The official G9 help thread

[ognam2]

Does it make sense to mount the power trafo to the case from the outside?
I mean to prevent Hum Problems....

Jonas

 

[steppenwolf]

I really don't understand, why you are so concerned about hum and noise.
I have two toroids inside the case and even on full gain no hum. A little bit of white noise but every high gain preamp does that. The G9 is a really quiet device.
Of course, if you analyse it with spectralab you'll notice some 50 and 100hz hum, but who cares if it is down -75dB?
Even my AD converter has 50hz hum, but at -120dB.
If I can't hear it on full gain, I don't care much.
By the way:
I have a little bit of hum, too, when I have a XLR cable connected without a mic attached. If you connect a mic, you have an impedance of only 200ohms on the primary, a lot less than without the mic.
Try to short Pin 2 and 3 of the XLR to ground and listen again. Is the hum gone?
Best,
Stefan

 

[ognam2]

well... I just thought "why not" ... because some people seems to have problems with hum and Jakob E. describes this method to search for the right place for the toroid...
So why not place it outside of the chassis?
I just want to know if there is any reason not to place it there....

Thanks Jonas

 

[steppenwolf]

If your toroid isn't potted, there are flying leads coming out of it that need to be routed through the chassis. I think, taht could be a bit vulnerable but I can't see any electrical reason not to do so. But I would use a potted one...

 

[ognam2]

it is in a black plastic pott and the cables are isolated....
to cut a small hole in the chassis to put the cables to the inside is not the problem.
Do you think this is a stupid idea?

 

[steppenwolf]

Well, I don't think it's a stupid idea, but I wonder, if it will be much better hum wise.
I would try that, if I run into problems using it inside the chassis, but I don't think that toroids radiate that much...
I just think that it is more rugged having a piece of ewuipment all in one box with no trannies or tubes on the outside, rack depth might cause problems, too. Are you thinking of bolting it on top or behind it (LA2A style)?

 

[ognam2]

in behind.
It fits perfectly to the size of the chassis.
Maybe I try....

 

[jarlehal]

Hello DIYers

I'm putting together a G9 - and ran into trouble. I'm quite unexperienced - so how deep I do not know yet :wink:

Well, I thought I'd ask here to see if any of you had run into this:

Using mic-input, when I turn the gain switch to the 11th and final step I seem to lose almost all gain (as I was back to 1st pos)!! Then when I plug in my SG and switch to DI/LINE, it works normally all the way up. This is true for both channels and both my control boards. Same mistake twice?? Arghh.. Haven't found any shorts or wiring mix-ups, but I might have overlooked something.

Any clues?

I thought I'd fix this one before I go further dealing with hum, osc and a really loud buzzing toroid. Speaking of that, Im using a 12V 50VA (step-up) and a 15V 30VA. Are you guys able to reach the required (regulated) HT with 2x 15V 30VAs?

And I read steppenwolf is using a resistor to somewhat limit the source for the step-up, and cured the buzz. Good idea?!? I'm a little afraid of messing with the toroids here..

EDIT: Cut the traces and connected the instrument-jack using shielded cable, now my gain switches are cured!!

 

[riggler]

Hi,

Can I use 31k resistors for R26 and R126 instead of 47k?

THANKS!

 

[riggler]

bump.... see my previous post in this thread please.

 

[CurtZHP]

Well, gentlemen, it's been a long time, but I finally have everything I need for the final assembly of my G9. The OEP iron showed up this week, and I might be ready for a smoke test by Friday if I don't lose my nerve.

Here's a question:
How critical is the spacing between the PCB and the large 2W and 1W resistors in the HT supply? I've seen no mention of that here, and I was wondering if I needed to worry about leaving a bit of space under the resistors to prevent them heating up the PCB. Any thoughts?

Once I have completed the final assembly, I'll be sure to post detailed pictures before the smoke test. I want several more pairs of eyes on this before I let the smoke out!

 

[flintan]

[quote author="riggler"]Hi,

Can I use 31k resistors for R26 and R126 instead of 47k?

THANKS![/quote]

Why not just stick with 47k? It's not a very uncommon value...

 

[CurtZHP]

One more question regarding the 78S12 regulator. I'm bolting it to the chassis for heatsinking purposes. Since the backplate of the 78S12 is basically its ground (same as pin 2), do I still need to bother with electrically isolating it from the chassis; or can I just put some heatsink grease under it, bolt it down, and call it a day?

 

[Harpo]

[quote author="CurtZHP"]One more question regarding the 78S12 regulator. I'm bolting it to the chassis for heatsinking purposes. Since the backplate of the 78S12 is basically its ground (same as pin 2), do I still need to bother with electrically isolating it from the chassis; or can I just put some heatsink grease under it, bolt it down, and call it a day?[/quote]
Doing so without isolation might give you a ground loop. just my 2ct.

 

[CurtZHP]

Here it is! My finished G9!

To see more pics:
http://www.zerrohouseproductions/preamps.html

I'd like to get everyone's $0.02 before I fire it up.

[/img]

 

[Deeply Silent]

Hi everyone, firstly thanks for all the valuable info to be found on this forum.
I have nearly finished building the G9 and have a question about adding 1/4 inch jack inputs and outputs.
Iv found a thread about this but the answer wasnt too clear - http://www.groupdiy.com/index.php?topic=13127&highlight=xlr+jack+cable
So with the output I can connect ground to the jack sleeve and the tip to P2 (hot) on the xlr? That would give me unbalanced out right? Now what if i connected the out to balanced TRS setup and then a mono jack was inserted would it be a problem if P3 (cold) on the xlr are shorted out?

Similarly with the input can i connect the xlrs P1 to the jack sleeve, P2 to the tip and P3 to the ring? Again would inserting a mono jack and shorting the ground to P3 (cold) be bad, I'm guessing so with the phantom power??
If not, could I connect the inputs in an unbalanced way for line in - ground to sleeve an hot to tip?

Sorry for the lengthy post and if the answers are obvious..

 

[CurtZHP]

It's DONE!!

I finally got up the nerve to fire up my G9. I plugged it in, flipped the switch, and immediately the fuse blew. :?

After a bit of head scratching and rechecking my work, I found the problem. Next to the main PCB I put a small two position solder terminal strip to facilitate the wiring for the main power transformers. It appears that I failed to notice that one of the terminals was same piece of metal that formed the mounting bracket for the strip. So I was basically connecting one side of the transformer's secondary to the chassis. Talk about an "Amateur Hour" mistake! I cut all the wires loose, soldered them together without the terminal strip (covering the joints with wire nuts), and prayed that I hadn't cooked the transformer.

Flip the switch. IT'S ALIVE!! :grin:

I let it burn in for a few minutes and then started checking supply rails. The 245V rail was showing 235V. I assume this is within tolerance. The 12V rail was showing 11.72V, and the 48V rail was showing 49.2V. After letting it idle for a while, I connected a tone generator to the left input and a small powered speaker to the output. Good tone. Exercised the controls. All good. Switched everything to the right channel. No audio. :?

Careful examination of the right channel control PCB revealed the problem. I had neglected to solder the center pins for the "phase" switch, so no audio was passing through it. Another "Amateur Hour" mistake. A quick touch-up with the soldering iron, and the right channel is in business.

Tried it with a Shure SM-58 just to make sure it worked, then with an Audio Technica AT4040(?) to check the phantom power. I also connected the tone generator via the instrument inputs. All is well! No hum, no oscillations, no weird noises, no smoke.

So it looks like I'm the latest proud new owner of a Gyraf G9 tube mic preamp!

A HUGE "TAK" to Jakob and all the rest at Gyraf. Thanks to all the guys here on the forum for your help as well. I'd also like to thank the Academy.... (Oops! Wrong speech!)

I can't wait to put this thing in action.

:guinness:

 

[gyraf]

Congrats!

Now go make some great music...!! :razz:

Jakob E.

 

[riggler]

My G9 is almost done and I'm about to order power transformers. I'm going to get Avel Y236103 30VA toroids. I'm in the U.S. and I've read elsewhere in the forums here that some people got 50VA toroids instead of 30.

Why is this necessary for some, but not others? Is this just a preference for more stable voltages / cooler running?

My bottom line question is: Should I stick with 30 or get 50VA transformers?
I can't find the thread mentioning 50VA transformers now, but I do remember reading it!

THANKS GYRAF FOR THIS ROCKIN DESIGN! :green:

 

[CurtZHP]

I used the Amveco toroids from Digikey. The model number is 62073, and they are 30VA.

In my initial tests, they seem to run pretty cool and all my supply rails are within tolerance and don't seem to fluctuate at all.

Somewhere on this thread (or elsewhere on the site) someone was kind enough to provide a detailed diagram of how to wire them to the main PCB, complete with color codes.


BTW, where in Pennsylvania are you?