The official G9 help thread

[Sergio]

Any suggestions on why I've got one channel on my G9 that isn't working?

Here are the symptoms:

-Output from the first channel does not have any gain.
      -With the Gain and Output settings at maximum I get about unity gain output. In other words I get an output that is the same level as the output from a mic that I feed in to it.
      -When I flip the phase (polarity) switch for the first channel I get no output. Nothing at all.

-The second channel is great. I guess that this isn't really a symptom, but I have no problems with the second channel which is producing nice clean signal with lots of gain, no hum, and very little noise.

Here are things I've tested or tried:

-I tried plugging the front panel controls from the second (good) channel in to the first (bad) channel. But I got the same results. So I guess that rules out a problem with the front panel.

-I've tried swapping the output transformers, but didn't get any change. (There was a post by someone with a similar problem a while back and it had turned out to be a bad output transformer for them.) Looks like both my output transformers are fine though.

-I appear to be getting the same voltage in the V1 tube sockets for both channels. I'm not really sure what I'm looking for here though.

Thanks again for any help offered here. I'd be lost without this help thread.

Ok.

You can change the tubes maybe one is broken.
After you have found out that the tubes are all right you should follow the signal path with a multimeter from one to the next component. Maybe you have a cold solder joint.....Check the frontpanel connection from the Point [ J1 ].
If there is nothing wrong you should check the resistors functionality and values....

Have fun and take care!

 

[sirblue52]

grounding problem?

New to this forum, lurking for a few months, but finally decided to post a question.  Really love this project.

So in my original build I failed to isolate the 78S12 from ground.  On power on I could hear the tiniest bit of hum coming through with gain 5 and output 6.  When I say tiniest bit, I could probably be convinced it was not there when mixed to a track.  The noise is louder when either gain or output level is raised higher towards 11 though.

Searching this forum on how to squash that tiny hum, I found that I was supposed to isolate the 78S12.  As it turns out, isolating it caused even more noise, in addition caused erratic behavior of the output dial - noisy and quiet spot.  Grounding 78S12 again turned it back.

I double checked that the XLR input was correctly grounded - yes.
I checked other 0V points on the board were leading XLR input ground - yes

I must be doing something wrong if I am not supposed to ground the 78S12....

 

[Sergio]

Hi Sirblue52!

I think the hum is normal because the power supply is not a high end design.
How loud is the hum? Can you quantify the hum in db on highest gain settings?

 

[gyraf]

Actually, the G9 powersupply is not bad at all, if made right.

I usually get diy-G9's to an equivalent noise distance of some 120dB, i.e. some 65dB s/n @ 55dB gain, unweighted..

But you need to take care of grounding (and make sure the whole chassis is grounded) - and be aware that some types of power transformers are quite noisy.

Jakob E.

 

[sirblue52]

I'll check on how to properly quantify the hum.  If anyone knows a quick and easy way, please let me know. 
I'll also double check that the whole chassis leads to ground.
Hopefully its that simple.  Thanks!

 

[Sergio]

Hi Sirblue / Aarhus!

I know that there are some G9 out there without any problem with hum and noise.
I had a lot of problems with my preamp. Maybe because i am not a specialist....
Now after two years of construction (not all the time) all my problems with the G9 are gone.

I have used two very cheap power transformers (4€ each) and they are not really quiet. So i decided to put them out of the chassis. After this experiment the hum was not gone but quieter and i have used the G9 on some records....After a while i decided to change the psu design by using my old phono pre power supply. I figred out that 12.6V for heating my tubes is better than 12.0V and the high voltage part should be exactly 250V and not 238V or similar.
After my mod i had a little problem with some noisy resistors but by now i have a unit with more gain and less hum (just a little at the highest gain settings).
This are just my experiences with the G9 and i think with some experiments you should get what you want.




Question to Aarhus:
How much gain delivers the G9 by maximum settings? On which position will this preamp deliver 55db gain?
How is it possible to check the linearity of the unit?


regards!

 

[gemini86]

Just a note, some tubes can be sensitive to heater voltage, but most tubes are NOT that sensitive to B+... the difference between 250 and 238 is negligible, in my experience. 

 

[Maynard]

Hi again!

Well, the preamp is working pretty nice so far but I am having some anoying problems with the voltage regulator TL783. A few days ago when no signal input start to hear like a far away low hit so I check the high voltage to the plates with an oscilloscope and saw how the voltage was not steady. It was fluctuating randomly a few volts up and down with an average of around 190Vdc.

Checking the TL783 data sheet decide to include a cap in order to avoid fluctuations due to the distance of C15, no result. Finally change the TL783, the voltage was steady at 220Vdc and no more anoying noises. But, after a few days the problem appear again. The fluctuations now seem less frequent but I feared it get worst since the TL783 might be degrading again.

Anybody experienced something like this? So far I have rechecked the PWS and everything seems right. Any suggestion is really appreciated.

 

[braeden]

Hi guys, I'm having a bit of trouble.

I've moved the power supply off the main board onto the separate B+ PSU provided. I believe everything is connected properly.

The first transformer supplying the mains voltage and it's first secondary winding (15Vac/0V) is connected to the 15VAC/1A connection on the B+ board.
That second secondary winding on that same transformer is then connected to the main board's second 15VAC input (for the +48V phantom power) which also has the second transformer's secondary windings also attached to it.

The CAP1 +/- section is connected to C14 and the CAP2/245V out is connected to C15 capacitor and the C15 CAP2+ input on the main board. The 0V from the PSU in the same section is connected to the 0/- part of the main board as well. I am getting a sufficient voltage at the HT link on the main board but I don't believe the +12V heater supply is working and the tubes aren't glowing. Have I connected the +12V out from the B+ PSU in the right spot?

I'm not sure what the problem is, I'm hoping this is easy enough to understand.

Thanks,
Braeden

If someone else has used the B+ supply could you provide photos of your G9 and how you wired it? Also, is it possible that the bridge rectifiers that I have used are of the wrong values and this could be preventing the heater supply from working?

 

[braeden]

Nevermind, problem solved... all hum issues gone as far as I know!

 

[gyraf]

the voltage was not steady. It was fluctuating randomly a few volts up and down with an average of around 190Vdc

Try changing the protection diodes around the 783?
Make sure that the second high-voltage reservoir electrolytic (the one after the 783) is in good working order.
Check input voltage to 783 while failing - should be at least 275V DC.
Measure HT current draw (dc voltage drop across the 470R resistors in series with HT)?

Jakob E.

 

[Maynard]

Thanks Jakob for those hints.

Actually I just built one channel and my power transformers are two toroidal 15VA (220 - 2x15) and may be I am a little short in that  :-\
I was checking voltage and currents like you suggested and you are right, I am getting around 240Vdc in the TL783 input, actually the transformer is giving around 200Vac......so, I guess it can not keep up with the juice requirements. Could that be the problem?
If I tested the power supply with no load the transformer output is 230Vac wich rectified gives me around 285Vdc. Also find out that at turning on the preamp the high voltage is around 245Vdc until  the tubes start to conduct and it falls to 220Vdc.

I though 15VA would be enough since two channels are working with 30VA transformers. May be I should go to 20VA. Thanks in advanced.

Got a second though since right now buying new transformers is over my budget. I was thinking about reducing plate voltage (wich already is....) to around 224Vdc replacing R35 and R36 for a single resistor of 560 Ohms. I hope that will make it easier on the transformers, not sure yet.
I know the load lines will change but I guess the bias point will be cooler and as far as I do not get too much distortion for cuttoff in early gain positions is OK with me. Could this high output voltage adjustment be viable? Thank you!

 

[gyraf]

Could this high output voltage adjustment be viable? Thank you!

It seems simple enough - try it and see (hear) what happens?

Jakob E.

 

[braeden]

It's finished! Thanks Jakob for wasting a year of my life! ... just kidding ;-)

... but seriously.

 

[Maynard]

Sorry late reply, didn't got too much time to deal with the problem  but it is resolved.
I adjust plate supply and the feeding improved in exchange for headroom. I recal power requirements since I did some mods to the original design and no way it needed more than 15VA....so as Jakob said I checked the zeners and susrprise one of those guys was open, no wonder why the TL was having trouble keeping up. Everything is smooth and quiet now! Thanks Jakob!

 

[redmojosteve]

Just got my G9 up and running. Both channels seem fairly similar in performance. One of my transformers (15V T2) seems to be getting very hot and I was wondering if this was normal. I haven't had this with other power transformers on other projects so far, apart from when I incorrectly connected my first GSSL! Both the transformers are 50VA. I am only getting 235Vdc rather than 245Vdc on the board. However T1 (12V) is pushing out around 280AC, which seems like it ought to be enough after rectifying for the regulator to work on.
My front panel instrument inputs need some working on so I might go for the direct to PCB mod before going any further.

 

[MeToo2]

Just got my G9 up and running. Both channels seem fairly similar in performance. One of my transformers (15V T2) seems to be getting very hot and I was wondering if this was normal.

HT voltage looks normal (280V). 235V sounds low. But voltage drop doesn't say much due to the regulator.

I would say that was definitely not normal for a 50VA transformer to get hot in a G9.

Check the current draw on each of the different portions of the PSU and then we can compare values.

You can check the current draw:
on the HT indirectly by measuring the voltage drop across the 470R R33 using a multimeter in voltage mode.

on the 12V heater directly at the PCB connector with a multimeter in AC current mode (be careful! and only connect and disconnect with power off and reservoir caps C14 C15 discharged) Check the heater voltage is 12V.

on the 48V directly at the PCB connector with a multimeter in AC current mode (be careful! and only connect and disconnect with power off and reservoir caps C14 C15 discharged) Check your 48V phantom is 48V.

Check that the mounting bolt of the transformer is only touching the chassis at one end. Check you've got your rectifier diodes correctly oriented. Also the caps on the voltage tripler can be confusing with their orientation.

You may some oscillation at high gain settings if you haven't done the PC board mod. Does the transformer still get warm at lower gain settings? But I'd get the thing stable at low gain settings first before doing any mods.

 

[redmojosteve]

Thanks for the helpful response. I haven't got the time to do the checks right at the moment but I will do as soon as possible. As a quick reply, I can confirm I am getting 48V and I have checked the caps and diodes and they are in the correct orientation. I didn't really get much time at high gain settings as I was concerned that the transformer was a bit hot even at low gain settings. There was some oscillation at high gain. I haven't bolted the transformers in place yet as I was going to complete that task when I was sure they were in the best location to keep hum pickup to a minimum.
For information the unit did pass audio signals both from the line/mic input (from a sig gen) and also the inst input (a Gibson Les Paul with P90 single coil pickups). Both gain and noise seemed reasonably satisfactory for a first trial  but I am sure it isn't right!
Thanks for your help
Steve

 

[MeToo2]

Thanks for the helpful response. I haven't got the time to do the checks right at the moment but I will do as soon as possible. As a quick reply, I can confirm I am getting 48V and I have checked the caps and diodes and they are in the correct orientation. I didn't really get much time at high gain settings as I was concerned that the transformer was a bit hot even at low gain settings. There was some oscillation at high gain. I haven't bolted the transformers in place yet as I was going to complete that task when I was sure they were in the best location to keep hum pickup to a minimum.
For information the unit did pass audio signals both from the line/mic input (from a sig gen) and also the inst input (a Gibson Les Paul with P90 single coil pickups). Both gain and noise seemed reasonably satisfactory for a first trial  but I am sure it isn't right!
Thanks for your help
Steve

OK if it's that bad a simpler test is just to disconnect all 3 power supply connectors to the PCB (assuming you're using the Gustav's PCB) and see if the problem clears for the transformer getting warm when powered up with no load. And then connect the connectors for the supplies one at a time. 1st the 48V on its own (2 pin connector on side of the board). Then the heater on its own (2 pin connector on the back of the board). Finally the HT (3 pin connector on the back of the board) + heater. Then check for excessive warming each time. Shouldn't cause any problems to run them one at a time and check like this, but be sure the HT caps C14 C15 are discharged before connecting/ disconnecting. Also if the heater current isn't flowing properly the HT supply voltage might measure high (as there'll be little load on the HT regulator which needs ±15mA before it starts regulating)

 

[redmojosteve]

At last I have been able to get back to the G9. I am using Gustav's board incidentally. I rechecked my transformer connections, swapped a couple over and I can confirm the voltages on the PCB seem OK, that is, 12V, 48V and 265V on the HT. Following discharge of C14, C15 I have disconnected the various connectors and with no load the 15V toroid is still getting warm. I then disconnected the other 15V secondary (which feeds the 12V secondary on the other transformer and provides 15V to the side connector on the board, thus providing no load at all to the transformer) and it runs cooler.  It doesn't produce any mechanical hum either. On this basis it would seem that I have either got something connected wrong between the two transformers or the transformer just gets rather warm.  Thanks for the wisdom and support
Steve