The official G9 help thread

[gyraf]

240VACis some 339V DC after rectifier.

If you have 380VDC, you have at least 270V AC

The zeners are described in the G9 circuit-run-up: they protect IC from differential in/out voltages higher than the 120V it can chew.

you probably want to keep R37 before C15in order not to load the zeners too much at power-up - if they give in, the ic dies..

TL783 is rather sensitive when used at high voltages - may be dead?

ps: for G9 you need some 20-30mA max...

Jakob E.

 

[Spencerleehorton]

Hi,

Just tried some ECC81 's in my g9 and it sounds amazing,  had some dragon ecc82 in there from eBay which didn't sound that great.
Made such a difference, really clear punchy sound with guitar plugged in the line in, only needs a touch of reverb.

Regards

Spence.

 

[andow]

I finally finished my G9 - voltages are ok and signal passes through  8)

However, I am having troubles with the Lundahl output trafo (I think):
I am sending a sine with 70mVpp into the Instrument input and trace the signal with an oscilloscope. One side of the trafo puts out 6Vpp and the other one only 350mVpp with the same polarity! (measured on either side of R32 to ground)
I also tried disconnecting the switchboard to make sure it doesn't have any influence on the output of the trafo, but no difference...
This behaviour is the same on both channels

Any idea what might be the problem? Thanks in advance!

 

[andow]

Nobody? 

 

[Harpo]

One side of the trafo puts out 6Vpp and the other one only 350mVpp with the same polarity! (measured on either side of R32 to ground)

There is NO reference to 'ground' (whatever your idea of 'ground' might be). You measure across the transformer.

Any idea what might be the problem?

Ignoring previous answers already given in this thread ...

 

[andow]

Thank you very much for the reply! Of course it is not referenced to ground, how embarrassing  . I must have spent too much time in the basement inhaling solder fumes....

That leaves me with a buzzing trafo and one relais that's not switching, but I'll get there 

 

[sam system-d]

Hi guys,

I've opened my G9 today just to check if everything was OK, it has been used by friends in studios because I was on tour.

I've got 4 burned resistors :

R9, R25, R109, R125.

47k for everyone.
I was wondering if anyone had this issue before ?
Any idea on the issue ? Short probably ? I think the resistors were 1/4W.

Thanks for your help.

 

[gyraf]

Does it still work? If so, don't worry.

These resistors are for anode load, and dissipate some 0.2W each - so they will be browned over time in normal function.

Measure them? If they're about right, don't worry.

Only thing that can blow them is way-too-high HT voltage - you may want to check that voltage under load (=unit heated up) to be around +245V DC

Jakob E.

 

[sam system-d]

The guys that used it didn't feel any changes, so I will just replace them.
I will check the HT this afternoon. Thank you for replying, just to let you know that I'm currently bulding my second one with Lundahl transfos  .

 

[hoeye440]

Recently I inherited a half finished g9.
I found time for fitting the parts of the puzzle  - that is as far as my knowledge about electronics goes...

What should I do next? Can someone point me to the right info about what/how to check/measure before trying to hook it up...

 

[Jamie1234]

So... I have finally dug out my G9 which has been sitting around for about a year as it had a few problems I didn't have time to solve.

I had a strange crackle on one channel, which sounded like a dodgy valve and wasn't, and a very low level on both channels.

I've just jumpered out the instrument input and the strange crackle has thankfully gone, but I still have my level problem and I was wondering if anyone might have any answers?

It's really quiet - almost no level on dynamic/ribbon mics, and I have to basically swallow a condenser mic and shout to get my voice to show much level at all on my converters (phantom power is working fine).

I've just spotted that the output gain pots I used were 50k rather than the 47k specified (no idea why that happened), do you think this could be the cause of the problem? I don't have any 47k pots at the moment to check.

Or if not where should I start looking?

Many thanks

Jamie  )

 

[bernbrue]

Hi,
50K pots are fine and definitely not the reason for your low output.  Have you checked voltages, especially the B+ and heater voltage?  Which input / output transformers did you use?  Check the wiring from main board to control boards.  A picture could help as well.
Bernd

 

[stmartin]

Hello out there,

I would like to build a G9 preamp. But there are some question…..

Is there a G9 kit out there? Should I use Oep or Lundahl transformers? Where can I buy a good case for the g9?

Thank you and regards.

 

[andow]

Today I did the final tweaks to my G9 and finally I can say: it is finished  ;D
Thanks to everybody who helped me out and of course to Jakob for making this project possible!
And becuause I am so proud of it here are some pics: 

https://www.dropbox.com/s/pacvkxf4tpnw5tq/20150407_115400_800x600.jpg?dl=0
https://www.dropbox.com/s/8xghh0no3vj4e70/20150407_115430_800x600.jpg?dl=0
https://www.dropbox.com/s/by6v7wuj23p6trp/20150407_115508_800x600.jpg?dl=0
https://www.dropbox.com/s/bjvym10ilbn45pg/20150407_115537_800x600.jpg?dl=0
https://www.dropbox.com/s/iyl035igbdy1nun/20150407_115621_800x600.jpg?dl=0
https://www.dropbox.com/s/6ok13rgxy905d0z/20150407_120102_800x600.jpg?dl=0

 

[Jamie1234]

Hi,
50K pots are fine and definitely not the reason for your low output.  Have you checked voltages, especially the B+ and heater voltage?  Which input / output transformers did you use?  Check the wiring from main board to control boards.  A picture could help as well.
Bernd

Thanks Bernd.

I've been checking it over for incorrectly orientated/value components and no luck so far. It was pretty much the first thing that I ever started building so I'm expecting the cause is some sort of silly mistake but can't find it yet and I spent most of yesterday looking. I've triple checked the wiring between the main board and the control boards and it's all good. I've also replaced all the shielded cable for the input/output XLRs.

My HT voltage is reading 248V and heater voltage 12.06V, which I'm guessing is ok. I've used Lundahls on input and output.

Here's a bit more info just in case anyone has any ideas:

I have really low level on both channels. With everything set to maximum I still can't get my loudest condeser mics to show above about 40% on my interface even with shouting/ close miced snare etc and I can barely get dynamic/ribbon mics to register a signal. Phantom power is working fine.

As I turn the input gain up from 0, the level increases up until the 4th or 5th click, and then gradually decreases (and gets fuzzy I think) the further clockwise I turn it. I expect the fuzziness with the gain set to max but I don't know why it's getting so much quieter. Also not sure why both channels have exactly the same problem if it's something to do with the gain circuit.

I originally did the mod with cutting the traces/ adding shielded wire for the instrument input, but I have since jumpered out the instrument input on both channels as I thought this may have had something to do with it.

I have bypassed the phase switch and the HPF on channel 1 to see if it made any difference but nothing changed.

Output gain pot seems to work fine (although I've no reason to turn it less that maximum at the moment!).

It was about a year ago I started this and it's been shelved since, but I seem to remember when I first put it together I had some cock up with the power transformers (but can't remember what exactly), so I may have fried something then (but don't know where to start looking).

Hopefully someone may have some answers!

Cheers

Jamie

 

[bernbrue]

Hi Jamie,
A few things you should verify, before going into detail.
1) Are the relays working? Listen for the clicking when changing from line to mic. 
2) repair the hpf, since audio passes through these caps
3) your first gain stage doesn't seem to work correctly, check if the front panel pcbs have a proper ground connection
4) check wiring and orientation of the 5 pin and 10 pin connector. I'm quite shure the wiring of the 5 pin connector is somehow messed up.

Regards
Bernd

 

[Jamie1234]

1) Are the relays working? Listen for the clicking when changing from line to mic. 
2) repair the hpf, since audio passes through these caps
3) your first gain stage doesn't seem to work correctly, check if the front panel pcbs have a proper ground connection
4) check wiring and orientation of the 5 pin and 10 pin connector. I'm quite shure the wiring of the 5 pin connector is somehow messed up.

So..

I have repaired the HPF and phase switch, and I'm now back to where I started.. 

The relays work (or at least they sound like they do).

All the connections to the front panels seem fine - I've checked with my meter to make sure that there is continuity along each wire from one board to the other.

The only slight confusion I had was with the 10 pin connector - The out +/- and transformer +/- connections are swapped around on the 2 boards. I originally had all of the wires in the same order at both ends as I didn't read the diagram properly, but as far as I can tell it wouldn't make any difference other than which way it would go through the phase switch.
Here's how it's hooked up now, presumably this is correct -

I've checked and checked and checked and the 5 pin connectors seem to be hooked up properly. Obviously I've only got 3 wires connected to it though as I haven't got the instrument jack connected.

Here's a pic of the underside. Ignore my scribbles sorry, that was just me trying to follow the traces and work out where things went. You can see where I've cut the traces to do the shielded wire mod for the instrument input, and the 2 bits of wire I added to bypass the instrument inputs when I removed them.

I've checked everything I can think of now, but still no luck.. 

 

[bernbrue]

Hi,
I can't see any pictures. Have you soldered in the wire bridge next to the 5 pin connector?  Actually it's the only way the front panel pcb get a ground connection.
Bernd

 

[Jamie1234]

Yes I have soldered the wire bridge.

On my screen the pictures are showing up as a blue square with a question mark but if I right click on them and 'open in new tab' they seem to work fine.

Thanks again for your help.

Jamie

 

[Gustav]

Fried D15 on a build, and after a little trouble shooting, it ended up working with a simple replacement.

I would like to know whats happening here, so a question.

1N400x is rated for an average of 1A. So is this just a case of being on the edge of that capacity and stumbling on a part rated in the low end?

A search here revealed that others have had the same problem, and solution (simple replacement). Would it be wise to upgrade to a 1N540x in this spot?

Or is this a symptom that my transformer is underpowered (should have 1.5A, according to the specs).

Gustav