The official G9 help thread

Racked up!

Guts

Last one: Transformer box (to keep everything hum-free). We felt that since we were using the best components and going all out custom, the supply should be separate.

Hi guys !
Yesterday, I decided to make the external HV PSU mod to reduce the noise of my G9.
While remounting everything, I checked if my heater regulator was still isolated from the chassis (I mounted the 78S12 regulator directly to the case with an electric isolation kit) and I saw that I had a shortcut between the regulator heatsink surface and the case...
I removed all tubes and unscrewed the regulator from the chassis but still I had 0 ohm between the heatsink surface and the case...
So I checked the 78S12 datasheet just in case and read that the heatsink surface is actually connected to the ground pin (which is connected to ground in the G9). So I got my answer ! 

But my question now is why do we read everywhere (and in particular on Gyraf's G9 page) that the 78S12 should be electrically isolated from the case ? This does not apply to TL783 or LM317/337 regulators that have their heatsink surface connected to the out pin but in the 78S12 case, I think this can be done.
Actually, I think it would be a good idea to connect it directly to the case to send this noisy power supply current to the starground on a least resistance path (though it would require a small mod to avoid a ground loop).

What's your opinion on this ? Maybe I am missing something.

why do we read everywhere (and in particular on Gyraf's G9 page) that the 78S12 should be electrically isolated from the case

Click to expand...

Reason is simple: We want one - and only one - connection from 0V to chassis ground.

And that is preferably the 0V connection at the input connector.

More than one chassis ground connection is asking for trouble - specifically ground loop hum.

Jakob E.

Ok.
So this recommendation is indeed to prevent ground loops, not for safety reason. Just wanted to understand why this insulation was needed.

Thank you for the explaination, Jakob !

I've recently finished my gyraf g9 project with the instrument mod. (thanks for everybody on the forum)
Besides working on some last details (wiring etc) the preamp seems to be working fine.
There is only one thing that probably has been talked about, but keeps me guessing and that is the instrument input level.

When plugging a synth into the instrument in the signal is coming trough clearly,
but when I plug in a passive bass guitar the signal is just barely above silence.

Since I've no comparison to other g9's in my area I'm just curious if the G9 would normally be capable by amplifying a bass or electric guitar.

If needed I can post some pictures or sound samples.

All the best,

Luax

The G9 inst. input is (extremely) suitable for passive bass guitar.

Check your wiring and/or resistor values around input stage?

Jakob E.

Hi Jakob,

That's also what I hoped. I'll check all the components (again and verify if everything is ok.

Another thing that never hurts to mention is that I removed the two from- and to- instrument wires from the control panel to the pcb.
Could this be of any influence to the sound?

All the best.

Luax

@thelabyrinth: Nice build, very impressive

Luax said:

Hi Jakob,

That's also what I hoped. I'll check all the components (again and verify if everything is ok.

Another thing that never hurts to mention is that I removed the two from- and to- instrument wires from the control panel to the pcb.
Could this be of any influence to the sound?

All the best.

Luax

Click to expand...

It's influence will be seen quite clearly as a lack of oscillation at high gain settings, which is a good thing and why the mod was implemented. Your output on your bass should be pretty hot, even though it's passive. Is it doing this on both channels?

Thanks for all the replies,
I was away for the weekend but i'll post my findings as soon i've checked everything.

Hi. I'm very new to this forum and i want firstly thank you all. It's an incredible resource of amazing ideas. Thank you Gyraf for all these amazing gears and for the passion you share them with. 

Last year I successfully built a GSSL compressor. No issue, wonderful sound. I'm not new to DIY. Built and modded a lot of guitar stompboxes and various basic electronic stuffs. So i decided to go for a mic pre since i'm planning to record an album in my own home studio. So I built a G9. You can see some images here (my project site, in Italian sorry):
https://sites.google.com/site/dokdude/progetti/g9-2ch-tube-preamp

Now I'm stuck with this unit that seems to suffer with serious problems: hum and hiss (loud). I first thought about the oscillation issue but it's  closer to a white noise. Very irregular.
Here is a noise sample recorded:
http://soundcloud.com/dokdude/2011-10-31-g9-hum-hiss
In the sample, after increasing volume (@ Gain 5) when you clearly hear the 'pop' (0:15), i'm switching source from line to mic input, then to line again and decrease volume. No instrument nor mic plugged into the unit. In the past i got some similar problems with some stompboxes and i found it to be the pcb traces not really clear (some dust, hairs, soldering paste, and so on), so i've cleaned very accurately. And still the problem persist. Checked connections, external condensers voltage, and moved power trafo while listening. Nothing.

Have you got some suggestions before I begin to re-solder components again and again?

D.

Did You make shielded wire mod

If you run into problems with oscillation on the highest gain settings, try running the connections to/from the front panel High-Z jack connector "directly" from the relevant points on the PCB - by cutting the PCB traces, and running a length of shielded cable directly to/from the jack.

Not yet. And i will since i want to cancel the hum and the oscillation problem at high gain settings.
But now the biggest problem is the hiss. So loud. Do you think it's because of the shield? A white/brown irregular noise?
Maybe I'm wrong. Surely confused.

Thanx.

First I will make shielded wire mod just to be sure, and of course  check all ground connections.
some pictures may help

Well, i've cleaned main pcb with an iron tool point per point, trace per trace, and then with an iron brush all over the surface.
At fist attempt i got a HUGE noise. Waited a minute, moved something and tried again: only hum (oscillation problem).
Tried with mic and guitar: all Ok. Only oscillation hum.

So it's mainly a contacts problem.

I will clear panel PCBS, make the osc mod and then let's see.

Check wiring from mainboard to switch boards.
regards
Bernd

Hey guys,

I just got my G9 working .. but not all perfect yet ;-)
I have the oscillation problem on the instrument jack wich i will fix later.
Just tested the normal mic input with an sm57: works on both channels, just the right channel has some more noise etc - don't know if the oscillation mod can fix this too?

Also my bigger problem is, that i get only 5 volts when i turn on the phantom power.
I don't really know what to measure or test right now to find out where the solution for this problem could be ..

hope somebody can help here

thx again in advance!

Phil said:

Also my bigger problem is, that i get only 5 volts when i turn on the phantom power.
I don't really know what to measure or test right now to find out where the solution for this problem could be ..

Click to expand...

  I think you test between XLR input pins 1 and 3 , or 2 and 3  ,  and adjust the 10k trimmer ( PR1 ) till you get 48v
.. hope this helps , good luck , I know you'll love this mic pre

EDIT :  reference the G9 PCB layout and schematic on the Gyraf web page