Fender G-DEC Signal Path and grounds

[walkman]

Hi ,

I've bought a G-Dec and will be getting it later in the week. For the moment I decided to look up the schematic.

http://support.fender.com/schematics..._schematic.pdf

Can anyone tell me whats going on with inputs? Why is there a resistance to ground and a 0.01 cap. Also why did they put 10uf polarized caps in the signal path? can these be removed.

While this is a modeling practice amp and I don't expect too much from it, are their any changes that could be made to improve the sound, besides changing the speaker or messing round with cab.

Can anyone suggest how to and if it is practical in this amp to change the voltage drive to current drive so the damping factor is reduced, to give more speaker to amp interaction



Cheers

Walkman

[teemuk]

Why is there a resistance to ground and a 0.01 cap. Also why did they put 10uf polarized caps in the signal path? can these be removed.

The resistor is actually zero-ohms in value and the beads were only used while prototyping so the "resistors" are proably either for layout issues (e.g. jumper wires) or for prototyping purposes (not fitted when the designers wanted to experiment with beads, and fitted because beads were not found important in production version).

The cap is an RF bypass, bypassing the isotated ground node to chassis earth at high frequencies. Basically it's an extra measure to make sure the amplifier doesn't turn into a radio receiver. Beads are also commonly also used for attenuating HF noises, like those produced by switching power supplies etc. Apparently they were not needed in the amp.

All this naturally begs a question, why on earth would you want to remove these components in the first place? They do absolutely no harm whatsoever, more the contrary.

Can anyone suggest how to and if it is practical in this amp to change the voltage drive to current drive so the damping factor is reduced, to give more speaker to amp interaction

It's is very practical and so easy you can not believe. You need to do absolutely nothing to the amp.

[walkman]

Thank Teemuk

I was wondering about cap C2 , why its there. There no need to remove bits if they cause no impact , however as i'm mainly familiar with PTP tube amp input stages i wondered what was going on.

Can you explain the output section for me. Is it current drive already? anything you 'd do to improve the amp or just leave it as is?

cheers

walkman

[teemuk]

I was wondering about cap C2 , why its there.

AC coupling. Fender doesn’t want DC offsets in the input getting to the gain stage, or DC offset in the following stage to mess up the operation. (The CODEC driver stage has a DC offset of 2.5V). I’m pretty sure they do not use polarized capacitors in the places where they should not be although the schematic seemingly portrays them. It’s probably just schematic capture software’s standard notation for any electrolytic capacitor playing tricks on you.

Can you explain the output section for me. Is it current drive already?

The output by principle is actually pretty much identical to the circuit diagram you posted. A low-resistance in series with the loudspeaker samples the output current generating a corresponding voltage drop across the said resistance (Fender uses two resistors in parallel for power sharing purposes). This “sampled” voltage is used as an additional feedback signal (“current feedback”), attenuated to suitable proportions by a voltage divider. In the said Fender amp this feedback loop section is also AC coupled. See, another seemingly polarized capacitor there as well… likely not.

anything you 'd do to improve the amp or just leave it as is?

I got an impression you don’t even have the amp yet? Why on such a hurry to mod it already. Get the amp, spend a nice honeymoon with it and maybe then you have a clearer idea about the aspects you like to modify in it – or if there’s any sense in modifications to begin with.

To me the amp looks like a pretty decent design from the ground up. They even use nice IC regulators in the power supply circuit, unlike in many other Fender amps where the current limiter resistors of shunt zener diodes overheat and unsolder themselves as time passes. At quick glance this looks like a very, very good design for basically a very inexpensive amp.

[walkman]

I'm picking the amp up shortly. Ive got it for very cheap as it apparently has problems booting.

http://www.mygdec.com/index.php?opti...=2&func=sb_pdf

Gdecboot.pdf

Suggests that it can be solved by re soldering or replacing D32,33 or some of the caps in the power supply.

I though that if it could do with any upgrades i'd do them at the same time .

[Jazz P Bass]

Wait until you see the insides of this amp!
Surface mount city.
Forget 'modding' it.

[Enzo]

Agreed, it would be like modding your cell phone.

[g1]

Can anyone suggest how to and if it is practical in this amp to change the voltage drive to current drive so the damping factor is reduced, to give more speaker to amp interaction

It's is very practical and so easy you can not believe. You need to do absolutely nothing to the amp.

I'm still waiting expectantly for some clarification here. I see there is a switch and pot in the block diagram labelled "current drive management". Not sure if this is what Teemuk is referring to, or rather that the amp is "current drive" to begin with.

[Steve Conner]

They are trying to say that no mod is required because the amp already has current drive to the speaker. Any half-decent solid-state guitar amp does, they have been doing it since the days of the Polytone Mini-Brute.

I got an impression you don’t even have the amp yet? Why on such a hurry to mod it already.

[walkman]

Hi Teemuk


On the weekend I first re flowed the solder on rectifier diodes , that seem to fix the booting problem then it appeared again. I then replaced the 1000uf 16v filter caps again appeared to fix the booting problem but then started happening again then next day.

So last night i decided to replace the all 4 diodes in the bridge. Well it was late and i was tired and my soldering skills aren't the best on PCB so i made a bit of a mess of it , traces lifted where the components enter the pcb etc. Now when it powers up it flickers very rapidly and the transformer seems to hum or vibrate. Naturally i turned it off again very quickly to avoid damage.

Anyway I think that i've somehow connected leads p16 and p15 at c240 . C240 looks like a tiny brown smd . Is it necessary? I'm thinking that i should veroboard a new bridge and join it with new leads to TP59 and TP60 and the appropriate power connector pins.

What else could be wrong , seems like a pity to trash it just cause of the power issue , though if the booting problem does not in fact lie in the power section , ive spent several hrs already now just to make a new problem or the original problem worse.


I have to keep reminding my self don't try to do anything with electronics when i'm tired . But as I'm tired i become careless and forget that i shouldn't do anything when i'm tired . Bit of a catch 22