The official G9 help thread

[andow]

Ok guys, I solved my C4 problem  8)

Here is another short question regarding the rack case: I think I'll get one with a depth of 220mm, do you think this is enough or would you recommend a bigger one?

 

[APL]

I used 225 mm wide and you can get everything in, including separate HT board, but if you have option I would go wider. I gives you more flexibility where you place things and a little more distance between transformers and he sensitive parts.

 

[andow]

I used 225 mm wide and you can get everything in, including separate HT board, but if you have option I would go wider. I gives you more flexibility where you place things and a little more distance between transformers and he sensitive parts.

Thanks for the fast reply! I'll go with 250mm then...

Meanwhile I received the PCBs from PCB Grinder and am kind of surprised that there is no spare PSU board included. Would have been nice since I'll do the PSU section off the main board...
Also it says rev#1, is this ok? I thought the PCB has been revised?

 

[Alexandru marian]

The HT bit is really simple, you can build it on a piece of plywood in a few minutes. 

 

[andow]

The HT bit is really simple, you can build it on a piece of plywood in a few minutes.

Sure, no problem. Still it would have been neater on PCB...

 

[Justin Mayer]

Hi everyone. This is my first post. I'm finishing up building a G9 and I have only one problem. On the highest gain setting the right channel completely cuts out. Is that caused by oscillation? I have not done the mod with the cut traces and shielded cable.
Thanks, Justin

 

[gyraf]

Try with the trace mods. It seems to work very well - and is recommended.

Let us know.

Jakob E.

 

[collinTHEbrewer]

Hey guys. It's been a while! Finally solved my regulator heating issues by removing the IC from the PCB and attaching it to a heat sink mounted in the chassis. I made sure to isolate it an such, but removing it from the PCB allowed me to use a much larger heat sink.  8)

Now I'm on my next issues. Channel 1 is getting some weird buzz at certain OUTPUT settings. When the knob is at 12 o'clock, it is silent. As I turn the output up, noise appears, but here's the weird part... When I turn it up all the way, the noise disappears! The output volume still increases as I turn the control all the way up.

My other issue is when I'm using my SM58 to test the preamp. I get some noise (oscillations?) at the last two gain settings. This only occurs when the SM58's on/off switch is in the off position. I understand oscillation can be an issue, but I honestly don't know what that sounds like. I also think this is a weird problem that won't affect me with most mics, but this doesn't happen on any other preamp... 

Alright experts... any ideas on where to start?

 

[andow]

Hey guys, just a short question regarding bleeder resistors over C14 and C15:

If I calculate the Watts needed for an 1M resistor over the 245V I get 0,06W. I guess this means I'll be fine using my standard 1/4W metal film resistors, am I right? (Even if the IC doesn't regulate and there are 325V over C15 I just get 0,11W....)
Just to make sure...

 

[collinTHEbrewer]

Since you're talking power supply, I would maybe just go 1/2 W just to be safe. But that's just me!

 

[andow]

Ok, thanks! That makes sense of course. It's just that I don't have any 1/2 W resistors at home....

 

[collinTHEbrewer]

Now I'm on my next issues. Channel 1 is getting some weird buzz at certain OUTPUT settings. When the knob is at 12 o'clock, it is silent. As I turn the output up, noise appears, but here's the weird part... When I turn it up all the way, the noise disappears! The output volume still increases as I turn the control all the way up.

My other issue is when I'm using my SM58 to test the preamp. I get some noise (oscillations?) at the last two gain settings. This only occurs when the SM58's on/off switch is in the off position. I understand oscillation can be an issue, but I honestly don't know what that sounds like. I also think this is a weird problem that won't affect me with most mics, but this doesn't happen on any other preamp... 

Alright experts... any ideas on where to start?

Anyone?

 

[gyraf]

No guess from symptoms. Go through the noise-reducing ideas in this thread...

Jakob E.

 

[Pusch3l]

Hey, thats my first Tube MicPreAmp build and i´d like to share some photos with you.

Board is almost fully loaded. just C25/C125 and the XFR are still missing.
Did the Trace-Cut-Mod and hooked up the Output-wire directly to PCB.
Will do that with the Inputs too. Wire used is Mogami Neglex.
Hope i am not running into any Noise problems when running the cables
underneath the PCB.
For the 220n Polyester Caps i used 0.22/630v Axial ones (LCC/Mullard?) i had laying
around. hope this works out
the 78S12 will be Desoldered as soon as the case arrives and screwed on the back of the case.

But let the pics speak for themselve





Tell me what you think!

Greets

 

[bruce0]

You probably won't notice it because the output are low impedance and not amplified, and thus are not particularly sensitive to noise.

But be aware that the back center and back right of the PCB are the PSU.  Those four diodes are a bridge rectifier and the AC mains input is there too.  It looks like you are running your output cabling right under those.

As a general precaution you should try to have cabling avoid those areas.  In particular you should not run cables close and parallel to AC mains because of picking up hum.

So for the outputs, rather than running them under the PSU, you could run them in an L shape along the front and then back.  That way you will be under the DC portion of the PSU and that would be better.


One good thing about running the cables under the PCB is that you will need to use nice high standoff's.  And that will move the sensitive DI circuitry and input transformers farther from the case, which is good to avoid noise coupling with the power transformer.

 

[Pusch3l]

Thanks for the hint with the AC mains.
i will run the cables around that area to avoid running into an open knife.

The standoffs can not be very high because i am sticking with a 2U unit and it doesnt look
very pretty if the tubes reaches over the top of the case 

so no complaining about the 150M axial caps and i can stick with them?
that saves me a lot of headache  ;D

 

[gyraf]

...no complaints. Just make sure their wire ends do not short to the wire-links on top of the PCB...

Jakob E.

 

[late]

I have noise problem with the plug inputs. When wire is connected to plug i got some noise. If i remove the cable from the plug and connect wires together. Then the noise is gone and sounds good. Do i need better plugs or what this mean? Have did mods and it helped a lot.

 

[gyraf]

This is normal behavior for the instrument inputs. They must be terminated to a source to be quiet. This actually also goes for mic/line inputs, but on a different scale.

Jakob E.

 

[buildafriend]

Hey guys,

I self etched this circuit and I'm not sure why I'm getting the wrong results. I was hoping maybe someone in here could help me find what I'm doing wrong.

I made a few changes so maybe it's because of my substitutions?

I used a 240VAC secondary rated at 130mA (instead of 250V @100mA)
The chip I used is a TL783C
The electrolytic caps are 270uF instead of 220uF
R37 has been put post C15 (not sure how much that matters)

I think everything else is spot on.. The problem is I'm getting 380VDC on the output, instead of the ~290 that I want. This is the same amount that I get at the output of the bridge rectifier. It's not shorted, I checked that.

While I'm asking questions here, what are the zener diodes doing?

Any guesses? Maybe change the values of R34, 35, and 36?

Thanks,
-JP