The official G9 help thread

[innercityman]

Thanks TillM,

I used the expensive one, but the spec seems to have changed in the newer version. Here's what's written on mine :

secondaries :

Red 275V 0,14A
Blue 12V 0,5A
Yellow 12V 3A

I saw pictures on the net of builds with the older version of this transformer and I saw that blue was 15V 0,3A . Would it change anything ?

And I did the correct wiring... Blue to 15VAC and yellow for 12.

But guess what... I've posted my problem on a Facebook group dedicated to pro audio, and a guy contacted me saying that he has exactly the same problem with exactly the same power transformer bought at don-audio.

I'll change the diodes D17 to D20 and see what happens. If it blows again, I'll try with the classic two transformers build.

 

[TillM]

Strange ... diodes d17 to d20 are for 275VAC area.
In my mind the 12VAC 3A are for the heater section. (Jakob prove me if I’m wrong)

 

[innercityman]

The guy who gets the same issue on FB told me yesterday that he contacted don-audio and they said that blue is 15V, not 12 like printed on the trafo, it's a printing mistake. He also said that he is pretty sure that the problem comes from the 220V. D5 zener has strange measurement on his G9.

I fired it up yesterday and no smoke. (strange). I've been able to put a mic on it and both channels work, but I have a hum on both sides. I suspect a ground problem. Here's a picture of my build...

 

[TillM]

Your grounding is wrong.
Just connect pin 1 from the input xlr with ground from IEC.
You separate it. That will causes ground loops.
Why you connect booth 12VAC wires to the pin ?
Your other 12VAC has no connection.
(Connection next to 275VAC)

 

[innercityman]

For the ground connection, this is what I was thinking about and decided to modify it today, so thanks for confirming me that. For the yellow 12V...

Why you connect booth 12VAC wires to the pin ?

What do you mean ? Do I have to connect only one wire and let the other floating with heat shrink ?

Your other 12VAC has no connection.

You mean the 15V blue wires ? They are connected to a socket. They come from under the PCB that's why you can't see it on the picture.

 

[innercityman]

I connected the trafo as it's indicated on don-audio's page for European 230V rating

 

[TillM]

For the ground connection, this is what I was thinking about and decided to modify it today, so thanks for confirming me that. For the yellow 12V...

What do you mean ? Do I have to connect only one wire and let the other floating with heat shrink ?

You mean the 15V blue wires ? They are connected to a socket. They come from under the PCB that's why you can't see it on the picture.

Thanks, I need glasses, I don’t see it first, that they went underneath the board.
The Hum should gone with new grounding.

 

[innercityman]

But you didn't answer my question about the 12V wiring...?

Why you connect booth 12VAC wires to the pin ?

What do you mean ? Do I have to connect only one wire and let the other floating with heat shrink ?

 

[TillM]

But you didn't answer my question about the 12V wiring...?

Your wiring from the power transformer is correct.

 

[Philip_BlueFX]

Hello dear community. I am new here as this is my first post ever.
I have decided to study on valve amplifiers and my first attempt is this G9. I am also planning on building it (of course!) but in lack of a budget (yet) and in excess of interest I am still trying to figure out the math and physics behind this (simple as it seems) design.

Now dear fellas, for my questions:
I am not 100% optimistic about my calculations but bear with me. Life's a journey of trial and error after all.

First of all, I have big trouble understanding the point of R7=47KΩ. I have calculated (for HT=245V) my V1 Bias Point at -6V for the 2.2k resistor. I also understand that the R7 resistor provides a voltage for the grid being a little lower than the cathode at all times. What is the purpose of this though? Studying valve theory I've seen the 1M Grid Leak and the 2.2k Cathode Bias shunting to the ground.

My second issue is about gain. I have calculated (my Bias Point is still 6V) my closed loop gain for the 1st Stage being Acl1=37 (that is for max input on the rotary, that is R11=1K). I have also calculated the 2nd stage gain being A=22.
I believe a microphone voltage output is about 15mV (right?) so I get about 100mV after the input trafo.
Considering my gain calculations, this all adds up to 77V  for the 2nd stage output. That is about 35V on the output trafo, right?
I am fairly positive that I have mistakes in my math and understanding.
What's an ideal output (before output trafo) voltage on an audio preamp like this?

I've spent some time in the forum reading posts about this design and I have noticed that most people's troubles are about the PSU voltage regulation and such. I more concerned about the actual amplification circuitry.
I know I am not but an infant in a group of war veterans but, in my defense, valves are considered ancestral technology in my university (Electronics Engineering) and I also got s**t for brains  :

 

[TillM]

Hey guys,
I get in trouble with my third Gyraf G9.
I get all the voltages and double and triple check all the parts.
I use OEP transformers in this build.
The channels have a difference from
nearly 12db.
What is a good starting point to „match“ the channels.

 

[evil grill]

Hey guys,
I get in trouble with my third Gyraf G9.
I get all the voltages and double and triple check all the parts.
I use OEP transformers in this build.
The channels have a difference from
nearly 12db.
What is a good starting point to „match“ the channels.

Check feedback resistors and cabling.

 

[gyraf]

..and check transformer types, they look the same..

/Jakob E.

 

[TillM]

Thanks for the instruction.
I get the unit non working and rebuild it.
I’ve no problem with hum or anything else.
It act like it should.
The transformers was glued on the pcb so it’s nearly impossible to change them.
The thing is. Sometimes it works and the channels have nearly the same output, sometimes I turn the unit on and I get this huge difference.
Then I turn it off and on and then it’s nearly matched.

 

[gyraf]

..swap the tubes and see if error follows a particular one...

 

[TillM]

Thanks !
I will try every suggestion you guys made  and see what happens.
Tube swapping I try first, but it makes no difference.
I will also check the output potentiometer, maybe the tolerances are to high.

 

[gyraf]

The thing is. Sometimes it works and the channels have nearly the same output, sometimes I turn the unit on and I get this huge difference.
Then I turn it off and on and then it’s nearly matched.

this is not tolerances or wrong components - it's something unstable or broken

Re-solder pcb, check and resolder wireing, run an ohmmeter across switches to verify good contact, check continuity in front-panel jacks when empty

..or start comparing AC levels at different points of the circuit: before input transformer and after it, after the first amplifier stage, at top of pot and wiper of pot, after output amplifier, before and after out transformer and after phase reverse switch..

/Jakob E.

 

[raul_ghiran]

Hello guys. I'm in the process o building my first Gyraf G9. Can you please recommend me some tube models that I can find in Europe at a decent price but still good quality. Do they all four tubes need to be matched? Thank's

Raul

 

[innercityman]

The tubes don't need to be matched as it's said many times in earlier posts. You can buy tubes from Thomann, they seem to be valuable and good quality ones.

https://www.thomann.de/fr/tad_vorstufenroehre_12au7.htm

https://www.thomann.de/fr/eh_nn065_roehre_12au7_eh.htm

 

[gyraf]

Can you please recommend me some tube models that I can find in Europe at a decent price but still good quality.

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

Do they all four tubes need to be matched?

No, not at all