The official G9 help thread

[dinesh]

HI,

Can I use 7812 instead of 78S12??

 

[gyraf]

I'd prefer 78S12 - that's why it is specified in stead of the easier-to-get 7812.

Jakob E.

 

[MrShhh]

[quote author="INDI03"]@Mrshhh

maybe this could help you - check the R34 , R35 , R36[/quote]

Cheers, Indi03, I've checked they are the correct values. I also checked some voltages:

783:
Vin = 264
Vout = 243
Vadj = 239

272V from the rectifier
242V at the zeners

R33:
Vin = 272
Vout = 264

R34 239V

R35 243V

R36 243V

R37:
Vin = 243
Vout = 235

Vc14 = 264
Vc15 = 235

Should I be losing that much voltage across R33 & R37? I will check the current going through the 783 if that helps...

 

[Soeren_DK]

Hey Foks.
I just put a G9 preamp together and it sounds so great. Thanks again Jakob for a great project.

The next is:
There is a little hum but not like a grounding problem. I have tried that a lot and this is a little different.

I can only hear the hum when I set the volume on my mixer yo max and its stays when all knobs on the G9 is in 0 position.. Weird. I discovered when I put my ear to the 12v trafo I can hear that same hum is comming from this.
Could there be a connection?

Soeren Sailor

 

[MrShhh]

I think I've found the source of the low gain problem I spoke of earlier.

I have 16.6mA HT current which is in the 15-20mA range, but only 157mA heater current (should be around 600mA).

No heater = no sound right?

I will try to work out where the problem originates with the 12V supply.

 

[MrShhh]

There's 610mA at the output pin of the 78S12 so I'm not sure how it's falling to 157mA when measured at the +12V bridging wire just above C24.

Could it be the valves? Or the valve sockets?

Looking at the schematic, the 12V supply circuit is very simple, The only other components I can see after the output pin of the 78S12 are C19, C24 and the light.

I've ruled out the light so far.

Just a thought, is it correct to assume that the heater current should be the same through all four valves? In other words does it drop lower after each valve? I know squat about valves, but if the heater is acting as the cathode and emitting electrons, would that then explain the current loss?? 157 is curiously approx. 600 / 4 and where I measured the heater current, it had yet to pass through 1 valve. Am I just babbling nonsense? Please advise, this is fascinating stuff!

Thanks again

 

[laiben]

Hi,

I am still building the G9 but my experience of DIYing a guitar tube amp tell me that the heater current should be very close if not the same through out the four tubes. Is the tube lighting up normal when you power on? I think if there is only 1xxmA current you can't see the tube light up, or it may be very dim.

How do you measure the heater current? with the tube all in sockets? desolder some parts? measuring current should be put the meter in series with the component.

laiben

 

[MrShhh]

[quote author="laiben"]Is the tube lighting up normal when you power on? I think if there is only 1xxmA current you can't see the tube light up, or it may be very dim.[/quote]

Hi laiben, thanks for getting back :grin: All 4 tubes do light up, not very brightly however, but uniformly bright.

[quote author="laiben"]How do you measure the heater current? with the tube all in sockets? desolder some parts? measuring current should be put the meter in series with the component.laiben[/quote]

I measured the current (with all tubes in place) by cutting the long 12V bridge wire just above C24 and then placing the multimeter in series with the two cut ends of the wire.

I also measured the current at the output pin of the 78S12 by cutting the pin in half and attaching crocodile clip probes across the 2 halves being very careful not to short anything with the clips.

If the heater current should be the same through all 4 valves then that gives me something to work on.

I've checked for shorts on the main board but found none. I noticed that 12V goes to the switch boards as well so maybe there is a problem there.

 

[gyraf]

Measure voltages, not currents - that makes many things easier.

If the tube gets the needed 12V for the heater, it'll draw the right current (providing it's functional, and the right type).

 

[MrShhh]

[quote author="gyraf"]Measure voltages, not currents - that makes many things easier. [/quote]

Thanks Jakob, I started off down this route thinking it was the "easy" cheats way of doing things :!:

There is 11.73 heater voltage which is down to the 78S12. Some of them seem to like putting out around 11.7V. There is 19V at the input pin so I'm fairly sure the 78S12 is working correctly.

The tubes are 2 x Amperex and 2 x Valvo ECC82. The dealer claims they are in very good condition but I can't know for sure without a tester.

Would it be a good idea to check the voltages at the valve pins?

 

[enthalpystudios]

if i want to use a standard tube power transformer for my build, what should i be looking at?

I'm thinking that a 250-0-250 at 250ma each winding, with a 2A 12.6v winding.

Will each of those ac voltages be enough, after rectification, to properly keep the regulators working?

 

[dinesh]

Hi,

I am not able to get 78s12 in India. Can I use LM317?

or any other equivalent??

Regards
Dinesh

 

[gyraf]

You should be able to find 78S12 nearly anywhere - it is not rare at all. TL783 is much, much more hard to find...

Jakob E.

 

[dinesh]

Hi Jakob,

I have searched these in my 400km area without any success. some comman parts are not easily available in India, & I am from a very small town. I got 783 as Samples. Thanks to TI for samples

 

[gyraf]

Hi Dinesh,

In that case you may want to try with a standard 7812 regulator - but make sure that it has very good heatsinking.

What part of India are you in? I lived most of my childhood in Orissa, India..

Jakob E.

 

[dinesh]

WOW Jakob,

Thats great :grin: What was you doing there??

I am in North (Punjab), I am going to delhi in couple of days in search of

some parts.

Thanks for your help

Regards
Dinesh

 

[gyraf]

What was you doing there??

In fact, very little.

My father was a lone doctor on a far-out countryside hospital in Bissamcuttack, Rayagada district, for six or seven years - he was sent there by some help organization to start up a hospital from scratch.

But for me there was very little to do - almost no technology around :razz: - one telephone in the village (in the post office) - absolutely no mains power until 1972 (and we lived there from '66 or so), no tv, no libraries... Oh yes, we had a battery-driven transistor radio that would occasionally pick up something other than static noise... ..but not often..

We spoke the Orya dialect locally - but sadly I've forgotten most of it because of no use the latest 30 years..

Nice to see someone from that part of the world in this forum!

Jakob E.

 

[nielsk]

I wound up seperating the heater supply from channel 1 & 2, and running them off seperate regulators. Even with the "s" series, & lots of heat sinking, they get very hot. This approach seems to be holding strong

 

[dinesh]

Hi nielsk,

Thanks I was thinking of same. I am going to use two 7612 for testing.

Regards
Dinesh

 

[madreza]

Hi everyone

I built A G9 1 year ago
It perfectly worked.
I wanted to change a few things and now I only have 1 channel working

My question is :
How do I have to troubleshoot the problem ???
( what to begin with, how to test ....... )

Thanks