The official G9 help thread

[MeToo2]

At last I have been able to get back to the G9. I am using Gustav's board incidentally. I rechecked my transformer connections, swapped a couple over and I can confirm the voltages on the PCB seem OK, that is, 12V, 48V and 265V on the HT. Following discharge of C14, C15 I have disconnected the various connectors and with no load the 15V toroid is still getting warm. I then disconnected the other 15V secondary (which feeds the 12V secondary on the other transformer and provides 15V to the side connector on the board, thus providing no load at all to the transformer) and it runs cooler.  It doesn't produce any mechanical hum either. On this basis it would seem that I have either got something connected wrong between the two transformers or the transformer just gets rather warm.  Thanks for the wisdom and support
Steve

Can you draw diagram of how you wired the transformers?
Do they have split/dual secondaries?
And if so, did you make sure the phasing was correct?

 

[culteousness1]

Short question on C23:

I could only source a 22uF instead of a 10uF cap at Reichelt.

Will this be a problem?

Best,
Carsten

 

[redmojosteve]

It's taken me a bit longer than I expected to draw the transformer connections but here they are.
Sorry but the yellow isn't too prominent against the other colours
Thanks
Steve

 

[bernbrue]

 

[MeToo2]

It's taken me a bit longer than I expected to draw the transformer connections but here they are.
Sorry but the yellow isn't too prominent against the other colours
Thanks
Steve

Sorry. Can't read the doc. Can you attach as gif, jpeg or png?

Presume you are located in the UK (Warrington) with 240V 50Hz mains.

If so, I suggest you do NOT follow the example posted above by bernbrue.
That is for a 115V supply as far as I can see (primaries of first transformer in parallel)

 

[bernbrue]

Yes, your are absolutely right as far as the primary is concerned. So, Steve, connect the first transformer as you did in your drawing and do the rest according to my picture and you should be allright.
regards
Bernd

 

[MeToo2]

Yes, your are absolutely right as far as the primary is concerned. So, Steve, connect the first transformer as you did in your drawing and do the rest according to my picture and you should be allright.
regards
Bernd

Exactly.

Managed to read the doc now (browser issue at my end)

Steve, I don't know why you've marked many transformer terminals "0V" on your diagram. They are NOT 0V, and must NOT be connected to ground, or to each other in your wiring outside of the PCB. The mains neutral is normally grounded at your electricity meter in most old style UK installations, but may not be the same 0V as your Earth pin. This could be very dangerous if you've connected mains neutral to the chassis. Some mains sockets may be wired incorrectly in your house, which would then connect live to the chassis. If you're still in doubt, please post detailed photos and transformer model numbers.

 

[redmojosteve]

Thank you MeToo2 and Bernd for the quick response. You are correct and I have 240V 50Hz mains. Believe it or not, the diagrams I used for the transformers were cut and pasted from the Nuvotem/Talema transformer datasheet (the transformers I am using). I couldn't find a nice schematic for a transformer quickly enough to get the sketch on the forum so I used the diagram on the datasheet, which could be misleading. Nonetheless, thank you for your very important message regarding safety, and to put everyone's mind at rest, including mine, I have not grounded the terminal marked "0V"!
I'll connect up as per Bernd's diagram and let you know how I get on.
Cheers
Steve

 

[MeToo2]

Thank you MeToo2 and Bernd for the quick response. You are correct and I have 240V 50Hz mains. Believe it or not, the diagrams I used for the transformers were cut and pasted from the Nuvotem/Talema transformer datasheet (the transformers I am using). I couldn't find a nice schematic for a transformer quickly enough to get the sketch on the forum so I used the diagram on the datasheet, which could be misleading. Nonetheless, thank you for your very important message regarding safety, and to put everyone's mind at rest, including mine, I have not grounded the terminal marked "0V"!
I'll connect up as per Bernd's diagram and let you know how I get on.
Cheers
Steve

Cool.  8) I'm surprised at that data sheet. Normally they mark the transformer connections as normal and inverted phase with a dot. But those points sure ain't 0V DC relative to ground, and they way they were wired would explain the heating and vibration. Don't hesitate to ask questions if you're not 100% sure.

 

[redmojosteve]

Again, thanks for your greatly appreciated assistance. I have reconnected the transformers, but I am still getting a warm 15V one. It all works and passes audio nice and quietly. As before, the 12V, 48V are OK but I am getting just 235V on the HT. I checked the voltage across R33 which was 9.15V which suggests a current of around 19.5mA. The comment on grounding got me curious. I have only grounded the build at the No 1 channel input XLR ground, and the incoming IEC earth connection to the case of course. I'll come back to it in a couple of days and check it over again as I am sure I must have missed something.
Cheers
Steve

 

[bernbrue]

Hi Steve,
you can try using only one secondary winding of the transformer that transforms 15V to 236V. I did this in my build and transformer noise/heat went away. Proper grounding is essential for correct voltages. There are a few ground bridges on the main board as well. Verify that you soldered them in.
regards
Bernd

 

[MeToo2]

Hi Steve,
you can try using only one secondary winding of the transformer that transforms 15V to 236V. I did this in my build and transformer noise/heat went away.
regards
Bernd

hmmm would that suggest non-identical 15V secondary windings on your step up transformer?
One winding shouldn't be a problem for the amount of HT power used if you're only drawing 20mA through R33 (which sounds about correct) 20mA *240V = ±5W.

 

[redmojosteve]

Thanks gents for patience and the ongoing helpful suggestions. I have disconnected one of the secondaries of the step up transformer and the first transformer seems a little cooler, but not much. I had a spare 230/15V traffo for my planned second G9 and tried that and it is the same, so it rules out a transformer fault. This begs the question, and I am sorry if this sounds stupid, but how hot would you expect the trafo to get?
Thanks
Steve

 

[MeToo2]

Thanks gents for patience and the ongoing helpful suggestions. I have disconnected one of the secondaries of the step up transformer and the first transformer seems a little cooler, but not much. I had a spare 230/15V traffo for my planned second G9 and tried that and it is the same, so it rules out a transformer fault. This begs the question, and I am sorry if this sounds stupid, but how hot would you expect the trafo to get?
Thanks
Steve

Pretty much standard 40VA toroidal transformers here. Nothing special. Block RK40/12 & RK40/15. Can't hear them (no buzz at all). Can't feel them (not even hand hot) The only things that get warm in my build are the 12V heater regulator heat sink, and the tubes themselves. [Don't be tempted to touch the heatsink on the TL783. It'll be at ±240V!]

 

[redmojosteve]

Good evening guys. I have gone back a few steps and taken some measurements of current. HT current draw about 20mA, on the 12V connector around 1.05A, and at the 48V connector it's 0.175A. I think the noise will be minimal by effective mounting and putting the lid on the case! Interestingly I have also found that hum pickup at max gain is minimised to a very low level by rotating the toroidal transformers by about 20 Deg so I'll create an angled mount for them to take advantage of this.
Thanks again
Steve

 

[Majestic12]

I just checked my second G9 build and I'm wondering why the polarty knob changes the frequency response.
With the knob in reversed position, I get more bass and top end. Same behaviour on both channels. (Of course every channel was tested on its own, no summing of phase reversed signals)
Can anybody give me an explanation for this ?

 

[daal]

Have you recorded the same signal in each position to see if it actually nulls or not ? Because in my case, everytime I pull a "reverse phase" button, I have the sensation that the new sound is much more plain, but it's a psychoacoustic effect due to the phase change. If I record both, I can see that they null correctly.

 

[Majestic12]

I can clearly see the difference in the two frequency responses..
I'm taking exact measurements this afternoon, but I have no idea where this stange behaviour is caused....

EDIT: Problem solved. It was just a mistake in my measurements...

 

[redmojosteve]

Re my ongoing saga of the hot transformer, I have just checked the current drawn by the secondary of the second transformer (12V-12V) from the secondary of the first (15V-15V) and it is around 4.4A which is rather more than I would have expected. I also think this is greater than the manufacturer's data sheet, which would explain the excessive temp.

 

[MeToo2]

Re my ongoing saga of the hot transformer, I have just checked the current drawn by the secondary of the second transformer (12V-12V) from the secondary of the first (15V-15V) and it is around 4.4A which is rather more than I would have expected. I also think this is greater than the manufacturer's data sheet, which would explain the excessive temp.

On the PCB you have:
12V heater 1.05A = nominal
48V = 175mA = nominal
so they don't look like the problem.

Neither is the HT supply pulling 20mA. (9V drop on the 470 ohm resistor R33) also pretty nominal

Assuming the step up transformer is acting something like 80% efficient (which is poor)
HT 20mA @ 260V would imply 260/12*20mA /0.8 => ± 550mA RMS on the 12V side.

Certainly not 4.4A sustained (although you may get peaks over a portion of the cycle as the smoothing capacitors charge).
But 4.4A sustained @ 15V is 66W and would certainly explain excessive heating on the 15V transformer.

So it looks to me like there's something very suspicious either with that 2nd 12v step up transformer or your HT bridge rectifier (D17 - D20).
Is your TL783 isolated from the case?
Are your HT rectifier diodes (D17 - D20) correctly oriented?
Is the point where D19 & D20 meet isolated from 0V (chassis)?