# Barely audible signal when amp cranked



## bcmatt (Aug 25, 2007)

So, a changed my BF Vibro Champ Clone into this this week:
http://bp0.blogger.com/_4u9l5q2yFBA/SFE-kx8WNgI/AAAAAAAAAaw/t69nJRcDhw0/s1600-h/Latest.jpg

Of course, I used my VC xformers and tubes that worked fine for my champ last weekend.
I would like help to narrow down where my problem may be before I start posting pictures of the guts:

1) If I crank everything, I can get a very faint signal from the guitar coming through the amp speaker(like a soft whisper).
2) The Master volume pot does come through loud and clear with some scratchiness as I turn it.

How can I narrow down where my problem is coming from?
Since the MV knob has decent volume on it's scratchiness, would this indicate that the problem is somewhere in the preamp or tonestack?


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## bcmatt (Aug 25, 2007)

I don't want to squelch the overwhelming popularity of this thread by withholding any information... so here are the voltages at each of my pins:
V1 (12ax7) 
P1: 213 
P2: 0 
P3: 2 
P4: 68 AC (91 on standby) 
P5: 68 AC (91 on standby) 
P6: 234 
P7: 0 
P8: 2 
P9: 68 AC (91 on standby) 

V2 (12ax7) 
P1: 250 
P2: 0 
P3: 3 
P4: 68 AC (91 on standby) 
P5: 68 AC (91 on standby) 
P6: -1 
P7: 0 
P8: 0 
P9: 68 AC (91 on standby) 

V3 (6V6) 
P1: 0 
P2: 68 AC (91 on standby) 
P3: 325 
P4: 322 
P5: 41 
P6: 0 
P7: 68 AC (91 on standby) 
P8: 47 

V4 (5Y3 GT) 
P1: 0 
P2: 353 DC (461 on Standby) 
P3: 0 
P4: 305 AC (331 on standby) 
P5: 0 
P6: 305 AC (331 on standby) 
P7: 0 
P8: 353 DC (461 on Standby)

Thanks for the help


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## Ripper (Jul 1, 2006)

The first thing to do is go back over all the changes you made for this mod and triple check that they are connected correctly, check component values and make sure that something didn't get swapped into the wrong place, check that the solder joints are all good.

Do you have a signal generator and a scope?


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## bcmatt (Aug 25, 2007)

Ripper said:


> The first thing to do is go back over all the changes you made for this mod and triple check that they are connected correctly, check component values and make sure that something didn't get swapped into the wrong place, check that the solder joints are all good.


OK. I'll be doing that.



Ripper said:


> Do you have a signal generator and a scope?


nope. The closest I have is a couple electric guitars and a DMM.


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## Wild Bill (May 3, 2006)

bcmatt said:


> OK. I'll be doing that.
> 
> 
> nope. The closest I have is a couple electric guitars and a DMM.


First off BCM, something's wonky with the wiring at pin 6 of V2. You show a voltage of 0. It should be close to the same voltage as pin 1, since they both are supposed to be fed from the same B+ point, each through a 100k resistor.

With no plate voltage you can't have any cathode bias voltage across that 2.7K resistor, which explains the 0 volts at that point.

Re-check your wiring and it will probably fix your problem.

Good luck!

:food-smiley-004:


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## Ripper (Jul 1, 2006)

bcmatt said:


> nope. The closest I have is a couple electric guitars and a DMM.


If you're going to be doing lots of amp work you'll find them to be a huge help for tough problems


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## bcmatt (Aug 25, 2007)

Wild Bill said:


> First off BCM, something's wonky with the wiring at pin 6 of V2. You show a voltage of 0. It should be close to the same voltage as pin 1, since they both are supposed to be fed from the same B+ point, each through a 100k resistor.
> 
> With no plate voltage you can't have any cathode bias voltage across that 2.7K resistor, which explains the 0 volts at that point.
> 
> ...


Great! Thanks Wild Bill! You nailed it. It works great now!
It was the 500 pF cap that also comes off pin 6. I had accidentally installed it blocking the input to pin 6. I just had to alter a couple connections and voila!
My problem is that I changed the layout too many times to make it fit on the Vibro Champ eyelet board.

I learned that things would have been less confusing (and more aesthetically pleasing) if only I had managed to track down my own new eyelet board to build on, rather than adapting one from a different amp. Even if I couldn't get any board material, I would have been better to go straight Point to point with just terminal strips. I'd consider that next time.

Anyways, this is one great and versatile little amp that has some great manageable volume and heaps of gain on tap if I need it.

Thanks guys!


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## Wild Bill (May 3, 2006)

bcmatt said:


> Great! Thanks Wild Bill! You nailed it. It works great now!
> 
> Thanks guys!


Great! Glad to hear you're up and wailin'!:smile:

:food-smiley-004:


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