The official G9 help thread

[raul_ghiran]

Anything of ECC82/12AU7/5814 family produced before 1990

No, not at all

Thank you!

 

[raul_ghiran]

Finished my second Unit.
For this build I use a red frontpanel and OEP transformer IN and OUT.
Could not make a comparison with the Lundahls but the bass DI sounds a bit different in my opinion.
Now I’ll take a build break

Hi! Where have you made the front panels?

 

[TillM]

Hi! Where have you made the front panels?

I got this from Frank Röllen.
http://www.frontpanels.de

 

[biosynth]

Hi!

I've auto oscillation at high gain so I make this mod:
08. Jan. 2007: If you run into problems with oscillation on the highest gain settings, try running the connections to/from the front panel High-Z jack connector "directly" from the relevant points on the PCB - by cutting the PCB traces, and running a length of shielded cable directly to/from the jack. See this image for reference!!
http://www.gyraf.dk/gy_pd/g9/G9-EDIT.GIF

Now I've no signal (a small hum with really low signal) on the two DI inputs, so I think my work is correct. I wonder if this mod is ok with REV1 pcb. Can someone can confirm?
I'm disappointed my preamp doesn't work anymore (In DI input only, It works with mics)  :-\ :-\ :-\

thanks a lot in advance.

 

[gyraf]

You have most probably reversed to/from connections...

 

[biosynth]

Hi,
thanks for your quick answer. It was that, thanks a lot

But I scrupulously done your mod, so I think there is a mistake on the schematic. Could you verify?

Working with this color code (I think FROM and TO are to be swapped on the plug):
https://ibb.co/qNJVjy7

 

[gyraf]

The pic showing the edit still looks right to me - it might have been your actual jack connector that was mirrored? I've only (so far) come across connectors that work with the guiding pic as-is..

/Jakob E.

 

[biosynth]

it's an amphenol jack plug but other noname plugs are the same.
Only writing has to be swapped, not wiring.

All in all thanks a lot for putting your mighty preamp to the DIY world.  :-* :-* :-*

 

[raul_ghiran]

Hello. I have started my first G9 and discovered that the 12 volt stabilizer (L78S12CV) heats up to 63-64°C. And the cooler is at 60-61°C. Is that normal?
Thanks

 

[volker]

What is the DC voltage before the regulator?

 

[gyraf]

.. Raw unregulated HT voltage usually some 255 to 265V DC when heated up (i.e. under load). Usually ca. 234V after regulation..

/Jakob E.

 

[mcrouch]

Hi, I am trying to get my power supply voltages straightened out and I am thinking my toroidal transformers might be bad. I am in the US (120v mains). Transformers are 2* 120v primary: 18v secondary. I can get the voltage to step down to 18v, no problem. But when trying to get the transformers to step up to 240v I can only get 70 volts on each winding on the primary side (would give 140v in series). Based on the specs, the transformer should step the 18v up to 120v (not 70v). Is this a common issue with cheap transformers? Or am I overlooking something simple? Thanks! 

 

[raul_ghiran]

What is the DC voltage before the regulator?

Hi! The voltage before regulator is 17.5v. Another thing that might be weird is, on the C14 cap voltage is about 296-300v and on C15 240v straight. Is that Ok?

 

[Harpo]

... Transformers are 2* 120v primary: 18v secondary.

Have a look at the schematic. There is a reason, the 1st transformer has dual secondaries. For this build you for usual want two 30VA 110+110VAC:15+15VAC or :12+12VAC transformers (and not a single secondary 18VAC heat generator, partly shorting out BR2).

 

[mcrouch]

Thanks for the reply Harpo. I do have dual secondaries. My language was misleading, I have two transformers (30va) with 2*120:2*18 2*115:2*15

Also, i am testing without a load on the output, could this be the issue? Update: With no load I am getting 17 or 18 volts on the secondaries of the first transformer, which is normal based on regulation specs (caused me to forget I actually have 115:15 transformers).

 

[Harpo]

My language was misleading,

...so filling in a location in your profile might be useful. Not only mains voltages differ between countries/continents...

I have two transformers (30va) with 2*120:2*18.
Also, i am testing without a load on the output, could this be the issue?

As previously said, dual 18VAC secondaries wasn't called for and will be a heat generator.
AC mains voltage isn't a constant and might vary by +/-10%, depending on location or time of day.
18VAC is the secondary voltage at rated load, that will be higher by maybe 10% unloaded (look up the datasheet of your transformer for the voltage regulation % number of your mains transformer).
For a high mains and transformer unloaded scenario, this 18VAC will give a raw DC voltage in front of the 78S12 voltage regulator of 18VAC * 1,1 for 10% high mains * 1,1 for no-load * SQR(2) for fullwave bridge rectification = 30.8VDC.
Are your -from schematic 25VDC rated- caps C16,C17,C18 at least rated for this 30.8 DC voltage or are they already damaged.
The voltage differential between raw VDC-in and VDC-out times connected load (light and tube heaters, about 350mA) is transfered to heat by the voltage regulator (78S12). For a 15VAC secondary, heavy heatsinking was already called for. For your 18VAC secondary transformer, generated heat for 30.8VDC-in - 12VDC-out = 18.8VDC no load differential (or 16VDC differential at rated load) times connected load, you'd need a river to cool it down.
Your parallel connected 120VAC primary /18VAC secondary windings transformer is a factor 6.666 step down ratio. For the HT rail, this 18VAC winding connected to a 2nd backwards connected same type transformer will give you a 13.333 step up ratio with the primary windings connected in series for about 240VAC between the outer winding ends on paper. This 240VAC then is fullwave bridge rectified to about 340V raw DC.
In your shoes, I'd disconnect the mains transformers from pcb and measure these seperatly for their real world no-load primary/secondary ratio or voltage.

 

[mcrouch]

Thanks again Harpo...I had more issues with my post :-X, corrected above. Still have an issue stepping up to 240v, and am talking with the manufacturer. 

 

[sarakisof]

Hello folks. Could this work as output transformer or 114Ω is too low for 2400Ω?

114Ω : 27Ω
40-15K, ±2dB

 

[gyraf]

Ohm number does not compute by itself

you need to know the primary inductance, and from that calculate your low-frequency cutoff point

..or you need to know "recommended/marked impedance", GIVEN that specification was made with a certain frequency range in mind. In your case, if you have a -3dB point of 40Hz with 114 Ohms impedance, you have very little low end when driving it with the 2500 Ohms or so from the G9's ECC82 SRPP stage.

If mental exercises fail, I usually just try it out in-circuit. Sometimes to be surprised.

/Jakob E.

 

[sarakisof]

Thank you Jakob, yeap right, ohm numbers say nothing from themselves. I will finally end up with OEPs. But i am thinking of also trying a pair from a local well known winder old man here, for one channel, just for experiment. He can  wind them for me in any ratio and specs desired for really low cost, a few bucks both like. He usually builds power and power out trannies for valve circuits and is well known for this here. Different story i onow but he has also built some  in and out audio trafos in tube pres and mics for other guys with  good results as they say. So why not taste too?
I know he uses M6 core, open type and told me he could get about 20-20K, 0db ( ???). Didn't spoke a lot about inductance when i mentioned to.
So according to your scheme what would you suggest i could tell him wind? I know again it's very generalized and vague but just a starting point. Don't hurt experimenting i guess. And if they don't "pass the tests" i could probably use them for smthng else in the future.
Told him about 1:5 (600:10K) for input and something like 4:1 (2400:600) for output. 250-300mV and passed him the scheme. He understood we have to do with mWatts here indeed. What other points should i mention to him?

Peace.