The official G9 help thread

[Spencerleehorton]

Hi all,

I have a 100hz - 120hz hum issue, I have use OPA transformers with cans and used 2 x toroidal transformers for the power, both toroidal a are on top of each other.
First question is where in the circuit should I use screened cable? Just make sure this is not the problem. Do I used screened cable on both the input and output, or is unshielded cable fine?
Could this be ripple from the 7812? should I change for the 78S12?
I will check my voltages but what is the preferred voltages before and after?
Should this unit sound absolutely clean or is there always going to be a slight hum?

Regards

Spence.

 

[gyraf]

Screened cable is probably only necessary to/from the front-panel jack connector (signal is unbalanced here, ins and outs are balanced and thus quite immune).

Look through this thread for ideas about tracking down hum sources, we've been there  often.

Jakob E.

 

[museic]

"Finished" my g9 about a month back. Channel 2 works amazing, definitely the best sounding preamp I have! It's amazing on anything I have plugged into it.

My issue is with channel 1. It has some kind of harmonic like distortion going on. I didn't notice it at first with the guitar playing lead lines, but once I started playing chords the distortion stacks up and the chords sound muddy and not particularly good.

I tried things like swapping the tubes between channels and swapping the controls to no effect. I also took some voltage measurements at the tube sockets and everything seemed fine. I did some searching and didn't see anyone with the same problem so I'm not sure what to look for next?

Also channel 1 is the farthest from the power transformers, I've done the di mod, used screened cable and my transformers are llundahl.

 

[Spencerleehorton]

Hi all,
Still trying to get to the bottom of this hum but have hit another problem ,
Changed over some of the leads as I wasn't using different colours and now everything is back how it was, I plug into either the line in or the XLR input and the gain and output controls are bypassed but signal is going through?
Also the right hand channel polarity when it is set opposite to the left hand channel there is oscillation ?

Any help getting this unit fully operation would be very welcome as I really love the sound.

Regards

Spence.

 

[Spencerleehorton]

Also forgot to mention that my pcb is rev #1

 

[gyraf]

I plug into either the line in or the XLR input and the gain and output controls are bypassed but signal is going through?
Also the right hand channel polarity when it is set opposite to the left hand channel there is oscillation ?

Double-check your wireing.

Jakob E.

 

[Spencerleehorton]

Hi all,

The unit shuts down now after about 10 seconds?
Have checked all wiring and looks good to my mind, can't understand why the output and gain controls don't work but I still get signal through?
I've taken a photo if anyone could help please.

Regards

Spence.

 

[Spencerleehorton]

The only thing I can see is that I don't think I have made the other 15v connection with the toroidal, but this connection has never been made and the unit has worked?
Any other things anyone can spot would be most welcome!

Spence.

 

[Spencerleehorton]

Well it seems d7 has gone and d107 as they both beep when in diode mode on MM, all other diodes check out, I have check them both ways, d7 long beep when touch with way, d107 single beep both ways.
Would d7 and d107 being faulty cause the gain and output control to not work?
The unit shutting down was due to the 78s12 not having the isolation material properly connected to the heatsink.
HT sitting at 233.6v, is this enough?
11.76v on the 12v

I have also connected the 15vac for the 48v now! In the picture it's before I connected it.
Will replace D7 and D107 and see if I get my controls back!!

Regards

Spence.

 

[museic]

Any help for my post from above here?

"Finished" my g9 about a month back. Channel 2 works amazing, definitely the best sounding preamp I have! It's amazing on anything I have plugged into it.

My issue is with channel 1. It has some kind of harmonic like distortion going on. I didn't notice it at first with the guitar playing lead lines, but once I started playing chords the distortion stacks up and the chords sound muddy and not particularly good.

I tried things like swapping the tubes between channels and swapping the controls to no effect. I also took some voltage measurements at the tube sockets and everything seemed fine. I did some searching and didn't see anyone with the same problem so I'm not sure what to look for next?

Also channel 1 is the farthest from the power transformers, I've done the di mod, used screened cable and my transformers are llundahl.

 

[Spencerleehorton]

i seem to have made a minor break through, bit of swapping round and i think ive got a working G9 with very little hum!!!
bass time!!!

regards

Spence.

 

[gyraf]

HT sitting at 233.6v, is this enough?
11.76v on the 12v

looks right...

 

[Spencerleehorton]

I may or may not have found another reason why i have hum?

on the schematic a 15v/0v/1v5 and a 12v/0v/12v toroidal is used, the 15v toroidal feeds the 12v one, i have used 2 x 15v, this is probably why i am only at 233v rather than 245v as per the schematic.
is this 12 or so Volts enough to introduce a bit of hum?
Im gonna try putting 12V toroidal there to see if the hum goes down anymore, not that the unit is un useable how it is at the moment as its so much better but as i turn up the unit i can hear the same hum still there just much much quieter than ever before.

regards

Spence.

 

[shabtek]

2x 15 is correct---feeding 15 into a 12 may burn up or saturate the transformer--barring that: there would be higher voltage on the "secondary" than 220.

is your 233 measurement dc or ac?

 

[Spencerleehorton]

233v DC on mine with 2 x 15v, the original schematic states a 12v and it should be 245v.

 

[shabtek]

the plans I have show 2 transformers, both "30va 110+110:15+15"
220vac in 1 15v winding goes to the filament dc supply --what do you measure here dc after diodes before regulator?

the other 15v winding is connected to paralleled 15v secondaries of the other 30va transformer, 'backfeeding' it for 220vac--what do you measure  here dc after diodes before regulator?

what happens to measurement s when 1 channels worth of valves are pulled?

not sure any of this is relate to hum--what are ac readings post regulators?

 

[Spencerleehorton]

Hi, shabech,

I will check all voltages that you have asked for tomorrow, been a busy weekend.
Do you have G9 and have you noticed any hum at louder levels?
For me it's important if there is even a little hum from the psu to get rid of it, especially as I want to use this for bass guitar and kick drum, as I will be boosting eq at around 100hz so don't want to be boosting hum up.

Regards

Spence.

 

[AyGuid]

Hi ! First post Here!! I was wondering if i could get some help with my G9 build. Will be posting a lot of questions!  ;D
First one... The 1.5Uf output Dc coupling caps.... I have some really nice 1.Uf caps,,, would it be ok to use them?
Thanks!!!

 

[Kari.M]

Good sound!

 

[gyraf]

The 1.5Uf output Dc coupling caps.... I have some really nice 1.Uf caps,,, would it be ok to use them?

There is a reason for most of the component values - at smaller caps you might loose a bit of low frequency response

Jakob E.