The official G9 help thread

I will wind one m6 transformer for 1:7 and test the inductance and I will wind one nickel lam 1:7 to see how the bottom end compares to the OEP 1:6.45 transformers on the input.
Thinking about it I can kill two birds with one stone, I'll wind some U67 transformers, they are 1:7 with a feedback winding, I just won't use the feedback winding.
I can then compare inductance and DCR.

Before i do that i have a couple of my 6.5:1 B5887 transformers which i'll throw in there and test, as these sound fantastic on my API-312 preamps. Fingers crossed it cures the lack of bottom end as this preamp is great otherwise.

well so far i have established that adding 20uf to output capacitor doesn't do anything, now for changing input transformers.
definitely a lack of bottom end here, A/B'ing agains Neve and API.

Happy to report, installed my B5887 6.5 ratio transformers on input and getting full bottom end now!!!
Dont use the OEP 1:6.45 transformers folks, sucks all your bottom end out!!!

Interesting. Is that your own transformer batch?

Yes, that is my U47 transformer which works great in U47 mic, API-312 and now the gyraf g9.
Sounds fantastic I have to say and I'd written of this build!
Can't wait to use it on the next albums now!

Good job

If I wasn't out of budget, I would have go with the Cinemag 75101 for this one.

Yeah nice , cinemag is £79.99 + shipping, mine is only £50, probably ends up being half price for me!

Spencerleehorton said:

Yeah nice , cinemag is £79.99 + shipping, mine is only £50, probably ends up being half price for me!

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Mu-metal shielded?

Yes it can come in a mu-metal can for a bit extra.

..going too off-topic, guys...

Sorted problem with 50mA lamp. I would have liked to understand if it was lamp or faulty tube, but its finished and sounding good.

Thanks to Jacob and everybody else who helped.

Hi everybody.

I come with a little voltage problem here. Previously to asking anything, I want to say I'm sorry about it to be noob questions if it is. 

So I finished populating the PCB and decide to test and measure voltage out of PSU before mounting tubes on the preamp.  EVERY measures were taken without tube on sockets (if it's important).

Some voltage are correct :

• 12 VDC on C24
• 11,9 VDC on the 4 heaters pin of tube sockets
• 48,2 VDC on C23

But other are really High :

• 276VAC out of T2
• 371VDC on C14
• 380VDC on C15 instead of 245VDC
• 88VDC on D12 instead of 60VDC

I don't know if it's because there is no tube mounted. And I fear damaging my tube (I'm a little on a budget with this tubes I rather prefer don't burn them  ;D...)

I first thought it was a problem with Transformers (I'm in France and my main is measured at 235VAC). Here is a link to their datasheet : http://www.farnell.com/datasheets/1909346.pdf

T1 is 1x230V to 2x15V 50VA (VTX-146-050-115).
I measured 17VAC on its output.

T1 is 1x230V to 2x12V 30VA (VTX-146-030-112).
I measured 276VAC on its output.

I wired this way : On T2 orange and black tied together and connected to T1's Orange.  T2's red and yellow tied together and connected to T1's yellow.

Doesn't someone have an idea of what happen ? Is it normal ? Did I fail my transformers mount in any way ? I see someone had a similar issue but there is no indication in the discussion on how he fixed the probleme.

Best regard
Bee

As we have mentioned repeatedly in this thread, it does not make sense to measure anything in a G9 without the tubes mounted. The reason is that voltages will keep climbing when there is no load, i.e. no current is flowing (it can only flow through the tubes, and only when they are heated). This voltage climb can even damage components, if voltage gets too high.

And although there is a theoretical risk of damaging a tube by wrong circuit, it's very very rare. And nearly impossible if you check your heater voltage not too long after power-up.

I have not yet managed to damage a tube by wrong-circuiting it, and I throw lots of errors around..

Jakob E.

Thank you Jacob. And sorry to make you repeat it again, but I really search and didn't find anything about that.  :-[

I will come back with usable measures...  :-X

Hello everybody,

I don't know if it's me or not, but each time i try to search for something with this website search engine, I don't find my answer.  :-\

Anyway, I just completed my G9 and just tested it. On both channel I have a really low hum that I will correct. Channel 1 is working perfectly. Channel 2 sound strictly the same except that I have a big buzz growing up following gain position : From position 1 to Position 5, Buzz is more and more loud, after position 5, it stay at the same level.

I remember that i saw a post about a similar problem just before beginning construction of my G9, but I didn't manage to find it again now.  :-[

Do someone know where i can find it ?

if the buzz-hum follow the gain, then it's behind the gain circuit, so probably around the input transformer, input connector, etc. Is your input cable shielded? How is your grounding scheme? Did you use shielded input transformer 9in a mumetal can) ? pictures? Thanks

Deepdark said:

if the buzz-hum follow the gain, then it's behind the gain circuit, so probably around the input transformer, input connector, etc. Is your input cable shielded? How is your grounding scheme? Did you use shielded input transformer 9in a mumetal can) ? pictures? Thanks

Click to expand...

Hello,
It follows gain only on the 4 first steps, when this is not "gain" but the attenuation part on the switch. and only on the 2nd channel.

- Input cable shielded : no they are not because inputs are symetrical, do they really need to be shielded ? Moreover if it was input that was the origin, buzz would get louder even after step 4 of gain switch.
- Instrument cable is shielded
- Ground scheme is in test (understand i did what was asked, but now i'm trying different things now that I have buzz)
- All trannies are in cans, cans are soldered and are the official cans for OEP (http://uk.farnell.com/oep-oxford-electrical-products/a262can/screening-can-transformer/dp/1172345?MER=sy-me-pd-mi-acce)
- I'm investigating something that could be the answer : it seems that the non-shielded power cable that go to the power switch is to close from the instrument input connector.

Does shielded cables works in the two direction ? I mean, if i put a shielded wire on power cable do that make this cable do less emission ?

I Add photo as soon as possible.

You don't have to shield your power cable, but twist it as tight as possible, and of course, keep it as far as possible from the sensitive part as possible. Input cable should be shielded ideally, if you get noise from the cable, it will get amplified throught out the circuit. Your ground scheme should be like the schematic recall it. So, input xlr is a bit like your star ground. You exit at only one place in the board, channel 1 input xlr ground pad. You connect it to pin 1 of xlr 1. Then, you send it to input xlr 2 (second channel) pin 1 and then, send it to the main bolt, near the IEC. You do not connect channel 2 input ground pad on the board to the xlr pin 1. Only on channel 1. Because you don't want to create a ground loop. The output xlr, you could try to only connect pin 1 straight to the chassis. It shouldn't be harder than that.

Deepdark said:

You don't have to shield your power cable, but twist it as tight as possible, and of course, keep it as far as possible from the sensitive part as possible.

Click to expand...

My guess was good. The problem comes from the main power switch cable that emite buzz to the control panel.
Because I wanted to have the on/off switch and brand logo on the center of the front panel and as i wanted to add a "standby" fonction, I put the power switch there. But this is where there is the instrument jack input, the gain switch and the HPF which are the three areas sensitive to main power emissions. this cable running with 220VAC is putting buzz everywhere...

So I will twist it, but anyway, the power switch also emite some buzz. I will find a solution.

It seem that I have no ground problem for the moment but I will double check my grounding. Thank you very much.

Can I ask (simple curiosity of the curious man) why twisting power cable works to make them not emite buzz ?