$299 Gibson tube amp - Made in USA!

[Guitarslinger1]

Originally Posted by Gee

...Changing that voltage divider ratio...

Not trying to be argumentative, but what is this "voltage divider" that you speak of? Please point it out on the schematic.

[Gee]

Originally Posted by specialty guitars

Not trying to be argumentative, but what is this "voltage divider" that you speak of? Please point it out on the schematic.

In the attached schematic, it is R6 and R7, but it is drawn to look different on the Gibson schematic, but it is exactly the same.

[Brent Hutto]

There's an annotation on the "Baby Vox" schematic that Gee attached saying that changing the power tube input resistor will change the "presence". What exactly does "presence" mean? I hear it used a lot in describing amp and pedal controls.

[Guitarslinger1]

Originally Posted by Gee

In the attached schematic, it is R6 and R7, but it is drawn to look different on the Gibson schematic, but it is exactly the same.

I see.

I can see how this could get confused, but the functions of these two resisters must be looked at correctly. And, the function of R6 in your schematic, and the 1M/22pF high pass filter in the GA5 are NOT the same.

That 100k in series with the signal (R6) is a remedial gain reduction circuit. It is acting as a voltage divider with the volume pot. It actually impedes tone, and is not necessary. I mean, would you stick a resistor in between your guitar's pickup and its volume control? No, because it would reduce the output (which is what the volume control is for) and it would permanently darken the tone (which is what happens when you turn your guitar's volume control down, which is what all of the various treble bleed circuits on volume controls are trying to overcome). These same concepts are the same with amps as well. So sure, decreasing that resister's value will brighten the tone and increase gain, the same way turning your guitar's volume to "10" will increase its output and likely sound brighter than when the control is turned down. Removing R6 completely will allow the full signal to the volume pot, which will sound best as it will not be impeding the signal/tone in any way.

The 100k in parallel with the volume control (R7) works to limit the full range of the 1 meg volume pot. This limits it's value when turned to "10", which ultimately controls how loud the amp can be cranked up. With that 100k in place, the value of the volume control is effectively a bit less than 100k, which limits the amp from being cranked very high. With the value of that resister increased or removed, the volume control has more range, and can be cranked up higher. Adding a resister in parallel with a pot is an ages old method of creating "effective" pot values where actual values of pots don't exist.

R7 also effects the taper of the volume control. So this method also allows to turn an audio taper pot into a linear taper pot, or a taper somewhere in between the two types

Again, both resistors are acting as kind of remedial "governors", that would not be necessary if too much gain were not being achieved elsewhere in the circuit. The R7 is valid to change the effective value of the volume pot and its taper, but at values as low as 100k it is simply a remedial governor. It and the volume pot could be replaced with a 100k linear taper pot, and you'd have the same thing.

In the GA5, the 1M and 22pF in parallel with each other are acting as a high-pass filter. The 1M blocks part of the signal from getting to the volume control (acting as voltage divider with the volume pot), but the 22pF cap allows the treble to bypass the 1M unaffected. This is a simple "tone shaping" circuit to reduce the bass and low mids. Changing the values of this cap and resister will change the tone shaping, or it can be removed, as I prefer, which allows the entire signal to pass to the volume control.

The 1M in parallel with the volume pot, is acting to reduce the 1M pot's effective value to 500K. As with R7 in your schematic, it's value could be decreased to reduce to how high the amp can be cranked, or it could be removed to allow the pot's full 1M range to be used. In my opinion, this works well at the effective 500K value, and since the volume control is plenty fast and hot, there is no need to remove that resistor.

[Gee]

Agreed. The mod is to replace both 1M resistors with 100K values to keep the gain the same or to alter the value of R7 to adjust the gain. The Valve Junior uses 2 x 1M just like the GA-5 yet sounds better with 2 x 100K.

[Guitarslinger1]

Originally Posted by Gee

Agreed. The mod is to replace both 1M resistors with 100K values to keep the gain the same or to alter the value of R7 to adjust the gain. The Valve Junior uses 2 x 1M just like the GA-5 yet sounds better with 2 x 100K.

This is being used as a "remedial" gain reducer, that negatively effects tone, and would not be necessary too much gain was not being produced by having cathode bypass caps of both preamp stages.

The R6 in the VJ is a general, across the board, gain reducer.

The 1M/22pF cap in the GA5 is a high pass filter. Sure, it also effects gain because it reduces the bass part of the signal, but R6 in the VJ and this filter in the GA5 are not serving the same function.

R6 and R7 are not acting as a voltage divider. R6 does act as part of a voltage divider, along with the volume pot. R7 acts to change the value of the volume pot.

With R6 limiting the amount of signal that gets to the volume pot, and R7 limiting the range of the volume pot, the two resistors are working at two different, but related functions.

[Gee]

Originally Posted by specialty guitars

This is being used as a "remedial" gain reducer, that negatively effects tone, and would not be necessary too much gain was not being produced by having cathode bypass caps of both preamp stages.

The R6 in the VJ is a general, across the board, gain reducer.

The 1M/22pF cap in the GA5 is a high pass filter. Sure, it also effects gain because it reduces the bass part of the signal, but R6 in the VJ and this filter in the GA5 are not serving the same function.

R6 and R7 are not acting as a voltage divider. R6 does act as part of a voltage divider, along with the volume pot.

I don't disagree. Yet the standard mod in the VJ even for the versions without a cap on the second pre amp, is to replace both 1M with 100K. Then you may find you don't need the extra cap across the 1M. The smaller the number of components that the signal passes through and the lower the resistance of those components is, the better the overall signal will be.

But, hey there are many ways to skin this cat. I'm just throwing out the highly evolved mods that are already out there.

Here is an example of what that last version of the VJ (Baby Vox) I posted, sounds like as an example of what a modified circuit can sound like.

Tele played through Baby Vox mod SE EL84 amp.

And isn't about time that we get some samples to listen to here. How can this thread grow so big without someone posting what it sounds like?

[Guitarslinger1]

Originally Posted by Brent Hutto

There's an annotation on the "Baby Vox" schematic that Gee attached saying that changing the power tube input resistor will change the "presence". What exactly does "presence" mean? I hear it used a lot in describing amp and pedal controls.

That R15 is yet another "remedial" gain reducer. The higher its value, the more it will interfere with the tone.

The more I look at this Baby Vox schematic, the more bizarre it seems.

Too much gain is being produced by the first preamp stage, so its output is reduced by R6. Too much gain is being produced by the second preamp stage, so it is being reduced by R15.

If the value of R2 was increased, and C4 removed, the output from the first stage would be more manageable, so R6 could be removed. And if C3 were removed, the output from the second stage would be more manageable, so R15 could be removed.

The 1K resistor it prescribed on the power tube screen is not necessary, because R12 is already serving that function.

And the value of 47uF for the preamp filter, C8, is incredibly high. The .1uF C11 is interesting as well.

Going to DC for the filaments is sure a fantastic idea.

[Guitarslinger1]

Originally Posted by Gee

The smaller the number of components that the signal passes through and the lower the resistance of those components is, the better the overall signal will be.

I agree completely.

Originally Posted by Gee

But, hey there are many ways to skin this cat.

I agree here as well, but I'll add that some ways are better than others, and some require additional, unnecessary "skinning".

Originally Posted by Gee

And isn't about time that we get some samples to listen to here. How can this thread grow so big without someone posting what it sounds like?

I couldn't agree more!

I need to get set up to record some clips myself.

[Rob DiStefano]

Originally Posted by Gee

...
And isn't about time that we get some samples to listen to here. How can this thread grow so big without someone posting what it sounds like?

Yeah man, I gotta work on some sound bytes ...

[Guitarslinger1]

[quote=Gee;1175299]Here is an example of what that last version of the VJ (Baby Vox) I posted, sounds like as an example of what a modified circuit can sound like.

Tele played through Baby Vox mod SE EL84 amp.

From what I can tell on this computer, that sounded very tasty! Very nice!

[petesguitar1]

For little fellas I have a Blackface VibroChamp Clone from Ceriatone! It currently has an AC Hum that I have to look at but apart from that it is an awesome little amp. Just waiting on a custom 12" (yes 12) cab.

[Joe K]

Like Boris, I just received my '06 blem from MF and am digging it just the way it is. Thus far, I like playing it a little dirty at 4-6 volume (or 11-1 if you call straight up 12 o'clock).

I noticed today that input 2 is a little quieter than input 1. Who here knows how #1 is wired? Is that a useful channel, or just stay with #1?

(e.g., does 2 work best for humbuckers, mic, etc.?)

I'm sure I will try these mods sometime. For right now though, I have a happy little amp and owner.

[Guitarslinger1]

Originally Posted by Joe K

Like Boris, I just received my '06 blem from MF and am digging it just the way it is. Thus far, I like playing it a little dirty at 4-6 volume (or 11-1 if you call straight up 12 o'clock).

I noticed today that input 2 is a little quieter than input 1. Who here knows how #1 is wired? Is that a useful channel, or just stay with #1?

(e.g., does 2 work best for humbuckers, mic, etc.?)

I'm sure I will try these mods sometime. For right now though, I have a happy little amp and owner.

I'm not trying to be dismissive of you, but almost everything you could want to know about this amp and more can be found in here somewhere: http://www.tdpri.com/forum/amp-centr...-made-usa.html

Like most every other amp that has two inputs per channel, it was never intended that anything be plugged into #2 unless something else was already plugged into #1. The fact that #2 is lower in gain when used by itself is just a product of the design. If you like the sound, nothing wrong with using it.

[androo]

Joe...welcome to the hip-est thread around! I have been just so overcome with knowledge...I never knew :)

Anyway, I've been pretty successful with using only input #1 with singles or buckers. #2 is nice, but lower volume and dynamics.

I've done one mod to this amp, the lift of the first stage bypass, and gotten a bit more headroom and less "over the top" distortion. I highly recommend this mod. Read this whole stinking thread, all 700+ posts. It's amazing.

What I've found useful, especially for buckers, is an eq pedal in line. I gig this little guy and use it as a monitor for me....sending it through the PA. It's a dark little amp in stock form, and the bypass mod, while giving a tad more headroom and taming the distortion beast, doesn't change the fact that it's dark to begin with. A speaker swap may be in order. I had a modded Boss GE7 laying around, and now I use it to tweak those parameters I either need, or lack.

Works for me, and my bass player loves this little fella. I'm a fan of gigging this guy...you want a Scofield grind? This is it. And at decent stage levels too boot!

droo

[boris bubbanov]

Originally Posted by Joe K

Like Boris, I just received my '06 blem from MF and am digging it just the way it is. Thus far, I like playing it a little dirty at 4-6 volume (or 11-1 if you call straight up 12 o'clock).

I noticed today that input 2 is a little quieter than input 1. Who here knows how #1 is wired? Is that a useful channel, or just stay with #1?

(e.g., does 2 work best for humbuckers, mic, etc.?)

I'm sure I will try these mods sometime. For right now though, I have a happy little amp and owner.

I hear that. It would seem to me that turning the knob beyond that just makes it fizzier.

I think it sounds better with the single coil pickups than the 'bucker.

I am fascinated that you can go from clean to dirty just off the pick attack.
It is a gas; I admit I was kinda dismissive of what I called 'practice amps' for years. I wish I'd discovered this sooner in my life.

[TNO]

Pete-I had a Ceriatone 5E3 that hummed until I separated the preamp and power amp grounds. Star grounding everything together was the problem in that amp.

[Jack]

Anyone want my GA-5 before I send it back? I've got a Fender Super champ XD on the way which is more to my taste. I did the first mod, but can solder it back (which I will have to anyhow before I send it back) or leave as is, your choice.

I've had it for about three weeks. Let me know. $300 + actual shipping charges from 78414 via FedEx for UPS. Money order or Paypal (3% fee for Paypal).

[zero2sixty]

I used an alligator clip temp jumper to test the sonics that results thru the jumper mod in the "trinity". That mod was going to shred the stock speaker. I am trying to keep this amps overall volume at dbs that are almost "office" quiet and that mod really opens up the volume...just pulled the gator clips off and back to "normal". I ordered a Weber Blue Pup to see if that will tame the overall rumbly-dark sonics of the GA-5. At $50 with shipping, I hope I don't find thta I should have paid the extra and got a 8A100T! Only experimenting will tell.

Btw: (Juan & Jesse signed off on my GA-5 Nov. 22, 2006).

When these amps get to vintage age, will all of the mods diminish the value of these amps?

[Rob DiStefano]

Originally Posted by zero2sixty

I used an alligator clip temp jumper to test the sonics that results thru the jumper mod in the "trinity". That mod was going to shred the stock speaker. I am trying to keep this amps overall volume at dbs that are almost "office" quiet and that mod really opens up the volume...just pulled the gator clips off and back to "normal". I ordered a Weber Blue Pup to see if that will tame the overall rumbly-dark sonics of the GA-5. At $50 with shipping, I hope I don't find thta I should have paid the extra and got a 8A100T! Only experimenting will tell.

You should test out adding the NFB resistor - that could be just what ya need. Personally, having heard the 8A100T and the Blue Pup, I like the Blue Pup much better. I've got an 8A125T coming - hey, it's all a crap shoot of sorts because of what each of us expects to hear, and if mods were done to the circuit.

Btw: (Juan & Jesse signed off on my GA-5 Nov. 22, 2006).

When these amps get to vintage age, will all of the mods diminish the value of these amps?

Ya know, when I first read that I thought "is he nuts?" but given the history of Fender, and that beat up '56 Tele I bought used in '59 that I GAVE AWAY in the late 60's, who's really nuts now?!?!?!?

....

[zero2sixty]

Has anyone tried swapping out the 25uf cathode bypass cap for a .68uf mallory? If I understand, that will reduce some of the low frequesncies form ulimately getting to the speaker.

What effect will changing the 22uf/350v filtering cap to a 30uf produce?

[zero2sixty]

Thanks, glad to hear that the Blue Pup seems to be the stronger choice...I'll have a better idea in a few weeks. Ted replied to me and confirmed that these speakers are made at his facility in IN.

I am sure I saw your Tele at Christi's in New York! Sorry 'bout that :-)