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Old 20th December 2004, 09:31 AM   #1
cedus is offline cedus  France
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Default New GC, old problems. Help required

Here is my little story.

After reading tons of articles and thread over the web during weeks and with my limited abilities with electronics I decided to go ahead and built my own amp.
After choosing the GainClone concept I finally chose to build the Dual Mono T-Network Inverted Valve Buffered GainClone (pfff ) of Joe Rasmussen : http://members.ozemail.com.au/~lisaras

After spending tens of hours creating, designing and building my own amp (I spent so much times on the case, burned so many fingers and checked so many times everything was OK !) yesterday I finally finished one of the channel.

I checked all the soldering again and again

I checked the DC offset with a 6R and found 100 mv (not great but I decided to give it a try).
I checked the PSU
7.8 V for the filament heater
+35V and - 35 V for the valve
+35V and -35V for the LM


Very confident but also very anxious I plugged everything and made a visual check twice and switch it on.

And....

The purest sound you can get : TOTAL SILENCE !

Nothing...

Not even a small buzzz. The filament turned red but no sound at all.

So frustrating !
I know I did not choose the simplest design for my first try but it is not so difficult (I mean it did not look like it).

My question is fairly simple and fairly common for a newbee. How can I check where the signal is lost.

Thank you in advance for your help !

Just for information : the speaker I try to drive is 6 ohm.
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Old 20th December 2004, 09:40 AM   #2
lohk is offline lohk  Europe
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Location: Northern Noricum, near the Limes
searchin a fault can work like that:
follow the signal flow and try to "circle in" the fault.

in your case: is there a signal after the tube stage? is there an output if you feed in some signal in the chips input directly?
And last but not least: Before all powering up and measuring check and double check all the wiring, most of the time it is just very "simple and stupid" mistake.

Klaus
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Old 20th December 2004, 09:55 AM   #3
cedus is offline cedus  France
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I am sure that must be something stupid! I checked the wiring so many times I hope for myself it is not coming from a wiring problem.

I will try to go over the tube stage tonight to see if I can get something from the chip alone.

Thank you
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Old 20th December 2004, 11:38 AM   #4
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Default New GC, old problems. Help required

That happen to me, it turn out that all the grounds( signal and power)where not connected. All the signal ground wires must be connected together then connected to power ground.
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Old 20th December 2004, 10:18 PM   #5
cedus is offline cedus  France
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Jaudio you were right ! It was a grounding problem. My cinch connectors were not isolated from the case ! Stupid me !
I finally heard sound (very very loud sound because apparently my stepped attenuator does not work) but amazing sound for one channel.

I am so happy. Just have to fix everything for a proper use and after that I will be able to post pictures here !!!
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Old 20th December 2004, 11:43 PM   #6
lohk is offline lohk  Europe
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As I said, it is often just simple, but sometimes it is also not there where you think it is.
I happend to make a MINI CHIPAMP about half a year ago, I think, using a LM1876. I reversed the polarities of the psu caps and the whole thing exploded right before my eyes after powering up. What a mess! Check and double check, but everything...

best wishes with your GC

Klaus
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