The official G9 help thread

Hi guys.

I talked to David from Cinemag yesterday to know which one of his transformer would work best has input transformer, and he gave couple of advise, by the way. He's really a nice guy. So, his personnal choice would be the CMMI-7C, instead of the CM-75101A. The CMMI will get better overall performance with a larger choice of microphones, even at lower spl. But we have to strap a load resistor on the secondary to adjust the impedance. Nominally, we want the input impedance to be around 1.5K to 2K (ten time the mic impedance) so in a 1:7 configuration, the output should be 2K (primary wired at 200ohm * 10) x 49 (the square of the ratio) = 98K (100K). Just take a look at the schematic I linked.

Looking at the output transformer, He suggest me the CM-2820 (2:1). I looked at the Edcor WSM 2.4K/600, too. I would like to know if somebody have tried this model from Edcor in output? What is the difference between the WSM and the XSM (instead of the core) in the G9? Is the WSM will work fine??

Finally, he point me out about C13 (4u7) being too small, resulting in a bass roll off around 100hz. He has a similar unit with a 10uf that roll off at 100hz and he just strap a 22uf in parallel and it's ok. Maybe folks who encounter the roll-off problem should give it a try? not that hard to made (and really cheap too) ;D

The G9 does not have the mentioned 100Hz roll-off, so there is clear a mis-calculation here.

Why not use the recommended, tried and tested LL5402?

Jakob E.

Thanks Jacob. The only reason why not using the lundhal is that i personnaly like cinemag transformers, and because i live in Canada, shipping is always a big issue. So far, cinemag isn't that expensive to ship here, and for those who want to try another trannys, cinemag could be a nice option.

For the roll-off, it seems to be an issue with several persons building the g9, personnaly don't know why. Maybe a miscontructions, who knows. David from cinemag just let me know that he had a similar unit on his work bench who had a roll off at around 100hz and eh just strapped a 20uf in parallele, so maybe the folks who encounter the bass roll of should try it out, since it's really not a big deal to make. G9 is an awesome project by the way.

by the way, what is approximatively the load input impedance of the OPT?? Can we figure out 2.4K?

Depends on transformer output loading.

With LL5402, I get a -3dB-point of ca. 30Hz with a 4u7 output coupling capacitor, pointing to some 1.2KOhm (although this figure must be taken with a big spoon of salt).

Jakob E.

ok cool. thank you. Anyway, I think this cap should be adjust according to our transformer choice and what we want to achieve

Finally Got rid of my noise problem.. Turns out The screening cans wasen't properly installed.

With that said, I've now Got a new problem.

Channel 1 is way louder than Channel 2.
Tried swapping tubes and the front control boards, same effect.
The DI dosen't have that issue, It's only when using a mic.

I've checked every risistors, and all of my voltage is perfect.

Any suggestions ??

S

Stupid me...

Switched one of the output tranny to the input...

Super noob question. Should the tubes glow without any signal passing through the pre-amp?

Just powered it up, voltages are a touch high, 260V AC instead of 245 coming off the primary of the toroid into the main board, but apart from that nothing burning but nothing glowing either  ???

Regards

Gareth

From the moment you fired up the unit, yes the tubes should glow. Even no signal pass through the circuit. The tubes are feed by the filament circuit so if nothing glow, you can check that out. Don't Forget it's not a christmas tree, the tubes won't glow as this. Close the lights and Check if you see the filaments inside the tubes glowing.

Ah, thanks Deepdark, I was waiting for a christmas tree display  : They are glowing ever so slightly, so on to testing it with signal now....!

Thanks, Gareth

You're welcome hope everything will works fine. Let us know!

Unfortunately it doesn't work  :-[ powers up fine, channel 1 does nothing, channel 2 just sends out a distorted hum/buzz signal like something is shorting. Everything looks fine, I'd even rebuilt most of the PSU after I blew it up the first time with a reversed cap.

Where do you start looking to diagnose the problem? All joints and wiring look fine. Also how do you connect a multimeter when its not powered on? There is not enough room to clip on to a lot of places, seems it has to be on then done with care...

Gareth

Hi, i am having some problems with my G9(one channel for now).It has been working wonderful with no hum or noise, but several days ago, it started making some hum when the Output is full right

Hum is -76 dBs FS measured with a 24dBu input converter, it may go to -66dBS if i move a little the gain knob(lorlyn, i think is kind of broken)
When i set max gain i have -58dB FS with a little hum, which 1 month ago i didnt have any kind of noise problem.

It is not much noise, but when i use mics which needs heavy amount of gain(like my D87 clone) i can hear it in tails of voices(and i dont want to).

Would it be a Switch problem?(it makes noises each time i turned 1 step)
Or Would it be a Tube problem?  I leave G9 turned on all the day in the studio as my other gear
Maybe a ground loop problem? Didnt have problems earlier with this.

I have already cut traces to eliminate hum a while ago, but now i have this problem.... what should i look for?

Edit: Some months ago i needed less gain for everything so i dont know if it is tube related, i dont have other tubes to test out.

Thanks in advance

I finished building my G9, and it worked on the first power up No explosions or smoke!
Freq response is very good, -3dB point is at 30Hz when using the XLR line input or DI instrument.  I used OEP transformers.

But I have a little problem with the HT voltage. I have 2 15V 30VA transformers, but after the second transformer, the voltage is only 200VAC.  After rectifying, C14 charges to 240VDC and after the TL783, C15 is only 230VDC which is the HT voltage for the tubes. From what I read, this is due to transformer losses and I should try a 12V transformer for T2 to get the HT voltage high enough to be well regulated.

But still, it does work. Are there any gain/sound color benefits from running the tubes at 245VDC instead of 230VDC?

Thanks!

230 should be in the ballpark. If it sounds good to you, I think I wouldn't be upset with this little 15V loss

..unless you get hum from loosing psu regulation. At some low point of HT voltage, you'll run into this.

Jakob E.

quick one. Am I ok if I use a SPST for the main switch?

Regarding switch question  I  cannot give a definitive answer but I would go with the schematic, also the recomended switch RS 316- 800 is Double pole.

I have just ordered boards from Gustav and I am working my way through the 84 pages of this thread for potential problems ahead. So far I found:
1. The instrument jack needs to be wired directly between input transformer output and C2 and the tracks need to be cut. Is that still true or has the board been ammended since ?

2 . I have just bought 100u/400v capacitors for C14 & C15 but I just found  people seem to be recommending higher values less hum?.  The next highest I can get from my supplier (for exchange) is 330u then 470u.  If you go higher do you need to change anything else?

3. Are there any other issues /improvements any body can suggest before I get started?

3
2 issues I

1) Yes, the mod with cutting PCB traces and wireing frontpanel jack directly is highly recommended, as it saves you from potentially hard-to-trace problems later (but it will sometimes work without, just not always).
2) 100u should be enough for HT - but ymmw
3) Lundahl LL5402 output transformer recommended - the OEP version is not really consistent in performance.

Jakob E.