one with different value coupling caps. Amp circuits are kind of a "balancing act".

I thought about this and decided to try some other value NFB resistor. First I used a 100k , because the squeal was so intense, I figured the 47k must be way off if what you told me is the case. There was still squeal, but not so immediate or scary sounding. Next I used a 1 meg resistor (overkill?) to just determine if anything stopped the squeal. No squeal, but no noticable difference in the amps sound. So now I think I will hook up a pot and see if I can dial in something. To your ears, just how does the NFB change the tone or sound of the amp, what should I be looking for?

[Gee]

Originally Posted by specialty guitars

With effects and all that in line, it's all the same. The resistance to ground is still there, so the circuit is fine.

With all of the circuit changes, layout and other design differences, etc. between the vintage GA5 and this new one, the likelihood that the lack of a grid return resistor could be an accidental omission, carried over from the 50s to this new design, is less than infinitesimal.

Strangely, Gibson did this on some amps but not others, i.e. the GA-6 had no pull down resistor yet the GA-40 and others did. The other advantage of having a 1M resistor in there is the improvement in signal to noise. It was how Leo did it and it is how everyone else still does it to this day and beyond.

[Rob DiStefano]

Carling off/standby/on added.

I pulled the GA-5 2-way power switch and removed the brown wire that goes to the IEC socket and the twisted pair of black and black/white wires from the PT.

I moved those two wires over to the Carling switch (see diagram below).

I removed the short white jumper between the rectifier diodes and the first filter cap, replaced that with a twisted pair of 18 gauge stranded wires (red and green).

I connected the diode/filter wires to the Carling switch (see diagram below).

I installed the Carling switch so that the end without lugs faces up (flipped the switch around as it looks in the diagram below) - and bundled all the loose wires together with mini tie wraps.

The switch works flawlessly.

[bdjr1]

Where can u buy the carling switch ?

[red57strat]

http://westlabs.com/Frames.HTML

DPTTP $ 9.00 Carling Super Heavy Duty

[Guitarslinger1]

Originally Posted by Jaybird

I thought about this and decided to try some other value NFB resistor. First I used a 100k , because the squeal was so intense, I figured the 47k must be way off if what you told me is the case. There was still squeal, but not so immediate or scary sounding. Next I used a 1 meg resistor (overkill?) to just determine if anything stopped the squeal. No squeal, but no noticable difference in the amps sound. So now I think I will hook up a pot and see if I can dial in something. To your ears, just how does the NFB change the tone or sound of the amp, what should I be looking for?

Again, something is not quite right with your amp. I believe the .1uF caps have made it unstable, and on the edge of parasitic oscillation. When you add the NFB resistor, it should not cause any problems, yet it does.

I can't say for sure without the amp in front of me, but my guess is that the .1uF cap installed between the second gain stage and the grid of the EL84 is causing the problem.

Until you get your amp under control, there is no point experimenting with NFB.

NFB smooths and evens out the frequency response. Early amps didn't have it at all. Tweeds acquired it, and blackface amps have more of it.

Less, or no NFB (high value resistor or none at all) makes the amp sound "raw". More NFB (lower value resistor) makes the amp sound more refined, with a more even response.

A common NFB resistor value for tweeds is 56K (some NFB). A common value for blackface amps is 820 ohms (a lot of NFB).

[Guitarslinger1]

Originally Posted by Rob DiStefano

Carling off/standby/on added.

I pulled the GA-5 2-way power switch and removed the brown wire that goes to the IEC socket and the twisted pair of black and black/white wires from the PT.

I moved those two wires over to the Carling switch (see diagram below).

I removed the short white jumper between the rectifier diodes and the first filter cap, replaced that with a twisted pair of 18 gauge stranded wires (red and green).

I connected the diode/filter wires to the Carling switch (see diagram below).

I installed the Carling switch so that the end without lugs faces up (flipped the switch around as it looks in the diagram below) - and bundled all the loose wires together with mini tie wraps.

The switch works flawlessly.

Good job Rob. Excellent pics!

[tc6969]

Ok so now that were talking about harmonicas, how would a harp player with a real Green Bullet get more headroom or "pre squeel" action out of a GA-5?

Even out on the end of a 25' cord, I cant even hit 12:00 without drilling out some ears!

[Guitarslinger1]

Originally Posted by tc6969

Ok so now that were talking about harmonicas, how would a harp player with a real Green Bullet get more headroom or "pre squeel" action out of a GA-5?

Even out on the end of a 25' cord, I cant even hit 12:00 without drilling out some ears!

First, install a grid return resistor on the input. 4.7meg should work, you might have to go down to 2.2 meg or 1 meg.

Have you done any of the mods to it that are discussed on this thread?

[tc6969]

Originally Posted by specialty guitars

First, install a grid return resistor on the input. 4.7meg should work, you might have to go down to 2.2 meg or 1 meg.

Have you done any of the mods to it that are discussed on this thread?

No, so far I'm just letting it sit on the floor and make friends with all the other animals.

I've only done the 10' HD computer cable mod which I thoroughly recommend if you can find one.

[JamonHamon]

Th